ESTRO 2021 Abstract Book

S396

ESTRO 2021

dimensional conformal RT (3DCRT) era and 2008-2014 representing the intensity modulated RT (IMRT) era. Post-treatment SHC data were collected up to 1-1-2020. Standardized incidence ratios (SIR) were calculated (adjusted for age and calendar year) to compare SHC rates to the Dutch male general population. In addition, the dose received by active bone marrow in the pelvic bones, delineated according to maps of McGuire et al, 2014 in 3DCRT (N=14) and IMRT (N=14) treatment plans (78 Gy) were quantified. Results The observed number of SHC was significantly increased by 9% for the complete PCa population, by 13% for patients having undergone prostatectomy and by 22% for those having received EBRT, compared to what would be expected in the Dutch general male population (Table). The risk for SHC after EBRT corresponds to an absolute excess risk (AER) of approximately 5 per 10,000 males per year. The risk for SHC was highest for patients treated in the 2DRT era followed by the 3DCRT era. Also in the IMRT era it remained significantly increased (Table). The average mean dose received by the active bone marrow area closest to the target (Figure-in pink) was 23.4 ± 3.6 Gy (1SD) for 3DCRT and 20.74 ± 3.4 Gy (1SD) for IMRT (p=0.06). Assuming a linear dose-effect, this dose reduction by -11% with IMRT vs 3DCRT would translate into a decrease in the SIR from 1.24 to 1.21, which is close to the actual SIR of 1.19 with IMRT (Table).

Conclusion EBRT is associated with an increased risk of developing a SHC. The risk remains significantly elevated over the years, although it moderately decreases with advanced EBRT. This is in accordance with the theoretical calculated risk reduction observed for IMRT compared to 3D-CRT. Further research into potential dose-volume effects is currently ongoing. OC-0513 Acute toxicity of hypo- and conventionally-fractionated radiosensitised bladder radiotherapy R. Huddart 1 , S. Hafeez 2 , A. Omar 3 , A. Choudhury 4 , A. Birtle 5 , I. Syndikus 6 , B. Hindson 7 , M. Varughese 8 , A. Henry 9 , D. McLaren 10 , F. Foroud 11 , A. Webster 12 , H. McNair 13 , A. Tolentino 3 , L. Webster 3 , H. Gribble 3 , L. Philipps 3 , A. Nikapota 14 , O. Parikh 15 , R. Alonzi 16 , R. Mahmood 17 , S. Hilman 18 , Y. Rimmer 19 , C. Griffin 20 , E. Hall 20 1 Institute of Cancer Research , Clinical Academic Radiotherapy, London, United Kingdom; 2 Institute of Cancer