WO2021072481A1

WO2021072481A1 - Medical linear accelerator calibration phantom

Info

Publication number: WO2021072481A1
Application number: PCT/AU2020/051046
Authority: WO
Inventors: Michael George MASTERS
Original assignee: Biline Calibrations
Priority date: 2019-10-14
Filing date: 2020-10-01
Publication date: 2021-04-22

Abstract

Medical linear accelerator calibration phantom (10) comprises a base plate (12) operatively mountable proximate a mechanical isocenter of accelerator (8), and includes first and second linear displacement sensors (14, 16). Phantom includes an X-axis slider (18) slidably arranged on the base plate (12), having an X-axis reticle (20), said X-axis slider (18) associated with the first linear displacement sensor (14). Also included is a Y-axis slider (22) slidably arranged on the base plate (12) perpendicular to the X axis slider (18) and having a Y-axis reticle (24), said Y-axis slider (22) associated with the second linear displacement sensor. The reticles complementarily form an X- and Y axes displaceable crosshair (26), a position of which is determinable via the displacement sensors, a light image indicative of a radiation isocenter of the linear accelerator, sightable by the crosshair in order to determine incongruence between the mechanical and radiation isocenters.

Description

MEDICAL LINEAR ACCELERATOR CALIBRATION PHANTOM

TECHNICAL FIELD

[0001] This invention relates broadly to the field of medical linear accelerators, and more specifically to a medical linear accelerator calibration phantom and an associated method of calibrating a medical linear accelerator.

BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application .

[0003] A linear particle accelerator (often shortened to 'linac') is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear beamline. Linacs have many applications, for example they generate X-rays and high energy electrons for medicinal purposes in radiation therapy, serve as particle injectors for higher-energy accelerators, and are used directly to achieve the highest kinetic energy for light particles for particle physics.

[0004] In medical linac applications particularly, the accurate measurement and calibration of a radiation isocenter is critical and will ultimately impact the quality of radiation therapy, especially high-precision techniques, such as stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT). The radiation isocenter is generally the point in space through which the central rays of the radiation beams pass, whereas the mechanical isocenter of a linac is defined as a virtual point at which the rotation axes of gantry, collimator, and treatment table intersect in an ideal condition, and is generally assumed to be within a virtual sphere in space due to mechanical limitations.

[0005] The American Association of Physicist in Medicine (AAPM) Task Group (TG) 40(1) provides quality assurance guidelines for radiation oncology and one important recommendation is that the coincidence of radiation and mechanical isocenter should be within a 2 mm diameter. Later in AAPM TG 142(2) it is recommended that the coincidence of radiation and mechanical isocenter should be within ± 1 mm from baseline for SRS/SBRT.

[0006] However, neither AAPM TG report provides guidance of quantifying the congruence of the radiation isocenters for different energy modes. The radiation isocenter of each energy mode can be within 1 mm from mechanical isocenter, in compliance with the TG-142(2) recommendation, but the relative distance among radiation isocenters can still be uncertain.

[0007] Because of different flattening filters, bending magnets and steering parameters used in medical linacs, the radiation isocenter of one energy mode can deviate from another if no special effort was devoted during the linac acceptance/commissioning phase. Conventionally, there is no established method to determine the relative radiation isocenter distance from the mechanical isocenter, thus major linac vendors do not generally provide the multi-radiation isocenters alignment test in their routine acceptance procedure.

[0008] In general, the projected light field image which emanates from the treatment head of a linear accelerator is a visual representation of where the focus of the radiation will be present during treatment. The accuracy of the targeting of this projected image is dependent on the calibration accuracy of the projected crosswire and the scale of the image.

[0009] As is known in the art, an imaging phantom is a specially designed object that is scanned or imaged in the field of optical and/or imaging instruments to evaluate, analyse, and tune the performance of such an imaging device. With medical imaging, a phantom is generally more readily available and provides more consistent results than trail-and- error or the use of, for example, a living subject or cadaver, and likewise avoids subjecting a living subject to direct risk.

[0010] Accordingly, Applicant has identified a shortcoming in the art of medical linear accelerators for accurately quantifying and configuring the accuracy and setup of the mechanical isocenter of a medical linear accelerator, generally determined by the treatment support system, with the radiation isocenter, as determined by an optical system. The current invention was conceived with this shortcoming in mind.

SUMMARY OF THE INVENTION

[0011] The skilled addressee is to appreciate that reference herein to the 'radiation isocenter' of a linear accelerator may refer to a coincidence between the actual radiation isocenter and a light field or light field image projected by the linear accelerator, said coincidence calibrated during manufacture, assembly, installation and/or commissioning of a linear accelerator. As known in the art, such a projected light field or light field image facilitates in alignment and placement of a treatment area of a patient, being in the visible spectrum, whereas the radiation focused on the radiation isocenter is generally invisible. In the present invention, such a projected light field or light field image may be referred to as, or used interchangeably with, a radiation isocenter, context depending.

[ 0012 ] According to a first aspect of the invention there is provided a medical linear accelerator calibration phantom comprising : a base plate operatively mountable proximate a mechanical isocenter of said linear accelerator and including first and second linear displacement sensors; an X-axis slider slidably arranged on the base plate and having an X-axis reticle, said X-axis slider associated with the first linear displacement sensor; a Y-axis slider slidably arranged on the base plate perpendicular to the X-axis slider and having a Y-axis reticle, said Y-axis slider associated with the second linear displacement sensor; wherein said reticles complementarily form an X- and Y-axes displaceable crosshair, a position of which is determinable via the displacement sensors, a light field indicative of a radiation isocenter of the linear accelerator sightable by the crosshair in order to determine incongruence between the mechanical and said radiation isocenters of the linear accelerator . [0013] The skilled addressee is to appreciate that misalignment or incongruence of the radiation and mechanical isocenters of the linear accelerator leads to radiation being directed elsewhere than anticipated, i.e. the radiation is expected at a particular position relative to a treatment support platform of the linear accelerator, but such incongruence leads to the radiation not being directed where expected. Generally, the radiation isocenter of the linear accelerator is sightable by the crosshair by means of sighting a projected crosswire and/or a projected light field image, indicative of such radiation isocenter, allowing calibration relative to rotation axes of a gantry, collimator, and treatment table (treatment support platform) of the linear accelerator .

[0014] Typically, the base plate is operatively mountable proximate a mechanical isocenter via mounting on a treatment support platform of the accelerator.

[0015] In an embodiment, the base plate comprises feet for securing the phantom to the treatment support platform, such as suction cup feet or similar support.

[0016] Typically, the phantom includes mountings for mounting the base plate proximate the mechanical isocenter.

[0017] Typically, the displacement sensors comprise linear encoders using the Vernier principle of interpolation.

[0018] Typically, the base plate defines a viewing window wherein the crosshair is displaceable. [0019] Typically, X-axis slider defines a viewing window which complementarily aligns with the viewing window of the base plate.

[0020] Typically, Y-axis slider defines a viewing window which complementarily aligns with the viewing window of the base plate.

[0021] Typically, each displacement sensor comprises a digital display for displaying a position of the crosshair from a virtual centre of the viewing window.

[0022] Typically, the sliders comprise slidable plates arranged on the base plate.

[0023] Typically, each slider comprises an adjustment screw, dial or knob configured to selectively displace or adjust said slider in order to facilitate sighting of the radiation isocenter via the displaceable crosshair in a controllable manner.

[0024] Typically, the reticles each comprise a straight- line reticle arranged along a relevant X or Y axis.

[0025] Typically, each reticle is arranged within a viewing window of a respective slider, the perpendicularly-arranged sliders forming the crosshair within complementarily aligned viewing windows.

[0026] Typically, the phantom comprises a camera configured to monitor an incidence of the radiation isocenter and/or projected light field of the linear accelerator. [0027] Typically, the camera includes an incidence surface on which the radiation isocenter and/or projected light field of the linear accelerator is operatively incident, said incidence surface monitored by the camera.

[0028] According to a second aspect of the invention there is provided a method of calibrating a medical linear accelerator, said method comprising the steps of: mounting, proximate a mechanical isocenter of the linear accelerator, a medical linear accelerator calibration phantom comprising a base plate and first and second linear displacement sensors; an X-axis slider slidably arranged on the base plate and having an X-axis reticle, said X-axis slider associated with the first linear displacement sensor; a Y-axis slider slidably arranged on the base plate perpendicular to the X-axis slider and having a Y-axis reticle, said Y-axis slider associated with the second linear displacement sensor; wherein said reticles complementarily form an X- and Y-axes displaceable crosshair, a position of which is determinable via the displacement sensors; and sighting a projected crosswire and/or a projected light field image, indicative of a radiation isocenter, produced by the linear accelerator with the crosshair in order to determine incongruence between the mechanical and radiation isocenters of the linear accelerator.

[0029] Typically, the method includes a step of correcting incongruence between the mechanical and radiation isocenters by means of suitable adjustment.

[0030] According to a third aspect of the invention there is provided a method of calibrating a medical linear accelerator, said method comprising the steps of: providing a medical linear accelerator calibration phantom in accordance with the first aspect of the invention; and determining incongruence between a mechanical and a radiation isocenter of the linear accelerator by means of such phantom.

[0031] According to a fourth aspect of the invention there is provided a medical linear accelerator calibration phantom and an associated method of calibrating a medical linear accelerator substantially as herein described and/or illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be made with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic perspective-view representation of a medical linear accelerator calibration phantom, in accordance with an aspect of the present invention;

Figure 2 is a diagrammatic top-view representation of the phantom of Figure 1;

Figure 3 is diagrammatic perspective-view representation of a medical linear accelerator having the calibration phantom of Figure 1 mounted to a treatment support platform thereof;

Figure 4 is a diagrammatic exploded perspective-view representation of the phantom of Figure 1, showing the constituent parts in accordance with one embodiment; Figure 5 is a diagrammatic exploded perspective-view representation of a medical linear accelerator calibration phantom, in accordance with an aspect of the present invention;

Figure 6 is a diagrammatic perspective-view representation of the phantom of Figure 5;

Figure 7 is diagrammatic top-view representation of the phantom of Figure 5; and

Figures 8 to 11 are diagrammatic perspective-view representations of example mountings for mounting the calibration phantom of the present invention to a treatment support platform of a linear accelerator.

DETAILED DESCRIPTION OF EMBODIMENTS

[0032] Further features of the present invention are more fully described in the following description of several non limiting embodiments thereof. In the figures, incorporated to illustrate features of the example embodiment or embodiments, like reference numerals are used to identify like parts throughout. This description is included solely for the purposes of exemplifying the present invention to the skilled addressee. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. Additionally, features, mechanisms and aspects well-known and understood in the art will not be described in detail, as such features, mechanisms and aspects will be within the understanding of the skilled addressee.

[0033] With reference now to the accompanying figures there is shown an embodiment of a medical linear accelerator calibration phantom 10. The phantom 10 generally comprises a base plate 12 operatively mountable proximate a mechanical isocenter of said linear accelerator 8 and includes first and second linear displacement sensors 14 and 16.

[0034] Phantom 10 further includes an X-axis slider 18 which is slidably arranged on the base plate 12 and has an X-axis reticle 20. The X-axis slider 18 is associated with the first linear displacement sensor 14. Similarly, the Y-axis slider 22 is slidably arranged on the base plate 12 perpendicular to the X-axis slider 18 and has a Y-axis reticle 24, said Y-axis slider 22 in turn associated with the second linear displacement sensor 26.

[0035] In the embodiment shown in Figures 5 to 8, each slider 18 and 22 also additionally comprises an adjustment screw, dial or knob 44 configured to selectively displace or adjust said slider 18 or 22 in order to facilitate sighting of the light field or light field image, indicative of a radiation isocenter, via the displaceable crosshair 26, as described below, in a controllable manner. Such adjustment of the crosshair 26 may require small and precise displacement of the sliders 18 and 22 which is facilitated via use of appropriate adjustment screws 44, or the like.

[0036] In this manner, the reticles 20 and 24 of the sliders 18 and 22 are able to complementarily form an X- and Y-axes displaceable crosshair 26, a position of which is determinable via the displacement sensors 14 and 16, so that a light field indicative of a radiation isocenter of the linear accelerator 8 is sightable by the crosshair 26, in use, in order to determine any problematic incongruence between the mechanical and radiation isocenters of the linear accelerator 8. [0037] The skilled addressee is to appreciate that variations on the above-described embodiments are possible and within the scope of the present invention. For example, the sliders 18 and 22 may be arranged at a different angle to each other, requirements depending, a different adjustment technique may be used, or the like.

[0038] The base plate 12 may be operatively mountable proximate a mechanical isocenter via mounting on a treatment support platform 28 of the accelerator 8. In an embodiment, the base plate comprises feet 30 for securing the phantom 10 to the treatment support platform 28. In the exemplified embodiment, the feet comprise suction cup feet, but other styles of feet 30 are possible, requirements depending.

[0039] Similarly, in a typical embodiment, the phantom 10 includes mountings 32 for mounting the base plate 12 proximate the mechanical isocenter at a desired or predetermined position. For example, such mountings 32 may allow mounting at a certain height above or distal from a patient treatment table of the linear accelerator, or the like. As described herein, the mechanical isocenter generally comprises a virtual point at which rotation axes of a gantry, collimator, and treatment table of the linear accelerator 8 intersect, said position or location of the virtual point being known. As such, mountings 32 may be configured to mount the phantom 10 to such gantry, collimator, and/or treatment table, requirements depending.

[0040] In the shown embodiment, the displacement sensors 14 and 16 comprise linear encoders using the Vernier principle of interpolation. Each displacement sensor 14 and 16 also comprises a digital display 36 for displaying a position of the crosshair 26 from a virtual centre of the viewing window 34, depending on where such Vernier sensors were 'zeroed', or the like.

[0041] As shown, the base plate 12 generally defines a viewing window 34 wherein the crosshair 26 is displaceable. Similarly, both X-axis slider 18 and Y-axis slider 22 define a complementary viewing window 34 which aligns with the viewing window of the base plate 12 when the sliders 18 and 22 are in position on the base plate 12. In other embodiment, the viewing window 34 may be formed from an L-shaped base plate, or a U- shaped base plate, or the like, i.e. the viewing window need not be round, but provides a frame wherein the crosshair 26 is displaceable via sliding of the sliders 18 and 22.

[0042] In the current example, the reticles 20 and 24 each comprise a straight-line reticle arranged along a relevant X or Y axis, as shown. Each reticle 20 and 24 is generally arranged within a viewing window 34 of a respective slider 18 and 22, so that the perpendicularly-arranged sliders form the crosshair 26 within such complementarily aligned viewing windows. Of course, other styles or configurations of the reticles 20 and 24 are possible, as will be appreciated by the skilled addressee.

[0043] In one embodiment, the sliders 18 and 22 comprise slidable plates arranged on the base plate 12, as shown. In such an example, the base plate 12 includes a pair of slider guides on either side of each slider 18 and 22 and configured to support and guide each respective slider therebetween, as shown. Again, variations hereon are possible, e.g. guiding slots or tracks, etc. [0044] In one example, the phantom 10 comprises a camera base 40 for a camera 38 which is operatively configured to monitor an incidence of the radiation isocenter and/or projected light field of the linear accelerator 8. The camera 38 may include an incidence surface 42 on which the radiation isocenter and/or projected light field of the linear accelerator is operatively incident, said incidence surface monitored by the camera 38. One example features a USB camera, as is known in the art.

[0045] The present invention further includes an associated method of calibrating a medical linear accelerator 8. Such a method broadly comprises the steps of providing a medical linear accelerator calibration phantom 10, as described herein, and determining incongruence between a mechanical and a radiation isocenter of the linear accelerator 8 by means of such phantom 10. Typically, the method includes a step of correcting any incongruence between the mechanical and radiation isocenters by means of suitable adjustment, as will be appreciated by the skilled addressee.

[0046] As described herein, the general purpose of the phantom 10 is to assist with (and accurately quantify) the accuracy and setup of the mechanical isocenter of the medical linear accelerator 8, the treatment support system 28 and the optical system of the linear accelerator 8, i.e. the radiation isocenter established thereby. Intended users of phantom 10 include installation and/or commissioning staff, breakdown and maintenance staff, medical physics and quality assurance staff and the machine users themselves (radiation therapists). The invention provides a means by which the crosshair 26 or cross wire and light field which are produced by the linear accelerator 8 are referenced to two adjustable perpendicular sliders 18 and 22 which is mounted at a predetermined position relative to the mechanical isocenter. When the user is satisfied that a precise reference between the light field and cross wires is achieved, a reading from the digital positional readout of the sensors 14 and 16 is taken and the positional errors can be quantified to two decimal places.

[0047] Further to the above broad method provided, the following description provides specific and practical examples of calibration steps and procedures that can be facilitated by means of the phantom 10. Reference to any specific components of the linear accelerator 8 will be appreciated as well-known in the art and specific details thereof will not be provided, e.g. gantry, collimator, and treatment table, etc.

[0048] In one example, a crosswire of the linear accelerator 8 can be calibrated by rotating the gantry of the linear accelerator to +180 degrees or -180 degrees, after which the treatment head collimator is rotated to +90 degrees or -90 degrees. Then, attach the patient support mounting 32 to the patient support platform or system 28, generally to extend the phantom 10 beyond the edge of the patient support platform. Affix the phantom 10 to the patient support mounting 32 and lower the patient support system to bring the phantom 10 in close proximity to the treatment head (while avoiding a collision and allowing sufficient space for the radiation head collimator to rotate without interference or damage).

[0049] Next, ensure that the X and Y-sliders 18 and 22 of the phantom are roughly in their central positions, and adjust the lateral and longitudinal movements of the patient support system until the projected crosswire align closely to the markings on the X and Y-sliders of the phantom 10. Now carefully and precisely adjust the X and Y-sliders 18 and 22 of the phantom 10 to align the crosswire markings with the projected crosswire.

[0050] Zero both the X and Y Vernier digital readouts, and rotate the collimator of the treatment head by 180 degrees. Without touching or moving the patient support system, readjust the X and Y-sliders 18 and 22 until a perfect coincidence is achieved between the projected crosswire and the crosshair 26 on the X and Y-slidersl8 and 22. Note the numerical value displayed on the X and Y Vernier digital readouts, and halve the noted values and set the X and Y-sliders 18 and 22 to this halved value.

[0051] Next, loosen the fixings securing the crosswire screen on the head of the linear accelerator 8. Move the crosswire screen to achieve a perfect coincidence between the projection of the crosswire and the markings on the X and Y- sliders 18 and 22 of the phantom 10. Once a perfect coincidence is achieved, tighten the fixings to prevent any further movement and recheck for perfect coincidence. Rotate the collimator of the treatment head by 180 degrees and confirm a perfect coincidence between the projected crosswire and the crosshair 26 on the sliders of the phantom 10. This concludes the procedure for calibrating the crosswire.

[0052] The following provides an overview of steps for linear accelerator light source calibration.

[0053] Ensure that the linear accelerator crosswire calibration above is completed prior to continuing. Ensure that there is no possibility of collision during gantry rotation. Rotate the gantry of the linear accelerator to 0 degrees. Rotate the treatment head collimator to +90 degrees or -90 degrees. Move the top of the patient support platform or system 28 to allow the light field to project onto the turntable below. Affix the phantom 10 to the turntable of the patient support system. Carefully and precisely adjust the X and Y-sliders 18 and 22 of the phantom tlO o align the crosshair 26 with the projected crosswire.

[0054] Zero both the X and Y Vernier digital readouts. Rotate the collimator of the treatment head by 180 degrees. Without moving the base plate 12 of the phantom, readjust the X and Y-slidersl8 and 22 until a perfect coincidence is achieved between the projected crosswire and the crosshair or crosswire markings 26 on the X and Y-slidersl8 and 22. Note the numerical value displayed on the X and Y Vernier digital readouts. Halve the noted values above and set the X and Y- sliders 18 and 22 to this halved value.

[0055] Next, loosen the fixings securing the light source in the head of the linear accelerator 8. Move the light source to achieve a perfect coincidence between the projection of the crosswire and the markings on the X and Y-sliders 18 and 22 of the phantom 10. Once a perfect coincidence is achieved, tighten the fixings to prevent any further movement and recheck for perfect coincidence. Rotate the collimator of the treatment head by 180 degrees and confirm a perfect coincidence between the projected crosswire and the crosswire markings on the sliders 18 and 22 of the phantom 10. This concludes the procedure for calibrating the linear accelerator light source.

[0056] The following provides an overview of steps for adjusting the gantry arm in the A/B direction. [0057] Ensure that the linear accelerator crosswire calibration is completed prior to continuing. Ensure that the gantry base has been levelled before continuing. Ensure that the gantry arm adjustment in the G/T direction has been completed before continuing. Affix the head mounting frame (of the phantom) to the treatment head of the linear accelerator. Affix the phantom 10 to the head mounting frame. Affix the camera base 40 to the phantom 10. Connect the USB cable of the camera base to a PC and monitor the live projected image.

[0058] Next, perform the gantry alignment test by setting up an external fixed pointer which is at isocenter and precisely aligned with the projected crosswire while the gantry is set precisely to +90 or -90 degrees. Ensure that there is no possibility of collision during gantry rotation. Rotate the gantry by 180 degrees and ensure that it is now at +90 or -90 degrees precisely. Adjust the Y-slider 22 until the crosswire markings on it align precisely with the projected crosswire. Zero the Y-slider Vernier digital readout. Adjust the Y-slider 22 until the crosswire markings 24 on it align precisely with the external fixed pointer. Make a note of this value (which is the difference between the external fixed pointer and the projected crosswire image and call it AB_error (x2). Divide AB_error (x2) by two to calculate the ABerror .

[0059] Adjust the Y-slider 22 until the Vernier digital readout displays the AB_error value. Adjust the 55mm gantry adjustment nuts at the rear of the gantry until a good coincidence is achieved between the Y-slider crosswire markings and the external fixed pointer. Once this is achieved, tighten all gantry arm fixings to ensure no further adjustment is possible. [0060] Ensure that it is safe to fully rotate the gantry without the risk of collision. Rotate the gantry to both of its extremes to allow the beam arm to settle. Repeat the gantry A/B alignment test as described above. This procedure concludes when the AB_error value falls within the manufacturer's specifications .

[0061] The following provides an overview of steps for mechanical isocenter test measurements. This is typically the procedure for accurately quantifying mechanical isocenter values during gantry rotation.

[0062] Ensure that the linear accelerator crosswire calibration is completed prior to continuing. Ensure that the gantry base has been levelled before continuing. Ensure that the gantry arm adjustment in the G/T direction has been completed before continuing. Ensure that the gantry arm adjustment in the A/B direction has been completed before continuing. Affix the head mounting frame (of the phantom 10) to the treatment head of the linear accelerator. Affix the phantom 10 to the head mounting frame 32. Affix the camera base 40 to the phantom. Connect the USB cable of the camera base to a PC and monitor the live projected image. Set up an external fixed pointer which is at isocenter and precisely aligned with the projected crosswire while the gantry is set precisely 0 (zero) degrees.

[0063] Ensure that there is no possibility of collision during gantry rotation. Rotate the gantry by 90 degrees and ensure that it is now at +90 degrees precisely. Adjust the X and Y-sliders 18 and 22 until the crosswire markings on it align precisely with the projected crosswire. Zero the X and Y-slider Vernier digital readouts. [0064] Adjust the X and Y-sliders 18 and 22 until the crosswire markings 20 and 24 on them align precisely with the external fixed pointer. Make a note of these values as X(@90) and Y(@90). Ensure that there is no possibility of collision during gantry rotation. Rotate the gantry by 90 degrees and ensure that it is now at +180 degrees precisely. Adjust the X and Y-sliders 18 and 22 until the crosswire markings on it align precisely with the projected crosswire. Zero the X and Y-slider Vernier digital readouts. Adjust the X and Y-sliders 18 and 22 until the crosswire markings on them align precisely with the external fixed pointer. Make a note of these values as X(@180) and Y(@180).

[0065] Ensure that there is no possibility of collision during gantry rotation. Rotate the gantry by -270 degrees and ensure that it is now at -90 degrees precisely. Adjust the X and Y-sliders 18 and 22 until the crosswire markings on it align precisely with the projected crosswire. Zero the X and Y-slider Vernier digital readouts. Adjust the X and Y-sliders 18 and 22 until the crosswire markings on them align precisely with the external fixed pointer. Make a note of these values as X(@-90) and Y(@-90). Apply these values to the manufacturer's test formula to verify that the tests are within specification. This concludes the mechanical isocenter test measurements procedure.

[0066] The following provides an overview of steps for light field size measurements. Load an appropriate field size on the linear accelerator 8 to project the crosswire onto the phantom 10. Ensure that there is no possibility of collision during gantry rotation. Rotate the gantry of the linear accelerator to 0 degrees. Rotate the treatment head collimator to 0 degrees. Attach the patient support mounting mechanism to the patient support system (for tabletop measurements). Affix the phantom to the patient support mounting mechanism. Adjust the patient support system 28 to place the phantom at isocenter.

[0067] The following provides an overview of steps for 10cm x 10cm field size checks.

[0068] Ensure that a 10cm x 10cm field is loaded on the linear accelerator. Ensure that the X and Y-sliders 18 and 22 of the phantom 10 are roughly in their central positions. Adjust the lateral and longitudinal movements of the patient support system until the projected crosswire align closely to the markings 20 and 24 on the X and Y-sliders 18 and 22 of the phantom 10. Carefully and precisely adjust the X and Y-sliders 18 and 22 of the phantom to align the crosswire markings with the projected crosswire. Zero both the X and Y Vernier digital readouts. Carefully and precisely adjust the X-slider 18 so that the XI 5cm field size mark aligns perfectly with the centre of the penumbra of the XI light field.

[0069] Note the value on the Vernier digital readout and add or subtract it from 5cm (depending on its polarity/direction of movement) . Carefully and precisely adjust the X-slider 18 so that the X2 5cm field size mark aligns perfectly with the centre of the penumbra of the X2 light field. Note the value on the Vernier digital readout and add or subtract it from 5cm (depending on its polarity/direction of movement). Rotate the treatment head collimator to +90 degrees. Carefully and precisely adjust the X and Y-sliders 18 and 22 of the phantom 10 to align the crosswire markings with the projected crosswire. Zero both the X and Y Vernier digital readouts. [0070] Carefully and precisely adjust the X-slider 18 so that the XI 5cm field size mark aligns perfectly with the centre of the penumbra of the Y1 light field. Note the value on the Vernier digital readout and add or subtract it from 5cm (depending on its polarity/direction of movement). Carefully and precisely adjust the X-slider 18 so that the X25cm field size mark aligns perfectly with the centre of the penumbra of the Y2 light field. Note the value on the Vernier digital readout and add or subtract it from 5cm (depending on its polarity/direction of movement).

[0071] The following provides an overview of steps for 30cm x 30cm field size checks.

[0072] Rotate the treatment head collimator to 0 degrees. Load a 30cm x 30cm field size on the linear accelerator. Ensure that the X and Y-sliders 18 and 22 of the phantom 10 are roughly in their central positions. Adjust the lateral and longitudinal movements of the patient support system until the projected crosswire align closely to the markings 20 and 24 on the X and Y-sliders 18 and 22 of the phantom 10. Carefully and precisely adjust the X and Y-sliders 18 and 22 of the phantom 10 to align the crosswire markings with the projected crosswire. Zero both the X and Y Vernier digital readouts.

[0073] Carefully and precisely adjust the X-slider 18 so that the XI 15cm field size mark aligns perfectly with the centre of the penumbra of the XI light field. Note the value on the Vernier digital readout and add or subtract it from 15cm (depending on its polarity/direction of movement). Carefully and precisely adjust the X-slider 18 so that the X2 15cm field size mark aligns perfectly with the centre of the penumbra of the X2 light field. Note the value on the Vernier digital readout and add or subtract it from 15cm (depending on its polarity/direction of movement).

[0074] Rotate the treatment head collimator to +90 degrees. Carefully and precisely adjust the X and Y-sliders 18 and 22 of the phantom 10 to align the crosswire markings with the projected crosswire. Zero both the X and Y Vernier digital readouts. Carefully and precisely adjust the X-slider 18 so that the XI 15cm field size mark aligns perfectly with the centre of the penumbra of the Y1 light field. Note the value on the Vernier digital readout and add or subtract it from 15cm (depending on its polarity/direction of movement). Carefully and precisely adjust the X-slider 18 so that the X2 15cm field size mark aligns perfectly with the centre of the penumbra of the Y2 light field. Note the value on the Vernier digital readout and add or subtract it from 15cm (depending on its polarity/direction of movement).

[0075] Generally, in addition to the above field size checks (which require visual alignment to locate the centre of the penumbra), the invention may also provide means whereby a visual and/or audible aid will assist in finding the centre of the penumbra (so as to increase accuracy and remove subjectivity) . This may be achieved by an electromechanical sounder and light emitting diode array which will emit a sound and light when the optimal penumbra position is reached.

[0076] The following provides an overview of steps for patient support system - automated table movement measurements. Load an appropriate field size on the linear accelerator 8 to project the crosswire onto the phantom 10. Ensure that there is no possibility of collision during gantry rotation. Rotate the gantry of the linear accelerator to 0 degrees. Rotate the treatment head collimator to 0 degrees.

[ 0077 ] Attach the patient support mounting mechanism to the patient support system (for tabletop measurements). Affix the phantom 10 to the patient support mounting mechanism. Adjust the patient support system to place the phantom 10 at isocenter. Carefully and precisely adjust the X and Y-sliders 18 and 22 of the phantom 10 to align the crosswire markings 20 and 24 with the projected crosswire. Zero both the X and Y Vernier digital readouts of the phantom 10. Note the patient support system (tabletop) X and Y digital readout values as displayed on the treatment room display.

[ 0078 ] Manually move the tabletop X and Y position away from the previously noted values. Ensure that the tabletop height (Z position) remains unchanged. With the use of the automated table movement feature, enter the previously noted X and Y values and automatically move the tabletop back to its previously noted position. Carefully and precisely adjust the X and Y-sliders 18 and 22 of the phantom 10 to align the crosswire markings with the projected crosswire. Note the values displayed on the X and Y Vernier digital readouts of the phantom. The values noted quantify the difference between the requested automated table movement and the actual movement achieved. This tells us that the automated table movement is accurate to within the manufacturer's specification.

[ 0079 ] The skilled addressee is to appreciate that variations on the above calibration steps are entirely possible and within the scope of the present invention. [0080] Applicant believes is particularly advantageous that the present invention allows the user to precisely calibrate and quantify any inaccuracies between the mechanical isocenter of the linear accelerator determined by the rotation axes of gantry, collimator, and treatment table, with the radiation isocenter (s) thereof, typically as indicated by a projected light field. By minimizing the setup and calibration errors via suitable calibration using phantom 10, it is possible to improve radiation treatment accuracies which in turn results in improved treatment conformity (which may translate to better patient outcomes).

[0081] Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. In the example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail, as such will be readily understood by the skilled addressee.

[0082] The use of the terms "a", "an", "said", "the", and/or similar referents in the context of describing various embodiments (especially in the context of the claimed subject matter) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising," "having," "including, " and "containing" are to be construed as open- ended terms (i.e., meaning "including, but not limited to,") unless otherwise noted. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. No language in the specification should be construed as indicating any non-claimed subject matter as essential to the practice of the claimed subject matter.

[0083] Spatially relative terms, such as "inner, " "outer, " "beneath, " "below, " "lower, " "above, " "upper, " and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature (s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0084] It is to be appreciated that reference to "one example" or "an example" of the invention, or similar exemplary language (e.g., "such as") herein, is not made in an exclusive sense. Various substantially and specifically practical and useful exemplary embodiments of the claimed subject matter are described herein, textually and/or graphically, for carrying out the claimed subject matter.

[0085] Accordingly, one example may exemplify certain aspects of the invention, whilst other aspects are exemplified in a different example. Variations (e.g. modifications and/or enhancements) of one or more embodiments described herein might become apparent to those of ordinary skill in the art upon reading this application. The inventor(s) expects skilled artisans to employ such variations as appropriate, and the inventor (s) intends for the claimed subject matter to be practiced other than as specifically described herein.

[0086] Any method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

Claims

1. A medical linear accelerator calibration phantom comprising : a base plate operatively mountable proximate a mechanical isocenter of said linear accelerator and including first and second linear displacement sensors; an X-axis slider slidably arranged on the base plate and having an X-axis reticle, said X-axis slider associated with the first linear displacement sensor; a Y-axis slider slidably arranged on the base plate perpendicular to the X-axis slider and having a Y-axis reticle, said Y-axis slider associated with the second linear displacement sensor; wherein said reticles complementarily form an X- and Y-axes displaceable crosshair, a position of which is determinable via the displacement sensors, a light image indicative of a radiation isocenter of the linear accelerator sightable by the crosshair in order to determine incongruence between the mechanical and radiation isocenters of the linear accelerator.

2. The calibration phantom of claim 1, wherein the base plate is operatively mountable proximate a mechanical isocenter via mounting on a treatment support platform of the accelerator.

3. The calibration phantom of claim 2, wherein the base plate comprises feet configured to secure the phantom to the treatment support platform.

4. The calibration phantom of any of claims 1 to 3, wherein the phantom includes mountings for mounting the base plate proximate the mechanical isocenter.

5. The calibration phantom of any of claims 1 to 4, wherein the displacement sensors comprise linear encoders using the Vernier principle of interpolation.

6. The calibration phantom of any of claims 1 to 5, wherein the base plate defines a viewing window wherein the crosshair is displaceable.

7. The calibration phantom of claim 6, wherein the X-axis slider defines a viewing window which complementarily aligns with the viewing window of the base plate.

8. The calibration phantom of claim 6, wherein the Y-axis slider defines a viewing window which complementarily aligns with the viewing window of the base plate.

9. The calibration phantom of any of claims 6 to 8, wherein each displacement sensor comprises a digital display for displaying a position of the crosshair from a virtual centre of the viewing window.

10. The calibration phantom of any of claims 1 to 8, wherein the sliders comprise slidable plates arranged on the base plate.

11. The calibration phantom of any of claims 1 to 10, wherein each slider comprises an adjustment screw, dial or knob configured to selectively displace or adjust said slider in order to facilitate sighting of the radiation isocenter via the displaceable crosshair in a controllable manner.

12. The calibration phantom of any of claims 1 to 11, wherein the reticles each comprise a straight-line reticle arranged along a relevant X or Y axis.

13. The calibration phantom of any of claims 6 to 9, wherein each reticle is arranged within a viewing window of a respective slider, the perpendicularly-arranged sliders forming the crosshair within complementarily aligned viewing windows.

14. The calibration phantom of any of claims 1 to 13, which comprises a camera configured to monitor an incidence of the radiation isocenter and/or projected light field of the linear accelerator.

15. The calibration phantom of claim 14, wherein the camera includes an incidence surface on which the radiation isocenter and/or projected light field of the linear accelerator is operatively incident, said incidence surface monitored by the camera.

16. A method of calibrating a medical linear accelerator, said method comprising the steps of: mounting, proximate a mechanical isocenter of the linear accelerator, a medical linear accelerator calibration phantom comprising a base plate and first and second linear displacement sensors; an X-axis slider slidably arranged on the base plate and having an X-axis reticle, said X-axis slider associated with the first linear displacement sensor; a Y-axis slider slidably arranged on the base plate perpendicular to the X-axis slider and having a Y-axis reticle, said Y-axis slider associated with the second linear displacement sensor; wherein said reticles complementarily form an X- and Y-axes displaceable crosshair, a position of which is determinable via the displacement sensors; and sighting a projected crosswire and/or a projected light field image, indicative of a radiation isocenter, produced by the linear accelerator with the crosshair in order to determine incongruence between the mechanical and radiation isocenters of the linear accelerator.

17. The method of claim 1, which includes a step of correcting incongruence between the mechanical and radiation isocenters by means of suitable adjustment.

18. A method of calibrating a medical linear accelerator, said method comprising the steps of: providing a medical linear accelerator calibration phantom in accordance with any of claims 1 to 15; and determining incongruence between a mechanical and a radiation isocenter of the linear accelerator by means of such phantom.