- Email: [email protected]
REAL-TIME DYNAMIC MLC TRACKING FOR ARC RADIOTHERAPY WITH VARYING MOTION MAGNITUDES
S 22
P ROFFERED PAPER
imaging modalities before the successful implementation of functional imaging methods in treatment planning.The use of FDG-PET in lung cancer is a field, where this process can well be illustrated. While the question of target volume delineation is presently being solved, the change of radiooncological concepts is underway, being tested in clinical studies on the potential benefit of patients from the integration of functional imaging in the treatment planning process.
M ONDAY, AUGUST 31, 2009
Proffered paper Rotational IMRT 55 oral USE OF A STANDARD CONSTRAINT SET FOR PLANNING OF RAPIDARC IN ADVANCED HEAD AND NECK TUMOURS W. Verbakel1 , M. Bieker2 , P. Doornaert3 , B. Slotman1 , S. Senan1 1 VU U NIVERSITY M EDICAL C ENTER, Academic Radiation Oncology Department, Amsterdam, Netherlands 2 VU M EDICAL C ENTER, Radiation Oncology, Amsterdam, Netherlands 3 VU U NIVERSITY M EDICAL C ENTRE, Academic Radiation Oncology Department, Amsterdam, Netherlands
Purpose: To evaluate the performance of RapidArc (RA, Varian Medical Systems), a novel technique for Volumetric Intensity Modulated Arc therapy, for treatment of locally advanced stage head and neck (HNC) tumors. As the full capabilities of the new optimization technique of RA for sparing organs at risk (OAR) are unknown, particularly when several OAR volumes overlap the PTV to a different extent in each patient, time for treatment planning may increase. Use of a standard constraint set for optimization may reduce the time considerably, and facilitate clinical implementation of RA. Materials: RA (v.8.2.23) was used for planning curative treatment in 70 patients with HNC tumors treated with a simultaneous integrated boost (SIB). For all plans, 2 arcs were used for delivery in order to achieve higher PTV dose homogeneity [Verbakel 2009]. A standard constraint set was developed to facilitate the RA optimization process by planning technologists. A 1 cm ring-shaped OAR structure was created around the PTVs to enforce a steep dose fall-off. Standard objectives were set for PTVs, spinal cord and brainstem, and the ring-shaped OAR. In addition, four dose objectives were set for parotid glands (PG), with objectives adapted to each individual patient. Low, medium and high dose objectives were set according to the amount of overlap between PG and PTVs, and to the relative volume of PG extending above the most cranial slice containing PTV. Each patient plan was measured with Gafchromic EBT films in a phantom to verify calculated doses. PTV coverage and OAR doses were analyzed in a sub-group of 27 patients with stage III-IV tumors treated with concurrent chemo-radiotherapy, and all received a PTVboost of 70 Gy in 35 fractions, and 1.65 Gy per fraction to elective PTV (PTVelect). Results: The standard constraint set used allowed for largely non-interactive planning, with only slight adaptations of PG objectives in the first phase of optimization. Typical RA plan optimization times were 20 minutes per arc, with a further 20 minutes needed for dose calculation. Sub-group analyses revealed excellent coverage of PTVelect and PTVboost, with a mean of >99% of both PTVs receiving > 95% of prescription dose. Only 0.1% of PTVboost received >107%. Mean dose to ipsilateral PG was 31 Gy versus 26 Gy to contralateral PG, depending on tumor localisation. The maximum cord dose was on average 46.8 Gy and mean delivery time (first beam-on to last beamoff) was 3 minutes, with a mean total of 427 MU per fraction. Conclusions: Use of a standard constraint set for optimization of RA for complex head and neck PTVs rapidly achieved highly conformal RA plans using 2 arcs for delivery, with excellent PTV dose homogeneity. 56 oral REAL-TIME DYNAMIC MLC TRACKING FOR ARC RADIOTHERAPY WITH VARYING MOTION MAGNITUDES M. Falk1 , P. Munck af Rosenschöld1 , P. Keall3 , B. C. Cho3 , J. Newell4 , J. Petersen4 , P. R. Poulsen3 , D. Ruan3 , A. Sawant3 , S. Korreman1 1 R IGSHOSPITALET, C OPENHAGEN U NIVERSITY H OSPITAL, Radiation Oncology Department, Copenhagen, Denmark 2 L UND U NIVERSITY, Department of Medical Radiation Physics, Lund, Sweden 3 S TANFORD U NIVERSITY, Radiation Oncology Department, Stanford, USA 4 C ALYPSO M EDICAL T ECHNOLOGIES, Seattle, USA
Purpose: Intensity modulated arc therapy (IMAT) with varying dose rate and gantry speed (RapidArc) is a novel treatment technique that offers a conformal dose delivery in a short treatment time. Using the RapidArc technique, the treatment is generally delivered in 1-2 gantry arcs using various gantry speed, MLC shapes and dose rate to create a homogenous coverage of the target while sparing healthy tissue. A promising method to account for intrafractional motion when using this technique is the use of dynamic MLC (DMLC) tracking, a method where the MLC is used to track the movements of the target during the treatment. The method has shown good compatibility with RapidArc in a feasibility study (Zimmerman et al, Acta Oncol. 2008). The purpose of this study was to evaluate the performance of the DMLC-tracking method in more complex situations, by varying the length of the peak to peak displacements of the target in RapidArc delivery. As a first approach, to eliminate the influences due to the change in dose rate, the DMLC-tracking’s performance was investigated for IMAT delivery with constant dose rate and gantry speed. We intend to present data from measurements where the dose rate and the gantry speed are varied as well.
M ONDAY, AUGUST 31, 2009
Materials: To simulate respiratory target movements, a 4D motion phantom with adjustable displacement lengths and cycle time was programmed to form sinusoidal motion. Studies have shown that lung tumour motion mainly occurs in the SI direction with about 0.2-25 mm peak to peak distance (Seppenwoolde et al, Radiation Oncology. 2002). The peak to peak distance was thus set to 5, 10, 15, 20 and 25 mm in the SI direction and the cycle time was set to 6 seconds for all measurements. The DMLC-tracking system received the real-time 3D displacement of the phantom from a Calypso system and was connected to the MLC to take over the control of the leaves. A RapidArc plan for a lung tumour case was created in EclipseTM (Aria ver. 8.5). The collimator angle was set to 90◦ to enable leaf motion parallel to the phantom movement, which has been shown to give the best result for DMLC-tracking (Sawant et al, Med. Phys. 2008). The gantry rotation angle was set to run from 210◦ to 150◦ to prevent intersection of the field and the rails of the couch. The MLC sequence was then exported and opened on the 4DTC with the Millennium MLC workstation (ver. 7.2) and run as a dynamic arc with a constant dose rate of 300 MU/min and constant gantry speed. The jaw settings were 13 x 13 cm to allow the leaves to track the moving target, and 349 MU was delivered accordingly to the original RapidArc plan. No prediction algorithm was used. Measurements were made with (1) static target no tracking, (2) static target with DMLC-tracking, (3) moving target no tracking and (4) moving target with DMLC-tracking. The results were evaluated by comparing the measurements in mode (3) to measurements in mode (1) and measurements in mode (4) to measurements in mode (2), using gamma analysis (3% dose difference, 3 mm DTA). Results:
P ROFFERED PAPER
S 23
(prostate and seminal vescicles) plus 0.8-1 cm margins. Fused MRI-CT images were used for Tomotherapy planning. The planning strategies were to deliver 80,90, 100 and 120 Gy to PTV_DIL while delivering 71.4 Gy to PTV outside PTV_DIL, in 28 fractions. The main limitation to dose escalation was rectum dose. The strategy used was to satisfy "safe" rectal constraintsderived from external radiotherapy (V65.5<20%, V68.5<5%) in combination with constraints translated from brachytherapy experience applied to the tail of DVH (V80<0.1cc; V75<1cc; V70<2cc); constraints for bladder (V75<0.1cc) and urethra (V80<1cc, V90<0.1cc) were applied as well. Rectal NTCP calculations were performed using the most recently available models data (Rancati 2004, Rancati 2008, Peeters 2006, Soehn 2007, Tucker 2008). Results: For all 5 patients it was possible to safely escalate the dose to at least 100 Gy (EQD2,α/β =10=113Gy). As shown in figure, for the 5 different planning strategies (PTV_DIL dose escalation) typical NTCP values were about 3% for G3 and about 5-7% for G2-G3 while the constraints of the other OARs were always satisfied;. V95% of CTV, PTV and DIL was up to 95% while for PTV_DIL V95% decreases in order to satisfy constraints.
Conclusions: Tomotherapy may permit to safely escalate the dose at least 100 Gy to DIL volume defined by T2WI and DWI without significantly increasing rectal NTCP compared to planning without dose escalation. 58 oral
The gamma analysis showed superior conformation between the measurements with DMLC-tracking and the benchmark measurements than the measurements without tracking. Conclusions: DMLC tracking enables improvement of the accuracy in dose delivery of arc plans with target moving in 1D, even for large motion magnitudes. 57 oral SAFE ULTRA-HIGH DOSE ESCALATION ON DOMINANT INTRAPROSTATIC LESIONS (DIL) BY TOMOTHERAPHY A. Maggio1 , C. Fiorino1 , P. Mangili1 , G. M. Cattaneo1 , S. Broggi1 , C. Cozzarini2 , F. De Cobelli3 , T. Rancati4 , A. Del Maschio3 , N. Di Muzio2 , R. Calandrino1 1 I STITUTO S CIENTIFICO S AN R AFFAELE, Department of Medical Physics, Milan, Italy 2 I STITUTO S CIENTIFICO S AN R AFFAELE, Department of Radiotherapy, Milan, Italy 3 I STITUTO S CIENTIFICO S AN R AFFAELE, Department of Radiology, Milan, Italy 4 F ONDAZIONE IRCCS I STITUTO N AZIONALE DEI T UMORI, programma prostata, Milan, Italy
Purpose: Local relapse after radiotherapy for prostate cancer mostly originates at original tumor location. This seems to be correlated to the presence of one or more DIL inside prostate that are characterized by high density radioresistant/hypoxic clonogens. The goal of this study is to verify the possibility of using Helical Tomotherapy to safety escalate dose to 2 Gy equivalent doses (EQD2) that are expected to control most hypoxic tumors (i.e.: EQD2>110-120 Gy). Materials: For 5 intermediate/high risk patients, submitted to T2WI and Diffusion Weighted MRI (DWI), imaging showed evidence of one DIL in four patients in the peripheral zone and two in one patient. DILs were contoured by an expert radiologist and the corresponding PTV_DIL was generated by 0.5 cm automatic expansion while PTV was created from CTV
PATIENT SPECIFIC QA FOR 54 CLINICAL RAPIDARC PLANS, A RETROSPECTIVE EVALUATION D. Hoffmans1 , W. Verbakel2 1 VU U NIVERSITY M EDICAL C ENTER, Department of Physics and Medical Technology, Amsterdam, Netherlands 2 VU U NIVERSITY M EDICAL C ENTER, Department of Radiation Oncology, Amsterdam, Netherlands
Purpose: RapidArc (Varian Medical Systems) is a novel approach for planning and delivery of highly conformal dose distributions using a volumetric intensity modulated arc technique. Since the clinical introduction in 2008, over 200 patients were treated. All treatment plans have been measured with film prior to patient treatment. We report the measurement results of 54 RapidArc plans: 40 plans for patients treated for head and neck (H&N) cancer, and 14 for patients receiving whole brain radiotherapy plus a boost to multiple brain metastases (WBRT+meta). Materials: Clinical RapidArc plans were made with Eclipse (v. 8.2.23, Varian Medical Systems) and delivered by a Varian Trilogy Clinac. The 40 H&N plans were planned to deliver a simultaneous integrated boost with dose levels of 1.65 and 2 Gy per fraction. The 10 WBRT+meta plans were planned to deliver a nominal dose of 4 Gy for the whole brain, and an integrated boost of 8 Gy for 2-5 brain metastases. All plans consisted of 2 non-identical arcs to achieve better dose homogeneity. Prior to treatment, all plans were measured in at least 2 coronal planes in a polystyrene phantom using double GafChromic EBT films. Comparison of dose measurements and calculations were made using OmniPro I’mRT (v. 1.6, IBA Dosimetry). 2D g-evaluations were made using limits of 2 mm and 3% of the typical PTV dose in the phantom (5 cGy for H&N plans and 12 cGy for WBRT+meta plans).For research purposes, 2 of the H&N cases were re-optimized with excessive constraints to achieve plans with a relatively high numbers of Monitor Units (MU). Results: All evaluated plans belong to the most challenging in terms of maximum modulation of leaf movement. Table 1 shows the 54 cases sorted by percentage of film surface with a g > 1. The average surface with g>1 for the H&N and WBRT+meta cases was 0.6 and 1.6%, respectively. None of the cases had a maximum g higher than 2.0.
P ROFFERED PAPER
imaging modalities before the successful implementation of functional imaging methods in treatment planning.The use of FDG-PET in lung cancer is a field, where this process can well be illustrated. While the question of target volume delineation is presently being solved, the change of radiooncological concepts is underway, being tested in clinical studies on the potential benefit of patients from the integration of functional imaging in the treatment planning process.
M ONDAY, AUGUST 31, 2009
Proffered paper Rotational IMRT 55 oral USE OF A STANDARD CONSTRAINT SET FOR PLANNING OF RAPIDARC IN ADVANCED HEAD AND NECK TUMOURS W. Verbakel1 , M. Bieker2 , P. Doornaert3 , B. Slotman1 , S. Senan1 1 VU U NIVERSITY M EDICAL C ENTER, Academic Radiation Oncology Department, Amsterdam, Netherlands 2 VU M EDICAL C ENTER, Radiation Oncology, Amsterdam, Netherlands 3 VU U NIVERSITY M EDICAL C ENTRE, Academic Radiation Oncology Department, Amsterdam, Netherlands
Purpose: To evaluate the performance of RapidArc (RA, Varian Medical Systems), a novel technique for Volumetric Intensity Modulated Arc therapy, for treatment of locally advanced stage head and neck (HNC) tumors. As the full capabilities of the new optimization technique of RA for sparing organs at risk (OAR) are unknown, particularly when several OAR volumes overlap the PTV to a different extent in each patient, time for treatment planning may increase. Use of a standard constraint set for optimization may reduce the time considerably, and facilitate clinical implementation of RA. Materials: RA (v.8.2.23) was used for planning curative treatment in 70 patients with HNC tumors treated with a simultaneous integrated boost (SIB). For all plans, 2 arcs were used for delivery in order to achieve higher PTV dose homogeneity [Verbakel 2009]. A standard constraint set was developed to facilitate the RA optimization process by planning technologists. A 1 cm ring-shaped OAR structure was created around the PTVs to enforce a steep dose fall-off. Standard objectives were set for PTVs, spinal cord and brainstem, and the ring-shaped OAR. In addition, four dose objectives were set for parotid glands (PG), with objectives adapted to each individual patient. Low, medium and high dose objectives were set according to the amount of overlap between PG and PTVs, and to the relative volume of PG extending above the most cranial slice containing PTV. Each patient plan was measured with Gafchromic EBT films in a phantom to verify calculated doses. PTV coverage and OAR doses were analyzed in a sub-group of 27 patients with stage III-IV tumors treated with concurrent chemo-radiotherapy, and all received a PTVboost of 70 Gy in 35 fractions, and 1.65 Gy per fraction to elective PTV (PTVelect). Results: The standard constraint set used allowed for largely non-interactive planning, with only slight adaptations of PG objectives in the first phase of optimization. Typical RA plan optimization times were 20 minutes per arc, with a further 20 minutes needed for dose calculation. Sub-group analyses revealed excellent coverage of PTVelect and PTVboost, with a mean of >99% of both PTVs receiving > 95% of prescription dose. Only 0.1% of PTVboost received >107%. Mean dose to ipsilateral PG was 31 Gy versus 26 Gy to contralateral PG, depending on tumor localisation. The maximum cord dose was on average 46.8 Gy and mean delivery time (first beam-on to last beamoff) was 3 minutes, with a mean total of 427 MU per fraction. Conclusions: Use of a standard constraint set for optimization of RA for complex head and neck PTVs rapidly achieved highly conformal RA plans using 2 arcs for delivery, with excellent PTV dose homogeneity. 56 oral REAL-TIME DYNAMIC MLC TRACKING FOR ARC RADIOTHERAPY WITH VARYING MOTION MAGNITUDES M. Falk1 , P. Munck af Rosenschöld1 , P. Keall3 , B. C. Cho3 , J. Newell4 , J. Petersen4 , P. R. Poulsen3 , D. Ruan3 , A. Sawant3 , S. Korreman1 1 R IGSHOSPITALET, C OPENHAGEN U NIVERSITY H OSPITAL, Radiation Oncology Department, Copenhagen, Denmark 2 L UND U NIVERSITY, Department of Medical Radiation Physics, Lund, Sweden 3 S TANFORD U NIVERSITY, Radiation Oncology Department, Stanford, USA 4 C ALYPSO M EDICAL T ECHNOLOGIES, Seattle, USA
Purpose: Intensity modulated arc therapy (IMAT) with varying dose rate and gantry speed (RapidArc) is a novel treatment technique that offers a conformal dose delivery in a short treatment time. Using the RapidArc technique, the treatment is generally delivered in 1-2 gantry arcs using various gantry speed, MLC shapes and dose rate to create a homogenous coverage of the target while sparing healthy tissue. A promising method to account for intrafractional motion when using this technique is the use of dynamic MLC (DMLC) tracking, a method where the MLC is used to track the movements of the target during the treatment. The method has shown good compatibility with RapidArc in a feasibility study (Zimmerman et al, Acta Oncol. 2008). The purpose of this study was to evaluate the performance of the DMLC-tracking method in more complex situations, by varying the length of the peak to peak displacements of the target in RapidArc delivery. As a first approach, to eliminate the influences due to the change in dose rate, the DMLC-tracking’s performance was investigated for IMAT delivery with constant dose rate and gantry speed. We intend to present data from measurements where the dose rate and the gantry speed are varied as well.
M ONDAY, AUGUST 31, 2009
Materials: To simulate respiratory target movements, a 4D motion phantom with adjustable displacement lengths and cycle time was programmed to form sinusoidal motion. Studies have shown that lung tumour motion mainly occurs in the SI direction with about 0.2-25 mm peak to peak distance (Seppenwoolde et al, Radiation Oncology. 2002). The peak to peak distance was thus set to 5, 10, 15, 20 and 25 mm in the SI direction and the cycle time was set to 6 seconds for all measurements. The DMLC-tracking system received the real-time 3D displacement of the phantom from a Calypso system and was connected to the MLC to take over the control of the leaves. A RapidArc plan for a lung tumour case was created in EclipseTM (Aria ver. 8.5). The collimator angle was set to 90◦ to enable leaf motion parallel to the phantom movement, which has been shown to give the best result for DMLC-tracking (Sawant et al, Med. Phys. 2008). The gantry rotation angle was set to run from 210◦ to 150◦ to prevent intersection of the field and the rails of the couch. The MLC sequence was then exported and opened on the 4DTC with the Millennium MLC workstation (ver. 7.2) and run as a dynamic arc with a constant dose rate of 300 MU/min and constant gantry speed. The jaw settings were 13 x 13 cm to allow the leaves to track the moving target, and 349 MU was delivered accordingly to the original RapidArc plan. No prediction algorithm was used. Measurements were made with (1) static target no tracking, (2) static target with DMLC-tracking, (3) moving target no tracking and (4) moving target with DMLC-tracking. The results were evaluated by comparing the measurements in mode (3) to measurements in mode (1) and measurements in mode (4) to measurements in mode (2), using gamma analysis (3% dose difference, 3 mm DTA). Results:
P ROFFERED PAPER
S 23
(prostate and seminal vescicles) plus 0.8-1 cm margins. Fused MRI-CT images were used for Tomotherapy planning. The planning strategies were to deliver 80,90, 100 and 120 Gy to PTV_DIL while delivering 71.4 Gy to PTV outside PTV_DIL, in 28 fractions. The main limitation to dose escalation was rectum dose. The strategy used was to satisfy "safe" rectal constraintsderived from external radiotherapy (V65.5<20%, V68.5<5%) in combination with constraints translated from brachytherapy experience applied to the tail of DVH (V80<0.1cc; V75<1cc; V70<2cc); constraints for bladder (V75<0.1cc) and urethra (V80<1cc, V90<0.1cc) were applied as well. Rectal NTCP calculations were performed using the most recently available models data (Rancati 2004, Rancati 2008, Peeters 2006, Soehn 2007, Tucker 2008). Results: For all 5 patients it was possible to safely escalate the dose to at least 100 Gy (EQD2,α/β =10=113Gy). As shown in figure, for the 5 different planning strategies (PTV_DIL dose escalation) typical NTCP values were about 3% for G3 and about 5-7% for G2-G3 while the constraints of the other OARs were always satisfied;. V95% of CTV, PTV and DIL was up to 95% while for PTV_DIL V95% decreases in order to satisfy constraints.
Conclusions: Tomotherapy may permit to safely escalate the dose at least 100 Gy to DIL volume defined by T2WI and DWI without significantly increasing rectal NTCP compared to planning without dose escalation. 58 oral
The gamma analysis showed superior conformation between the measurements with DMLC-tracking and the benchmark measurements than the measurements without tracking. Conclusions: DMLC tracking enables improvement of the accuracy in dose delivery of arc plans with target moving in 1D, even for large motion magnitudes. 57 oral SAFE ULTRA-HIGH DOSE ESCALATION ON DOMINANT INTRAPROSTATIC LESIONS (DIL) BY TOMOTHERAPHY A. Maggio1 , C. Fiorino1 , P. Mangili1 , G. M. Cattaneo1 , S. Broggi1 , C. Cozzarini2 , F. De Cobelli3 , T. Rancati4 , A. Del Maschio3 , N. Di Muzio2 , R. Calandrino1 1 I STITUTO S CIENTIFICO S AN R AFFAELE, Department of Medical Physics, Milan, Italy 2 I STITUTO S CIENTIFICO S AN R AFFAELE, Department of Radiotherapy, Milan, Italy 3 I STITUTO S CIENTIFICO S AN R AFFAELE, Department of Radiology, Milan, Italy 4 F ONDAZIONE IRCCS I STITUTO N AZIONALE DEI T UMORI, programma prostata, Milan, Italy
Purpose: Local relapse after radiotherapy for prostate cancer mostly originates at original tumor location. This seems to be correlated to the presence of one or more DIL inside prostate that are characterized by high density radioresistant/hypoxic clonogens. The goal of this study is to verify the possibility of using Helical Tomotherapy to safety escalate dose to 2 Gy equivalent doses (EQD2) that are expected to control most hypoxic tumors (i.e.: EQD2>110-120 Gy). Materials: For 5 intermediate/high risk patients, submitted to T2WI and Diffusion Weighted MRI (DWI), imaging showed evidence of one DIL in four patients in the peripheral zone and two in one patient. DILs were contoured by an expert radiologist and the corresponding PTV_DIL was generated by 0.5 cm automatic expansion while PTV was created from CTV
PATIENT SPECIFIC QA FOR 54 CLINICAL RAPIDARC PLANS, A RETROSPECTIVE EVALUATION D. Hoffmans1 , W. Verbakel2 1 VU U NIVERSITY M EDICAL C ENTER, Department of Physics and Medical Technology, Amsterdam, Netherlands 2 VU U NIVERSITY M EDICAL C ENTER, Department of Radiation Oncology, Amsterdam, Netherlands
Purpose: RapidArc (Varian Medical Systems) is a novel approach for planning and delivery of highly conformal dose distributions using a volumetric intensity modulated arc technique. Since the clinical introduction in 2008, over 200 patients were treated. All treatment plans have been measured with film prior to patient treatment. We report the measurement results of 54 RapidArc plans: 40 plans for patients treated for head and neck (H&N) cancer, and 14 for patients receiving whole brain radiotherapy plus a boost to multiple brain metastases (WBRT+meta). Materials: Clinical RapidArc plans were made with Eclipse (v. 8.2.23, Varian Medical Systems) and delivered by a Varian Trilogy Clinac. The 40 H&N plans were planned to deliver a simultaneous integrated boost with dose levels of 1.65 and 2 Gy per fraction. The 10 WBRT+meta plans were planned to deliver a nominal dose of 4 Gy for the whole brain, and an integrated boost of 8 Gy for 2-5 brain metastases. All plans consisted of 2 non-identical arcs to achieve better dose homogeneity. Prior to treatment, all plans were measured in at least 2 coronal planes in a polystyrene phantom using double GafChromic EBT films. Comparison of dose measurements and calculations were made using OmniPro I’mRT (v. 1.6, IBA Dosimetry). 2D g-evaluations were made using limits of 2 mm and 3% of the typical PTV dose in the phantom (5 cGy for H&N plans and 12 cGy for WBRT+meta plans).For research purposes, 2 of the H&N cases were re-optimized with excessive constraints to achieve plans with a relatively high numbers of Monitor Units (MU). Results: All evaluated plans belong to the most challenging in terms of maximum modulation of leaf movement. Table 1 shows the 54 cases sorted by percentage of film surface with a g > 1. The average surface with g>1 for the H&N and WBRT+meta cases was 0.6 and 1.6%, respectively. None of the cases had a maximum g higher than 2.0.