ESTRO 36 Abstract Book

S296 ESTRO 36 _______________________________________________________________________________________________

Amsterdam, The Netherlands 3 Radboud University Medical Center, Radiation Oncology, Nijmegen, The Netherlands 4 Leiden University Medical Center, Radiation Oncology, Leiden, The Netherlands Purpose or Objective In the Dutch phase III HYPRO trial (39x 2 Gy vs. 19x 3.4 Gy), the postulated non-inferiority of the hypofractionation arm with respect to the incidence of grade ≥2 late urinary toxicity was not shown. Moreover, a significant increase in grade ≥3 urinary toxicity was observed. In the current analysis we evaluated patient- reported urinary symptoms and possible relationships with hypofractionation and hospital of treatment. Material and Methods Patients with intermediate or high-risk prostate cancer from four hospitals applying image-guided IMRT protocols and recruiting >70 patients were analyzed, excluding patients with a baseline catheter. Long-term hormonal treatment (36 months) was prescribed to high-risk patients in hospital A-C but not in hospital D. A total of 561 patients (n=275 for standard fractionation (SF), hypofractionation (HF) n=296) with ≥1 follow-up symptom questionnaire were eligible (n=2355 total questionnaires). Treatment arm was balanced within hospitals. Local guidelines were applied for dose (in)homogeneity, margins (5-8 mm), and optimization. One hospital used MRI for prostate delineation (hospital A) and another hospital applied a rectal balloon (D). Hospital B and C varied in the applied safety margins of 5-6mm and 8mm, respectively. The study protocol did not provide dose constraints for the bladder; bladder delineation was done retrospectively. We calculated bladder and urethra dose (EQD2) with α/β ratios of 3 Gy and 5 Gy, and analyzed incidences of urinary symptoms between 6 months and 5 year. The impact of treatment arm and hospital on late urinary toxicity endpoints was calculated in a multivariate model including time and hormonal therapy. Results Dose to structures within the target volume (urethra, base of trigone area) was 78 Gy for SF vs 82.7 Gy for HF with α/β=3 Gy, and 78 Gy for both schedules with α/β=5. Average mean bladder dose was 29.2 Gy (SF) vs 29.9 Gy (HF) for α/β=3, (p=0.4), and 30.2 Gy vs 29.1 Gy (α/β=5, p=0.2), for SF vs. HF, respectively. Planned dose to the bladder varied significantly (p<0.05) between hospitals and was relatively low for hospital A and D (≈25 Gy vs. ≈33 Gy for hospital B and C, based on α/β=3 Gy). Symptoms of incontinence, straining, and weak stream were on average significantly more reported in the HF arm during follow- up ( FIG 1A-C ) and varied significantly between hospitals ( FIG 2A-C) . Hormonal treatment was not predictive in the current models. We established that baseline levels of urinary complaints were considerable as well ( FIG 1 ).

Conclusion