ESTRO 38 Abstract book

S155 ESTRO 38

distribution of each selected plan and the non-adaptive plan were used to calculate daily DVHs. To evaluate the dose levels as suggested in the QUANTEC papers (V15Gy, V30Gy, V40Gy, V45Gy), the corresponding dose levels per fraction were tested for significance: V0.6Gy, V1.2Gy, V1.6Gy, V1.8Gy and V95% for long, and V3.0Gy, V95% for short treatment schedules, respectively. Significance was tested using Wilcoxon signed ranks test for related samples. Coverage of the total CTV, as expressed by V95%, was also tested.

Purpose or Objective Volumetric modulated arc therapy (VMAT) combined with flattening filter free beams (FFF) is generally used for stereotactic body radiation therapy (SBRT). Compared to flattened beams (FF), FFF provides advantages clearly identified in the literature to treat small lesions. In this study, an analysis of the dosimetric impacts of large X6 FFF beams used in VMAT plans is developed for three major cancer locations. Material and Methods A retrospective cohort of 90 patients treated between 2016 and 2017 includes 30 lung, 30 brain and 30 head & neck (H&N) cancers. The prescribed doses are respectively ranging from 50 to 66 Gy, 30 to 50 Gy and 60 to 70 Gy, 2 Gy per fraction. Planned target volumes (PTV) range from 35.0 to 482.0 cm 3 . Patients treated with VMAT X6 FF plans with 2 opposed coplanar arcs at 600 UM.min -1 maximum dose rate on Varian Clinac® system. Plans are optimized on Varian Eclipse® TPS (AAA, PRO, V13.6). For each case, a new VMAT objective function and dose distribution are determined using X6 FFF beams at 1400 UM.min -1 . Both results are normalized so as 95% of the dose covers 98% of the PTV. VMAT X6 FFF and X6 FF plan comparisons included doses at the PTV and at 2 cm, specifics organ-at-risk (OAR), 50% isodose prescription volumes (V50%), number of monitor units (MU) and treatment time. 3D ɣ -indexes (3%, 3mm, local dose) on Varian Portal Dosimetry® software is used to validate the plans. Results PTV coverage do not differ significantly between the X6 FF and X6 FFF energies (<0.2%, p=0.5) for any given location. At 2 cm around PTV, an average dose reduction of -4.0% ± 0.1% (p <0.001) stands out for lung location, -2.75% ± 2.4% (p<0.001) for brain and -0.8% ± 1.7%; (p=0.004) for H&N targets. The V50% average dose decreases by 3.9% ± 2.1% (p=0.001) for lung but no significant difference for brain and H&N is found. For all lung plans, doses at OAR decrease between -2.9% and -6.5% for normal lung, heart, spinal cord and esophagus: the overall decrease is due to the lower energy spectrum and scattered radiation. However, lower doses to the OAR are found for only 24 plans in brain and 20 plans in H&N with X6 FFF beams. VMAT X6 FFF plans requires a global greater MUs, with +10.7% ± 4.9% than in X6 FF for the lung, +8.7% ± 10.9% for the brain, +28.3 ± 16.1% in H&N. X6 FF and X6 FFF ɣ -indexes do not show significant differences except for H&N: pass rate is less than 95% score for 70% of plans. Treatment time reductions using X6 FFF high dose rate is limited by the gantry rotation speed. To ensure dosimetric objectives, X6 FFF large field fluence is compensated by the segmentation algorithm. Except for lung location, X6 FFF does not improve outcomes systematically. Conclusion For lung localizations, the use of large X6 FFF VMAT plans improves dose conformity to the target volume and generate high dose gradients which spare surrounding normal tissues and organs at risk. However, large X6 FFF fields do not seem suitable for systematic brain and H&N plan management. OC-0305 Organ sparing potential and inter-fraction robustness of IMPT for cervical cancer E.M. Gort 1 , J.C. Beukema 1 , M.J. Spijkerman-Bergsma 1 , S. Both 1 , J.A. Langendijk 1 , W.P. Matysiak 1 , C.L. Brouwer 1 1 University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands Purpose or Objective Current chemoradiation (CHRT) for cervical cancer results in severe chronic bowel toxicity in one out of eight patients. Moreover, severe hematologic toxicity is a frequent cause of discontinuation of CHRT which may have an impact on treatment outcome. Our pencil beam scanning proton facility allows highly conformal intensity modulated proton therapy (IMPT), however inter-fraction

Results In total, 10 short and 10 long treatment schedules were included, resulting in 300 plan selections. The margin set of 25/15/0 mm margins was used for 13 patients, the margin set of 15/0/-15 for 7 patients. Overall the -15mm, 0 mm, 15, and 25 mm plans were selected in 2%, 41%, 40%, 17% of fractions, respectively. For bowel cavity, limited but significant reductions were found in favor of the adaptive strategy for the long treatment schedule for V0.6Gy, V1.6Gy, V1.8Gy, and V95% ( p <0.05). For the short treatment schedule the tested dose parameters (V3.0Gy and V95%) were not significantly different. For bladder limited but significant reductions were found in all dose parameters and both schedules ( Table 1 ). Target volume V95% increased from 98% to 99%, but this increase was not significant.

Conclusion The adaptive strategy with plan selection im

proved target coverage and lowered the dose to the organs at risk. Over all patients the clinical benefit seems limited. OC-0304 Dosimetric impacts of VMAT FFF large-field on standard dose fractionation D. Julian 1 , S. Muraro 1 , O. Lauche 1 , A. Moustamia 1 , K. Serre 1 1 Clinique Clementville, CCGM Unité de Physique, Montpellier, France