- Email: [email protected]
PO-0878 COMPARISON OF TWO MLC QA APPROACHES: INDIVIDUAL LEAF ANALYSIS OR GLOBAL PERFORMANCE?
S344
ESTRO 31
3 Royal Surrey County Hospital and National Physical Laboratory, Physics, Guildford and Teddington, United Kingdom
1
Purpose/Objective: The National Radiotherapy Trials Quality Assurance (RTTQA) group implemented an IMRT credentialing programme for NCRI portfolio clinical trials employing IMRT techniques. Credentialing has been on a trial by trial basis. With increasing numbers of multicentre IMRT trials in the UK it is of priority to improve efficiency by reducing workload for each centre to join multiple trials and to develop a streamlined programme for future IMRT credentialing. This preliminary study was carried out to rank IMRT plan complexity of the different trials to establish a hierarchy before assessing if this could be linked to quality assurance measurements performed on the plan. Materials and Methods: Three published IMRT plan complexity metrics: i) MU/Gy, ii) Plan Intensity Map Variation (PIMV) and iii) Modulation index were used to define the hierarchy of complexity in this study [1]. Plan complexities were assessed in 10 patients’ plans from each of four UK randomised multicentre IMRT trials (Head&Neck and Prostate) but limited to plans created using Eclipse TPS. For each IMRT plan complexity metric, Kruskai-Wallis was performed to detect if there is any significant difference between trials, followed by the Bonferroni-type multiple comparison to establish the complexity hierarchy. Results: The total number of fluences available for analysis was 190. Significant differences between trials were found in all complexity metrics (p<0.05). The resulting hierarchies based on these metrics are shown in table 1. A hierarchy appears to exist but would need further validation through comparison with plan quality assurance measurements. A new proposed streamlining process has been developed to consider initial trial audit measurements, audit frequencies and changes in centres’ status (e.g. major system upgrades). This will reduce the number of site visits required for IMRT trials resulting in a significant reduction in workload for UK centres for joining multiple trials.
Purpose/Objective The complexity of IMRT treatments calls for rigorous MLC QA procedures. Two opposite QA approaches are common in clinical practice: individual leaf quantitative analysis and global MLC performance. We compared the sensitivity and outcome of both approaches as implemented in our clinical setting using 2D EPID dosimetry. Materials and Methods We used three Clinac 2100 C/D (Varian Medical Systems) linacs equipped with Millenium 120 MLCs and a-Si EPIDs: aSi500-II for one linac and aSi1000 for the other two. EPIDs were calibrated in terms of absorbed dose to water following an in-house procedure. Both QA procedures consisted of (i) 6 MV x-ray irradiation of EPIDs with MLC test patterns and (ii) comparison of acquired portal images with reference images using the analysis tools of Portal Dosimetry v8.8 (Varian Medical Systems): - The metrics test used five static patterns and three well-known sliding-window technique tests (fence, speed and sweeping gap). We measured distances and dose differences to quantify errors on individual leaves. - The stress test used a complex fluence pattern only. We imposed a strict 1%-1mm gamma evaluation with a 95% acceptance criterion to guarantee an acceptable global MLC performance. To determine the sensitivity of the metrics test, we performed the MLC QA procedure as scheduled but using a modified set of MLC patterns including a wide range of leaf position and speed forced errors. For the stress test, we monitored the routine QA outcomes over a one-year period and compared these to the quantitative data of the metrics test of the day. Results: Metrics test. In all static MLC patterns and in the fence test, measured leaf position differences agreed with forced errors (r2 = 0.997) down to 0.1 mm. In the sweeping gap test, dose differences measured along a pair of leaf paths were linear with gap error (r2 = 0.996) down to 0.1 mm. Finally, in the speed test, dose differences ΔD measured halfway along a pair of leaf paths were linear with the speed error δv at first order:
Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain
where C is easily derived from the irradiation parameters. The slower the nominal speeds v, the smaller the error which could be detected, ranging from 0.013 to 0.33 mm/s for the typical routine values (3.33 to 16.67 mm/s). Stress test. All scheduled tests passed the gamma tolerance although sporadic but significant errors of up to 1.5 mm were found for the simultaneous metrics test.
Table 1: IMRT complexity hierarchy between UK IMRT trials using different plan complexity metrics. Bonferroni-type multiple comparison showed no significant differences between ARTDECO & PARSPORT in MU/Gy, CHHIP & COSTAR and ARTDECO & PARSPORT respectively in PIMV, and COSTAR & CHHIP in MI. Conclusions: This study suggested that some of the chosen IMRT trials showed similar complexity and individual trial credentialing could cover other trials. The new proposed streamlining approach considers previous QA programmes and focuses on audit frequency and changes in centres’ status. This reduces the number of tests and measurements required for subsequent trials (which could be done by postal audit) whilst still maintaining QA and trial standards. It is hoped to extend this approach to the international setting taking into account QA done for clinical trials in other countries thereby reducing unnecessary QA. Reference: [1] McGarry CK,et al. Assessing software upgrades, plan properties and patient geometry using IMRT complexity metrics. Med Phys, 2004; 38: 2027-2034 PO-0878 COMPARISON OF TWO MLC QA APPROACHES: INDIVIDUAL LEAF ANALYSIS OR GLOBAL PERFORMANCE? A. Latorre Musoll1, N. Jornet Sala1, P. Carrasco de Fez1, T. Eudaldo Puell1, A. Ruiz Martínez1, D. Rodríguez Latorre1, T. Martinez Jurado1, M. Ribas Morales1
Conclusions: We used only commercial solutions, without extra modelling or time-consuming image processing. Comparison of both QA methodologies showed that gamma analysis of a complex fluence pattern can miss significant MLC underperformances due to the small area which individual leaves project onto a large image, regardless of the clinical impact of the error. On the other hand, simple distance and dose measurements on individual leaves provide a reliable MLC QA procedure, with the submillimetric accuracy level which IMRT complexity calls for. 2D EPID dosimetry is a suitable technique to
ESTRO 31
implement such a QA procedure on a daily basis both in static and dynamic modes. PO-0879 VARIAN 120 MILLENNIUM MLC LEAF VELOCITY LIMITATIONS FOR SLIDING-WINDOWS IMRT TECHNIQUES S. Agramunt Chaler1, D. Jurado-Bruggeman1, C. Muñoz-Montplet1 1 Institut Català d'Oncologia, Servei de física mèdica i protecció radiològica, Girona, Spain Purpose/Objective: In sliding-windows IMRT techniques, to control the MLC motion parameters is essential for a proper treatment delivery. To be confident on plan integrity it is necessary to maintain such parameters within reliable and well-known limits. One of these parameters is leaf velocity. This work tries to establish the leaf velocity limitations for the Varian 120 Millennium MLC inner leaves (5mm width). A method to establish these limits is proposed by means of dynalog files analysis. Materials and Methods: By means of the MLC controller software a uniform 5 mm mobile-gap window was created for the inner leaves. It was intended to deliver uniformly the programmed dose in a 10cmx10cm field. Delivery was performed always at the same dose rate so different programmed doses implied different leaf velocities (uncertainties from dose rate stability were considered negligible). MLC capabilities were stressed by increasing leaves velocity gradually until response deviated from the expected one. Then a limit to this parameter might be established. Several fields were delivered. The doses were programmed from 5 to 550 monitor units (MU) at a rate of 300 MU/min (velocities ranging from 1 to 110 mm/s). An ion chamber (PTW TM30013) was placed behind build-up material in the field central axis to measure the output dose response per MU. Slow moving fields (high MU) were assumed to behave correctly so they were established as a reference. Response deviations relative to them were calculated for each velocity. At the same time dynalog files were obtained during irradiations using manufacturer software. A home-made software was used to nanlyze dynalogs. Differences from planned to real positions were obtained to calculate the area deviation for each leaves pair during irradiation. Since position deviations were very stable, area deviations were considered constant along the whole irradiation. Measured gap area deviations per MU divided by velocity should be proportional to the dose response deviations per MU measured with the ion chamber. These quantities were used to compare both methods. Results: Both sets of measured deviations were compatible within measurement uncertainty (figure, k=1). No significative deviation was found at slow velocities below 25mm/s, but slight deviations from expected (around 1%) were observed within de range from 25mm/s to 33mm/s. At higher leaf velocities deviations became quickly so important (>4%) that clinical delivery confidence was lost.
Conclusions: As shown in results, dynalog files can be used to establish leaves velocity limitations. Furthermore, they are suggested to be a useful tool to analyze integrity and robustness of slidingwindows IMRT plans. A limit of 25 mm/s for inner leaves velocity of a Varian 120 Millennium MLC was obtained to ensure a maximum 1% of dose deviation from planned treatment. This is a very restrictive value, as a small (5 mm width) moving-field was used in this work. Greater delivery fields, less sensitive to small deviations, should support faster velocities up to 33 mm/s.
S345
PO-0880 ON THE TRADE-OFF BETWEEN DMLC TRACKING ACCURACY AND TARGET CONFORMITY FOR PROSTATE INVERSELY OPTIMIZED ARC THERAPY T. Larsson1, M. Falk1, P.J. Keall2, R. O'Brien2, P. Munck af Rosenschöld1 1 The Finsen Center - Rigshospitalet, Radiation Medicine Research Center - Department of Radiation Oncology, Copenhagen, Denmark 2 Sydney Medical School University of Sydney, Radiation Physics Laboratory, Sydney, Australia Purpose/Objective: Applying constraints on the MLC configuration in the treatment plan optimization and reducing its complexity can potentially be used to make the plans more suitable for dynamic multileaf collimator (DMLC) tracking. The purpose of this study was to evaluate if an optimum could be found for the trade-off between dosimetric accuracy of DMLC tracking and target conformity and organs at risk dose for prostate inversely optimized arc therapy. Materials and Methods: Inversely optimized arc therapy plans were created in Eclipse™ treatment planning system for delivery with the RapidArc® technique. Plans were created for eight prostate cancer patients with 78 Gy in 39 fractions prescribed to the clinical PTV. During the optimization, a leaf position constraint limited the allowed distance (AD) between adjacent MLC leaves. The AD was varied in four steps between 0.8 cm and 2.7 cm to obtain a range of MLC configuration complexity. Strict optimization objectives ensured a high degree of intensity modulation regardless of the choice of AD. The plans used a full arc, 6 MV and 45° collimator angle. The plans were evaluated using dose-volume histogram parameters for the PTV, rectum and bladder. Delivery was done for plans for two of the patients on a Novalis™ TX linear accelerator equipped with a highresolution MLC. A dosimetric phantom was mounted on a motion platform (Scandidos Delta4® w., Hexamotion®), that could reproduce an arbitrary target motion in 3D. The plans were delivered with the phantom reproducing four prostate motion traces (average 3D displacement 2.7 mm) [Langen et al. IJROBP 71:1084-90, 2008]. Deliveries were done with and without DMLC tracking and evaluated with the gamma evaluation method using deliveries with a static target as reference. Statistical analysis was done with the paired ttest. Results: No clear impact of the AD was seen on the planned dose distributions. Although the average standard deviation of the dose to the PTV worsened from 1.72 Gy (no AD limit) to 1.87 Gy (AD = 0.8 cm), conversely the average rectum V70Gy decreased from 6.44% to 6.15% (p < 0.05 for both effects). The difference in bladder V70 was not significant. With target motion, the gamma pass rate was significantly improved with DMLC tracking (p < 0.001), and lower AD gave an increased average pass rate, from 90.8% (range 86.0% – 95.6%) (no AD limit) to 95.6% (range 93.1% – 97.9%) with AD = 0.8 cm, using 1% and 1 mm criteria. The difference in gamma pass rate was significant (p < 0.05) for AD < 1.5 cm compared to no AD limit. No such trend was observed when DMLC tracking was turned off. Figure 1. Average gamma index failure rate with DMLC tracking (1% 1 mm) and average PTV standard deviation for different allowed distances (ADs) between adjacent MLC leaves.
Conclusions: The AD limit tended to decrease the PTV dose conformity but, interestingly, also decreased the V70 rectum dose. DMLC tracking successfully compensated for the investigated prostate motion and the dosimetric accuracy was increased with lowered AD limit. No clear optimum between dosimetric accuracy and target conformity could be found.
ESTRO 31
3 Royal Surrey County Hospital and National Physical Laboratory, Physics, Guildford and Teddington, United Kingdom
1
Purpose/Objective: The National Radiotherapy Trials Quality Assurance (RTTQA) group implemented an IMRT credentialing programme for NCRI portfolio clinical trials employing IMRT techniques. Credentialing has been on a trial by trial basis. With increasing numbers of multicentre IMRT trials in the UK it is of priority to improve efficiency by reducing workload for each centre to join multiple trials and to develop a streamlined programme for future IMRT credentialing. This preliminary study was carried out to rank IMRT plan complexity of the different trials to establish a hierarchy before assessing if this could be linked to quality assurance measurements performed on the plan. Materials and Methods: Three published IMRT plan complexity metrics: i) MU/Gy, ii) Plan Intensity Map Variation (PIMV) and iii) Modulation index were used to define the hierarchy of complexity in this study [1]. Plan complexities were assessed in 10 patients’ plans from each of four UK randomised multicentre IMRT trials (Head&Neck and Prostate) but limited to plans created using Eclipse TPS. For each IMRT plan complexity metric, Kruskai-Wallis was performed to detect if there is any significant difference between trials, followed by the Bonferroni-type multiple comparison to establish the complexity hierarchy. Results: The total number of fluences available for analysis was 190. Significant differences between trials were found in all complexity metrics (p<0.05). The resulting hierarchies based on these metrics are shown in table 1. A hierarchy appears to exist but would need further validation through comparison with plan quality assurance measurements. A new proposed streamlining process has been developed to consider initial trial audit measurements, audit frequencies and changes in centres’ status (e.g. major system upgrades). This will reduce the number of site visits required for IMRT trials resulting in a significant reduction in workload for UK centres for joining multiple trials.
Purpose/Objective The complexity of IMRT treatments calls for rigorous MLC QA procedures. Two opposite QA approaches are common in clinical practice: individual leaf quantitative analysis and global MLC performance. We compared the sensitivity and outcome of both approaches as implemented in our clinical setting using 2D EPID dosimetry. Materials and Methods We used three Clinac 2100 C/D (Varian Medical Systems) linacs equipped with Millenium 120 MLCs and a-Si EPIDs: aSi500-II for one linac and aSi1000 for the other two. EPIDs were calibrated in terms of absorbed dose to water following an in-house procedure. Both QA procedures consisted of (i) 6 MV x-ray irradiation of EPIDs with MLC test patterns and (ii) comparison of acquired portal images with reference images using the analysis tools of Portal Dosimetry v8.8 (Varian Medical Systems): - The metrics test used five static patterns and three well-known sliding-window technique tests (fence, speed and sweeping gap). We measured distances and dose differences to quantify errors on individual leaves. - The stress test used a complex fluence pattern only. We imposed a strict 1%-1mm gamma evaluation with a 95% acceptance criterion to guarantee an acceptable global MLC performance. To determine the sensitivity of the metrics test, we performed the MLC QA procedure as scheduled but using a modified set of MLC patterns including a wide range of leaf position and speed forced errors. For the stress test, we monitored the routine QA outcomes over a one-year period and compared these to the quantitative data of the metrics test of the day. Results: Metrics test. In all static MLC patterns and in the fence test, measured leaf position differences agreed with forced errors (r2 = 0.997) down to 0.1 mm. In the sweeping gap test, dose differences measured along a pair of leaf paths were linear with gap error (r2 = 0.996) down to 0.1 mm. Finally, in the speed test, dose differences ΔD measured halfway along a pair of leaf paths were linear with the speed error δv at first order:
Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain
where C is easily derived from the irradiation parameters. The slower the nominal speeds v, the smaller the error which could be detected, ranging from 0.013 to 0.33 mm/s for the typical routine values (3.33 to 16.67 mm/s). Stress test. All scheduled tests passed the gamma tolerance although sporadic but significant errors of up to 1.5 mm were found for the simultaneous metrics test.
Table 1: IMRT complexity hierarchy between UK IMRT trials using different plan complexity metrics. Bonferroni-type multiple comparison showed no significant differences between ARTDECO & PARSPORT in MU/Gy, CHHIP & COSTAR and ARTDECO & PARSPORT respectively in PIMV, and COSTAR & CHHIP in MI. Conclusions: This study suggested that some of the chosen IMRT trials showed similar complexity and individual trial credentialing could cover other trials. The new proposed streamlining approach considers previous QA programmes and focuses on audit frequency and changes in centres’ status. This reduces the number of tests and measurements required for subsequent trials (which could be done by postal audit) whilst still maintaining QA and trial standards. It is hoped to extend this approach to the international setting taking into account QA done for clinical trials in other countries thereby reducing unnecessary QA. Reference: [1] McGarry CK,et al. Assessing software upgrades, plan properties and patient geometry using IMRT complexity metrics. Med Phys, 2004; 38: 2027-2034 PO-0878 COMPARISON OF TWO MLC QA APPROACHES: INDIVIDUAL LEAF ANALYSIS OR GLOBAL PERFORMANCE? A. Latorre Musoll1, N. Jornet Sala1, P. Carrasco de Fez1, T. Eudaldo Puell1, A. Ruiz Martínez1, D. Rodríguez Latorre1, T. Martinez Jurado1, M. Ribas Morales1
Conclusions: We used only commercial solutions, without extra modelling or time-consuming image processing. Comparison of both QA methodologies showed that gamma analysis of a complex fluence pattern can miss significant MLC underperformances due to the small area which individual leaves project onto a large image, regardless of the clinical impact of the error. On the other hand, simple distance and dose measurements on individual leaves provide a reliable MLC QA procedure, with the submillimetric accuracy level which IMRT complexity calls for. 2D EPID dosimetry is a suitable technique to
ESTRO 31
implement such a QA procedure on a daily basis both in static and dynamic modes. PO-0879 VARIAN 120 MILLENNIUM MLC LEAF VELOCITY LIMITATIONS FOR SLIDING-WINDOWS IMRT TECHNIQUES S. Agramunt Chaler1, D. Jurado-Bruggeman1, C. Muñoz-Montplet1 1 Institut Català d'Oncologia, Servei de física mèdica i protecció radiològica, Girona, Spain Purpose/Objective: In sliding-windows IMRT techniques, to control the MLC motion parameters is essential for a proper treatment delivery. To be confident on plan integrity it is necessary to maintain such parameters within reliable and well-known limits. One of these parameters is leaf velocity. This work tries to establish the leaf velocity limitations for the Varian 120 Millennium MLC inner leaves (5mm width). A method to establish these limits is proposed by means of dynalog files analysis. Materials and Methods: By means of the MLC controller software a uniform 5 mm mobile-gap window was created for the inner leaves. It was intended to deliver uniformly the programmed dose in a 10cmx10cm field. Delivery was performed always at the same dose rate so different programmed doses implied different leaf velocities (uncertainties from dose rate stability were considered negligible). MLC capabilities were stressed by increasing leaves velocity gradually until response deviated from the expected one. Then a limit to this parameter might be established. Several fields were delivered. The doses were programmed from 5 to 550 monitor units (MU) at a rate of 300 MU/min (velocities ranging from 1 to 110 mm/s). An ion chamber (PTW TM30013) was placed behind build-up material in the field central axis to measure the output dose response per MU. Slow moving fields (high MU) were assumed to behave correctly so they were established as a reference. Response deviations relative to them were calculated for each velocity. At the same time dynalog files were obtained during irradiations using manufacturer software. A home-made software was used to nanlyze dynalogs. Differences from planned to real positions were obtained to calculate the area deviation for each leaves pair during irradiation. Since position deviations were very stable, area deviations were considered constant along the whole irradiation. Measured gap area deviations per MU divided by velocity should be proportional to the dose response deviations per MU measured with the ion chamber. These quantities were used to compare both methods. Results: Both sets of measured deviations were compatible within measurement uncertainty (figure, k=1). No significative deviation was found at slow velocities below 25mm/s, but slight deviations from expected (around 1%) were observed within de range from 25mm/s to 33mm/s. At higher leaf velocities deviations became quickly so important (>4%) that clinical delivery confidence was lost.
Conclusions: As shown in results, dynalog files can be used to establish leaves velocity limitations. Furthermore, they are suggested to be a useful tool to analyze integrity and robustness of slidingwindows IMRT plans. A limit of 25 mm/s for inner leaves velocity of a Varian 120 Millennium MLC was obtained to ensure a maximum 1% of dose deviation from planned treatment. This is a very restrictive value, as a small (5 mm width) moving-field was used in this work. Greater delivery fields, less sensitive to small deviations, should support faster velocities up to 33 mm/s.
S345
PO-0880 ON THE TRADE-OFF BETWEEN DMLC TRACKING ACCURACY AND TARGET CONFORMITY FOR PROSTATE INVERSELY OPTIMIZED ARC THERAPY T. Larsson1, M. Falk1, P.J. Keall2, R. O'Brien2, P. Munck af Rosenschöld1 1 The Finsen Center - Rigshospitalet, Radiation Medicine Research Center - Department of Radiation Oncology, Copenhagen, Denmark 2 Sydney Medical School University of Sydney, Radiation Physics Laboratory, Sydney, Australia Purpose/Objective: Applying constraints on the MLC configuration in the treatment plan optimization and reducing its complexity can potentially be used to make the plans more suitable for dynamic multileaf collimator (DMLC) tracking. The purpose of this study was to evaluate if an optimum could be found for the trade-off between dosimetric accuracy of DMLC tracking and target conformity and organs at risk dose for prostate inversely optimized arc therapy. Materials and Methods: Inversely optimized arc therapy plans were created in Eclipse™ treatment planning system for delivery with the RapidArc® technique. Plans were created for eight prostate cancer patients with 78 Gy in 39 fractions prescribed to the clinical PTV. During the optimization, a leaf position constraint limited the allowed distance (AD) between adjacent MLC leaves. The AD was varied in four steps between 0.8 cm and 2.7 cm to obtain a range of MLC configuration complexity. Strict optimization objectives ensured a high degree of intensity modulation regardless of the choice of AD. The plans used a full arc, 6 MV and 45° collimator angle. The plans were evaluated using dose-volume histogram parameters for the PTV, rectum and bladder. Delivery was done for plans for two of the patients on a Novalis™ TX linear accelerator equipped with a highresolution MLC. A dosimetric phantom was mounted on a motion platform (Scandidos Delta4® w., Hexamotion®), that could reproduce an arbitrary target motion in 3D. The plans were delivered with the phantom reproducing four prostate motion traces (average 3D displacement 2.7 mm) [Langen et al. IJROBP 71:1084-90, 2008]. Deliveries were done with and without DMLC tracking and evaluated with the gamma evaluation method using deliveries with a static target as reference. Statistical analysis was done with the paired ttest. Results: No clear impact of the AD was seen on the planned dose distributions. Although the average standard deviation of the dose to the PTV worsened from 1.72 Gy (no AD limit) to 1.87 Gy (AD = 0.8 cm), conversely the average rectum V70Gy decreased from 6.44% to 6.15% (p < 0.05 for both effects). The difference in bladder V70 was not significant. With target motion, the gamma pass rate was significantly improved with DMLC tracking (p < 0.001), and lower AD gave an increased average pass rate, from 90.8% (range 86.0% – 95.6%) (no AD limit) to 95.6% (range 93.1% – 97.9%) with AD = 0.8 cm, using 1% and 1 mm criteria. The difference in gamma pass rate was significant (p < 0.05) for AD < 1.5 cm compared to no AD limit. No such trend was observed when DMLC tracking was turned off. Figure 1. Average gamma index failure rate with DMLC tracking (1% 1 mm) and average PTV standard deviation for different allowed distances (ADs) between adjacent MLC leaves.
Conclusions: The AD limit tended to decrease the PTV dose conformity but, interestingly, also decreased the V70 rectum dose. DMLC tracking successfully compensated for the investigated prostate motion and the dosimetric accuracy was increased with lowered AD limit. No clear optimum between dosimetric accuracy and target conformity could be found.