ESTRO 2021 Abstract Book

S1148

ESTRO 2021

prostate cancer radiotherapy (RT). Specific objectives were to identify dose and volume differences between the two strategies as well as possible implications for rectal toxicity.

[1] Olsson et al. PhIRO 2019:11:88 [2] Gay et al. IJROBP 2012:83:e353 [3] Salembier et al. RO 2018:127:49 Materials and Methods

A convenience sample of 121 patients were extracted from the Eclipse treatment planning system (ver. 15.6, Varian Medical Systems ) at one institution in Sweden. The patients had been treated for prostate cancer in 2018 using 6 MV photon beams to total doses of 50-70Gy@2-3Gy/fraction. Rectal volumes were defined on planning CT images clinically ( clinical ) and adjusted to fit the cranial/caudal borders by the proposed standard ( reference ). Volume and dose differences between these were investigated using dose-volume histogram (DVH) data and volume overlap was quantified using the Dice similarity coefficient. A subset of 57/68 patients with available post-RT data on rectal bleeding at 3 and 12 months from the Swedish National Prostate Cancer Registry (NPCR) was also assessed. The endpoint was dichotomized into no versus ≥mild toxicity assuming no rectal bleeding at baseline. Comparisons between groups were done with Student’s t-test with a two-sided p-value≤0.05 considered statistically significant. Results Clinical rectal volumes were on average larger than standard volumes (p<0.01; Table 1). More patients had volume differences in caudal direction (87%) than in cranial direction (78%), but cranial volume differences were larger than caudal volume differences (p<0.01; Figure 1). Dice ranged between 0.72-1.00. There were no differences in maximum dose but small differences in mean dose (<1 Gy). At 3 months, 5/68 patients (7%) reported rectal bleeding and at 12 months, corresponding figures were 13/57 patients (23 %). For patients reporting early toxicity 2/5 (40%) of volume differences originated from the clinical volumes being delineated on average 10 mm above the standard volumes in cranial direction (6-14 mm). Corresponding figures for late toxicity volume differences were 8/13 (62 %) with the same averaged shift in distance (range 2-18 mm).