ESTRO 38 Abstract book

S1008 ESTRO 38

J. Bertholet 1 , A. Hunt 2 , A. Dunlop 1 , T. Bird 2 , A. Mitchell 1 , U. Oelfke 1 , S. Nill 1 , K. Aitken 2 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom ; 2 The Royal Marsden NHS Foundation Trust, Department of Radiotherapy, London, United Kingdom Purpose or Objective Dose escalation to a biological effective dose (BED 10 ) of 100 Gy may improve overall survival in locally advanced pancreatic cancer (LAPC). The ability to dose escalate is limited by the risk of toxicity for organs-at-risks (OAR) close to the planning target volume (PTV). A BED of 100Gy may be easier to achieve in a moderate hypofractionated regimen (15 fractions (#)) compared to 5# stereotactic body radiotherapy (SBRT). We evaluated the potential for dose escalation in 5 or 15# for 10 LAPC patients and investigated the correlation between OAR/PTV overlap and achievable PTV coverage. Material and Methods The study included ten LAPC patients (table). The PTV was defined as the gross-tumour volume (GTV) plus 3 mm isotropic margin, assuming treatment delivery in breath- hold under active breathing coordination. The duodenum, bowel (large and small), stomach, spinal cord, liver and kidneys were delineated as OAR. Two regimens, 5 and 15#, were compared. The aim was to cover 95% of the PTV with a BED of 54Gy (base dose=33Gy in 5#, 42.5Gy in 15#) while respecting OAR constraints. For duodenum, stomach and bowel, previously published constraints (table) were used: V 15Gy <9cc, V 20Gy <3cc, V 33Gy <1cc in 5# and D 0.5cc <45Gy in 15#. Once the highest PTV coverage while respecting OAR constraints was achieved, dose escalation to a BED>98Gy (prescription dose=50Gy in 5#, 67.5Gy in 15#) was attempted up to 95% PTV coverage, limiting the PTV maximum dose to 130% of the prescription. Plans were generated using Raystation 6.99 (RaySearch Laboratories) with a single 6MV FFF VMAT arc (179-181° gantry rotation, collimator rotation 5°). The OAR/PTV overlap (PTV OAR ) and proximity (OAR prox ) volumes were calculated using eq.1 and 2 respectively (table) and their effect on PTV coverage was evaluated using Pearson’s correlation coefficients (significance level and OAR prox volumes leading to varying PTV coverage (table, Fig1). In 5#, 95% PTV coverage by the escalated dose was only achievable for patient 1 with PTV OAR =0. In all other patients, coverage even by 33Gy had to be compromised to comply with OAR constraints. In 15#, 95% PTV coverage by 42.5Gy was feasible for all patients and coverage by the escalated dose was higher than in 5# in eight patients. The target coverage by the base dose was correlated with PTV OAR in 5 and 15# and with OAR prox in 15#. The target coverage by the escalated dose was correlated with PTV OAR in 15# (Fig2). of 5%). Results The patients presented a range of PTV OAR

Conclusion The potential for dose escalation was higher in 15 than in 5# except for patients 4 and 7. Patient 4 had the largest PTV OAR volume and would likely benefit from the improved coverage by BED=54Gy in 15# compared to 5# (table). Patients with favourable anatomy (OAR>1cm away from PTV) can be treated in 5# to a high BED. Moderate hypofractionation is better suited if the PTV is overlapping with OARs. Using a daily adaptive approach, the PTV coverage by the escalated dose can be increased on days with favourable anatomy. EP-1857 Simulating the interaction of clinical electron beams with tissue-equivalent samples produced by FDM