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253 Invited Linear accelerator quality assurance for IMRT
Proffered papers
IMRT QA (LINAC ORIENTED) 253
Invited
Linear accelerator quality assurance for IMRT A. Boyer 1, L. Xing 1, C. Yu2, P. Xia 3, L. Verhey 3 1Stanford University School of Medicine, Dept. of Radiation Oncology, H01 44, Stanford, U.S.A. 2University of Maryland School of Medicine, Dept. of Radiation Oncology, Baltimore, U.S.A.
Friday, 20 September 2002 $85
lion is again. A variety of planes can easily be compared as appropriate for the plan being checked. We carry these out efficiently without requiring our Physicists to sacrifice even more time performing extra dosimetry! 255
Oral
Requirements on leaf position accuracy for clinical step and ShOot IMRT M. Sastre Padrd U. van der Heide, H. Welleweerd UMC, Radiation Physics, Utrecht, The Netherlands
3University of Cafifomia at San Francisco, Radiation Oncology, San Francisco, U.S.A.
In Intensity Modulate Radiotherapy Treatments (IMRT) the leaves of the Multi-Leaf Collimator (MLC) net only delimit the fields but also modulate
The implementation of IMRT requires that Quality Assurance procedures be
them, creating a 3-D dose distribution inside the target volume. In this study we determine how systematic leaf-positioning errors affect the dose distributions and whether these effects are clinically acceptable. The study was performed for three different class solutions, two for prostate (3 and 5 beams) and one for head and neck (7 beams). An analysis of the relation between the required leaf-positioning accuracy and class solution was also performed. When the leaf sequence has adjacent segments the dose delivered is very sensitive to leaf-positioning errors. To quantify this effect we used a strip test, 25 adjacent segments each 1-cm wide and 40 cm long. Films were exposed and dose profiles taken along the leaf-axis movement. To quantify the accuracy we defined a parameter called Relative Positioning Error that relates the close variations along the leaf-axis with the leaf accuracy. This test was performed on the Elekta SL20 accelerator as well as on a Varian Linac 600. For the Elekta the results showed underdosage on the border of adjacent segments while the Varian showed overdosage. The main reason for this is the difference in calibration procedure for the leaves. This could be addressed either by modifying the calibration procedure or by compensating for it on the Treatment Planing System (TPS). For the class solution studies we have recalculated a real clinical plan on a square water-equivalent phantom. This test was performed on the Elekta SL 20 accelerator using PLATO as TPS. We created different leaf positioning errors and exposed films at the isocenter plane of the phantom. Using in-house developed software the digitized films were compared to the isocenter slice calculated by the TPS. The results show better agreement when leaves have 0,5mm-gap error than with no-errors. This is in agreement with the previous results (underdose on the borders of adjacent segments) and can be explained by the asymmetric round leaf end that makes the effective segments smaller than those calculated by the TPS. Because the effects of leaf-positioning errors depend on the sequence of the leaves, we have created different clinical plans by modifying parameters such a step size, minimum field width, etc. The optimal planning parameters that minimize the effect of the leaf-positioning errors are presented.
in place uniquely designed to monitor computer-controlled delivery with an MLC. We discuss three classes of procedures that can be applied generally in spite of the design differences between MLCs offered by the linac vendors: 1) pre-clinical QA, which ensures the reliability and accuracy of the technical chain and also establishes the limitations of the technique to be used clinically; 2) patient specific QA, which ensures that the plan for a specific patient can be delivered accurately; and 3) routine system QA, which are checks performed on a weekly and monthly basis in addition to conventional QA procedures. The first class contains system QA measures of leaf positioning accuracy and alignment and, dose delivery not specific to any patient. These procedures use film and ionization chambers to measure system performance critical to the IMRT delivery technique. The secend class contains patient specific measurements to verify the accuracy of delivery. These measurements determine how closely the treatment delivered throughout the entire irradiated volume wilt be to the prescribed dose distributions based on a relatively small number of samples. The third class contains routine checks to verify that the performance of the system has not changed from its state when commissioned. Examples of applicalions with specific vendor's equipment illustrate the implementation of the principles. 254 Oral
Pre-treatment quality assurance for IMRT: the Princess Royal Hospital
protocol
A. Beavi~; 1, E. Bubula 2, V. Whitton 1 1princess Royal Hospital Radiation Physics, Kingston Upon Hull, United Kingdom 2Computerized Medical Sytems, Freiberg, Germany We have recently started an IMRT service, implementing the Inverse planning module within our CMS-FOCUS treatment planning system and delivering the treatments using a Varian 600CD Linac. A Step and Shoot method with a Millennium 120 MLC is utilised. Once the treatment planning software and delivery system have been commissioned the Quality Assurance question becomes: "What QA should be performed for each treatment before it is "signed off" as "ready to start" ". We will present the experience gained at our Cancer centre. There are three aspects to our QA protocol: 1) checking the monitor units, 2) checking the "dose maps" of each beam used in the plan and 3) measuring the dose distribution in axial planes. The measured MU comparisons agree within 1.5% (using a standard Farmer chamber) when "dosimetry unfriendly" segments are removed, These are those that just "clip" the ion chamber where partial volume effects cause large dosimetryerrors. By reviewing each beam these segments can be identified and deleted, the beam is then recomputed. Whereas we use the full beam for the treatment we believe that we have performed reasonable QA On it by verifying the majority of its component segments. Film dosimetry is performed, for parts 2 and 3, using KODAK EDR2 film. Calibrations enable comparison of measured to computed absolute doses, in general, tO within 2ram. This process has been possible by utilising the QA tools provided within the FOCUS treatment planning system, which include the ability to easily write ASCII files of 2-D dose planes which are then available via a SAMBA process to any networked PC. To QA individual beams we plan each beam, in turn, with perpendicular incidence to a cubic phantom computing the dose at 5cm depth. This dose plane is extracted from the planning system as an ASCII file. Films are exposed in the.geometry described. Absolute dose maps are computed from the scanned images and digitally compared using MATLAB code with the planned distributions, Plan Dose distributions are computed (and extracted as ASCII files) in the plane of the dose prescription point, with the beams centred in the cubic phantom. A film is exposed in this geometry and an absolute dose distribu-
256 Oral C o m p a r i s o n o f t w o verification strategies for radiotherapy
P.J. Reckwerdt 1, J.M. Kapatoes 1, G.H. Olivera 1, 2, K.J. Ruchala 1, T.R. Mackie 1, 2, R. Jeraj 1, 2, W. Lu 1, J.P. Balog 1 1TomoTherapy, Inc., Middleton, Wl, U.S.A. 2University of Wisconsin', Medical Physics, Madison, WI, U.S.A. Portal images are used routinely in the radiotherapy clinic for patient setup verification. A primary limitation is the lack of low contrast resolution available on these two-dimensional (2D) projection images. Such small contrast differences are common in targets located in the pelvic region (e.g., prostate). Moreover with a small number of 2D portal images, it is often impossible to capture the complexity associated with anatomical changes. It will be shown that this inability to distinguish low contrast objects and complex three-dimensional shapes ultimately limits the clinical potential of portal imaging for dosimetric verification purposes. One possible alternative to 2D portal imaging is acquisition of CT images at the time of treatment. This can be done with either a kilovoltage (kV) or megavoltage (MV) beam. A kV beam is preferable as it provides the lowest imaging dose for a given contrast resolution. However, since a diagnostic qualify CT image is not required for the purpose of radiotherapy verification, it will be shown that megavoltage images provide enough low contrast resolution to distinguish clinically relevant information at clinically acceptable imaging doses. Portal imaging devices can also be used to verify the intensify of the beam during delivery. However, in many cases it is impossible to distinguish between errors coming from anatomical changes versus those from the incident beam chain (linac, jaws, MLC, etc). A three dimensional (3D) representation of the patient (such as CT) at the time of treatment in conjunction with an exit dos@ detector allows for a clear split between errors that are
IMRT QA (LINAC ORIENTED) 253
Invited
Linear accelerator quality assurance for IMRT A. Boyer 1, L. Xing 1, C. Yu2, P. Xia 3, L. Verhey 3 1Stanford University School of Medicine, Dept. of Radiation Oncology, H01 44, Stanford, U.S.A. 2University of Maryland School of Medicine, Dept. of Radiation Oncology, Baltimore, U.S.A.
Friday, 20 September 2002 $85
lion is again. A variety of planes can easily be compared as appropriate for the plan being checked. We carry these out efficiently without requiring our Physicists to sacrifice even more time performing extra dosimetry! 255
Oral
Requirements on leaf position accuracy for clinical step and ShOot IMRT M. Sastre Padrd U. van der Heide, H. Welleweerd UMC, Radiation Physics, Utrecht, The Netherlands
3University of Cafifomia at San Francisco, Radiation Oncology, San Francisco, U.S.A.
In Intensity Modulate Radiotherapy Treatments (IMRT) the leaves of the Multi-Leaf Collimator (MLC) net only delimit the fields but also modulate
The implementation of IMRT requires that Quality Assurance procedures be
them, creating a 3-D dose distribution inside the target volume. In this study we determine how systematic leaf-positioning errors affect the dose distributions and whether these effects are clinically acceptable. The study was performed for three different class solutions, two for prostate (3 and 5 beams) and one for head and neck (7 beams). An analysis of the relation between the required leaf-positioning accuracy and class solution was also performed. When the leaf sequence has adjacent segments the dose delivered is very sensitive to leaf-positioning errors. To quantify this effect we used a strip test, 25 adjacent segments each 1-cm wide and 40 cm long. Films were exposed and dose profiles taken along the leaf-axis movement. To quantify the accuracy we defined a parameter called Relative Positioning Error that relates the close variations along the leaf-axis with the leaf accuracy. This test was performed on the Elekta SL20 accelerator as well as on a Varian Linac 600. For the Elekta the results showed underdosage on the border of adjacent segments while the Varian showed overdosage. The main reason for this is the difference in calibration procedure for the leaves. This could be addressed either by modifying the calibration procedure or by compensating for it on the Treatment Planing System (TPS). For the class solution studies we have recalculated a real clinical plan on a square water-equivalent phantom. This test was performed on the Elekta SL 20 accelerator using PLATO as TPS. We created different leaf positioning errors and exposed films at the isocenter plane of the phantom. Using in-house developed software the digitized films were compared to the isocenter slice calculated by the TPS. The results show better agreement when leaves have 0,5mm-gap error than with no-errors. This is in agreement with the previous results (underdose on the borders of adjacent segments) and can be explained by the asymmetric round leaf end that makes the effective segments smaller than those calculated by the TPS. Because the effects of leaf-positioning errors depend on the sequence of the leaves, we have created different clinical plans by modifying parameters such a step size, minimum field width, etc. The optimal planning parameters that minimize the effect of the leaf-positioning errors are presented.
in place uniquely designed to monitor computer-controlled delivery with an MLC. We discuss three classes of procedures that can be applied generally in spite of the design differences between MLCs offered by the linac vendors: 1) pre-clinical QA, which ensures the reliability and accuracy of the technical chain and also establishes the limitations of the technique to be used clinically; 2) patient specific QA, which ensures that the plan for a specific patient can be delivered accurately; and 3) routine system QA, which are checks performed on a weekly and monthly basis in addition to conventional QA procedures. The first class contains system QA measures of leaf positioning accuracy and alignment and, dose delivery not specific to any patient. These procedures use film and ionization chambers to measure system performance critical to the IMRT delivery technique. The secend class contains patient specific measurements to verify the accuracy of delivery. These measurements determine how closely the treatment delivered throughout the entire irradiated volume wilt be to the prescribed dose distributions based on a relatively small number of samples. The third class contains routine checks to verify that the performance of the system has not changed from its state when commissioned. Examples of applicalions with specific vendor's equipment illustrate the implementation of the principles. 254 Oral
Pre-treatment quality assurance for IMRT: the Princess Royal Hospital
protocol
A. Beavi~; 1, E. Bubula 2, V. Whitton 1 1princess Royal Hospital Radiation Physics, Kingston Upon Hull, United Kingdom 2Computerized Medical Sytems, Freiberg, Germany We have recently started an IMRT service, implementing the Inverse planning module within our CMS-FOCUS treatment planning system and delivering the treatments using a Varian 600CD Linac. A Step and Shoot method with a Millennium 120 MLC is utilised. Once the treatment planning software and delivery system have been commissioned the Quality Assurance question becomes: "What QA should be performed for each treatment before it is "signed off" as "ready to start" ". We will present the experience gained at our Cancer centre. There are three aspects to our QA protocol: 1) checking the monitor units, 2) checking the "dose maps" of each beam used in the plan and 3) measuring the dose distribution in axial planes. The measured MU comparisons agree within 1.5% (using a standard Farmer chamber) when "dosimetry unfriendly" segments are removed, These are those that just "clip" the ion chamber where partial volume effects cause large dosimetryerrors. By reviewing each beam these segments can be identified and deleted, the beam is then recomputed. Whereas we use the full beam for the treatment we believe that we have performed reasonable QA On it by verifying the majority of its component segments. Film dosimetry is performed, for parts 2 and 3, using KODAK EDR2 film. Calibrations enable comparison of measured to computed absolute doses, in general, tO within 2ram. This process has been possible by utilising the QA tools provided within the FOCUS treatment planning system, which include the ability to easily write ASCII files of 2-D dose planes which are then available via a SAMBA process to any networked PC. To QA individual beams we plan each beam, in turn, with perpendicular incidence to a cubic phantom computing the dose at 5cm depth. This dose plane is extracted from the planning system as an ASCII file. Films are exposed in the.geometry described. Absolute dose maps are computed from the scanned images and digitally compared using MATLAB code with the planned distributions, Plan Dose distributions are computed (and extracted as ASCII files) in the plane of the dose prescription point, with the beams centred in the cubic phantom. A film is exposed in this geometry and an absolute dose distribu-
256 Oral C o m p a r i s o n o f t w o verification strategies for radiotherapy
P.J. Reckwerdt 1, J.M. Kapatoes 1, G.H. Olivera 1, 2, K.J. Ruchala 1, T.R. Mackie 1, 2, R. Jeraj 1, 2, W. Lu 1, J.P. Balog 1 1TomoTherapy, Inc., Middleton, Wl, U.S.A. 2University of Wisconsin', Medical Physics, Madison, WI, U.S.A. Portal images are used routinely in the radiotherapy clinic for patient setup verification. A primary limitation is the lack of low contrast resolution available on these two-dimensional (2D) projection images. Such small contrast differences are common in targets located in the pelvic region (e.g., prostate). Moreover with a small number of 2D portal images, it is often impossible to capture the complexity associated with anatomical changes. It will be shown that this inability to distinguish low contrast objects and complex three-dimensional shapes ultimately limits the clinical potential of portal imaging for dosimetric verification purposes. One possible alternative to 2D portal imaging is acquisition of CT images at the time of treatment. This can be done with either a kilovoltage (kV) or megavoltage (MV) beam. A kV beam is preferable as it provides the lowest imaging dose for a given contrast resolution. However, since a diagnostic qualify CT image is not required for the purpose of radiotherapy verification, it will be shown that megavoltage images provide enough low contrast resolution to distinguish clinically relevant information at clinically acceptable imaging doses. Portal imaging devices can also be used to verify the intensify of the beam during delivery. However, in many cases it is impossible to distinguish between errors coming from anatomical changes versus those from the incident beam chain (linac, jaws, MLC, etc). A three dimensional (3D) representation of the patient (such as CT) at the time of treatment in conjunction with an exit dos@ detector allows for a clear split between errors that are