ESTRO 2021 Abstract Book

S395

ESTRO 2021

Conclusion By using a longitudinal dose-effect model, we found that an increased dose to the bladder and urethra will result in a significant increase in GU toxicity following EBRT. Although cumulative toxicity showed no significant difference between the treatment arms of the FLAME trial, dose-effect relations to the urethra and bladder were observed. We propose that focal boost treatment plans should incorporate a urethra dose- constraint in addition to the pre-existing bladder dose-constraints. We suggest a urethra dose-constraint of D0.1cm 3 ≤ 80 Gy to be used for the FLAME fractionation scheme. Further treatment optimization to increase the focal boost dose without increasing the dose to the urethra and other organs at risk should be a focus for future research, as we have shown that a focal boost is beneficial in the treatment for prostate cancer. OC-0512 Impact of modern radiotherapy on subsequent hematological cancer risk in prostate cancer survivors M. Jahreiß 1 , K. Aben 2 , A. Bertoen 3 , M. Dirkx 1 , M. Hoogeman 1 , L. Incrocci 1 , R. Nout 1 , W. Heemsbergen 1 1 Erasmus MC Cancer Institute, Radiotherapy, Rotterdam, The Netherlands; 2 Netherlands Comprehensive Cancer Organization, Research & Development, Utrecht, The Netherlands; 3 Inholland , University of Applied Science, Haarlem, The Netherlands Purpose or Objective External beam radiotherapy (EBRT) dramatically changed over the years, from simple two-dimensional rectangular fields to highly sophisticated conformal, intensity-modulated, image-guided techniques. Consequently, dose distributions around the target and associated risks of side effects have changed as well. We previously investigated modern radiotherapy in relation to solid cancer risks. In the current study, we investigated the impact of modern EBRT on the risk of developing subsequent hematological cancer (SHC) after prostate cancer (PCa) treatment. Materials and Methods All PCa survivors diagnosed between 1990-2014 were identified in the Netherlands Cancer Registry (NCR). Patients were categorized by treatment, i.e. radical prostatectomy, EBRT +/- adjuvant hormonal therapy (AHT), etc. The EBRT (+/-AHT) cohort, diagnosed with a T1-T3N0/X, M0/x PCa, was divided in three time periods; 1991-1996 representing the 2-dimensional radiotherapy (2DRT) era, 1998-2005 representing the 3-