ESTRO 38 Abstract book

S1191 ESTRO 38

performed. Median time to the development of G2 GU toxicity was 13.5 month (3‐63.6). The median duration of G2 events was 8.3 months (3.6‐30.6). Of the 13 patients presenting with G≥2 GI toxicity, 10 were free of toxicity by the time of the current analysis. Median time to the development of G≥2 GI toxicity was 11 months (3‐44.2). The median duration of the events was 6 months (0.2‐9.5). In univariate analysis, no variable studied was associated with late G≥2 GU toxicity, whereas previous cardiovascular disease (p=0.042), rectum D2cc (p=0.016) and rectum D1cc (p=0.017) were associated with G≥2 GI toxicity. Multivariate analysis showed that rectum D1cc (HR11.56; 95%CI 1.4‐92.1; p=0.021) and prior history of cardiovascular disease (HR3.6; 95%CI 1‐12.9; p=0.045) remained independent predictors of G≥2 GI toxicity. Conclusion Single fraction 15Gy HDR‐BT and hypofractionated EBRT is well tolerated with low incidence and prevalence of late GU and GI toxicity. Lower rectal D1cc doses may reduce GI toxicity. EP-2154 Efficacy of LHRH agonist-free cytoreduction prior to prostate seed brachytherapy N. Wallace 1 , A.M. Peoples 1 , S.J. O'Brien 1 , P.J. Kelly 1 1 Cork University Hospital, Radiotherapy Department, Cork, Ireland Purpose or Objective Our institutional practice for patients planned for prostate brachytherapy is to first perform a TRUS volume study to assess for technical feasibility. For patients who are unsuitable due to a large gland size or pubic arch interference (PAI), cytoreduction with androgen‐ deprivation therapy may convert patients to suitability for brachytherapy. In an effort to spare patients the known side effects of LHRH agonists, particularly erectile dysfunction, we have adopted bicalutamide 50mg once daily combined with dutasteride 0.5mg once daily for 12 weeks as described by Gaudet et al in Radiotherapy and Oncology in 2016. The purpose of this retrospective review was to establish the efficacy of this regimen in converting patients to suitability for a prostate seed implant. Material and Methods We treated 36 patients who were deemed unsuitable for brachytherapy based on their large gland volume and/or the presence of pubic arch interference at the time of their pre‐treatment TRUS assessment. Patients received a combination of dutasteride and bicalutamide for 12 weeks. We repeated the TRUS for each patient after 12 weeks of therapy to assess for the presence of PAI, gland volume and whether the patient had become suitable for treatment with brachytherapy. The data was collected prospectively at the time of the volume studies and analysed retrospectively. Results The median pre‐cytoreduction prostate gland volume was 53.5cc (IQR 47‐60cc). The median prostate volume after 12 weeks of dual agent cytoreductive therapy was 38cc (IQR:35‐42cc). The median reduction in gland size was 29.0% (IQR 25‐36.2%). 33 out of 36 patients (91.7%) had PAI on initial TRUS. Of those individuals with initial PAI, 13/35 (37.1%) had persistent PAI post cytoreduction and one had an obstructive pattern on uroflow studies so was not a suitable candidate. Of the 13 patients with persistent PAI post cytoreduction 9 were felt unsuitable to proceed to implant. Overall, 26/36 (72.2%) were suitable candidates for brachytherapy following 12 weeks of cytoreductive therapy Conclusion The percentage of our patients who converted to suitability for brachytherapy following LHRH‐free

calculated in the TPS and the dose measured in vivo with over 98% of correlation among them. The blood patch thickness will decrease in 50% in 10 to 15 days after the application. All the rectal dose parameters above the V20 ‐ V80 where significantly improved by the blood patch, also the Dmax and it was correlated with the homogeneity of the blood patch application, V5‐V10 weren’t significant because this isodoses are 5 to 6 cm far from target. The average pre rectal space obtained was 0.83 cm. The dosimetric potencial advantages with the blood patch are that, the mean dose to 0.1 cc of rectum was limited to 57.4% and mean dose to 2 cc of rectum was 40%, also parameters such as D90 and V100 were significantly improved. Conclusion The use of a blood patch can reduce the integral radiation dose to the rectum and may help to decrease the amount of possible acute and late rectal toxicities due to prostate HDR brachytherapy procedure, letting us to do a safe dose escalation treatment in order to improve outcomes. This technique could be particularly beneficial in patients with minimal peri‐rectal fat, appearing to be a cost‐effective way to improve outcomes in developing countries with limited resources. EP-2153 Late toxicity after single dose HDR-BT and EBRT for prostate cancer: clinical-dosimetric predictors D. Büchser 1 , F. Casquero 1 , J.M. Espinosa 1 , F. Perez 1 , P. Minguez 1 , L. Martinez‐Indart 2 , F. Suarez 1 , A. Gonzalez 1 , J. Cacicedo 1 , I. San Miguel 1 , P. Bilbao 1 , A. Gomez‐ Iturriaga 1 1 Hospital de Cruces, Radiation Oncology, Baracaldo- Vizcaya, Spain; 2 Hospital de Cruces, Bioinformatics and Statistics Department, Baracaldo-Vizcaya, Spain Purpose or Objective To describe the genitourinary (GU) and gastrointestinal (GI) late toxicity profile and to analyse the clinical and dosimetric predictive factors in a prospective trial of the combination of EBRT and high‐dose‐rate (HDR) prostate brachytherapy (BT) for localized prostate cancer. Material and Methods Between January 2012 and May 2017, 210 patients were included in a prospective protocol. Treatment consisted in HDR‐BT (15Gy single fraction) and supplemental 3DCRT (37.5Gy/15 fractions) +/‐ androgen deprivation therapy according to risk stratification. Exclusion criteria included previous pelvic radiotherapy and pubic arch interference. Acute and late urinary complications were assessed using the CTCAEs v 4.0 definition. A descriptive analysis was performed. Univariate and multivariate logistic regression was used to analyse the impact of these variables into late toxicity. All variables with a P value <0.15 in the univariate analysis were selected and included in the stepwise selection procedure of the multivariate analysis. In this last analysis, a P value <0.05 was considered statistically significant. A Hosmer‐ Lemeshow test and receiver‐operating characteristic analysis was performed to calculate area under the receiver operator curve (ROC) (AUC) to compare strength of predictors. Results Median age was 71 (56‐82), 12.4% of patients had low, 44.3% intermediate and 41% high‐risk prostate cancer. Median prostate volume was 28.4cc. Median V100, V150, V200 were 98.2%, 27% and 7.4% respectively. Median urethra Dmax, rectum D1cc and D2cc, were 113.5%, 62.2%% and 54.2% respectively. After a median follow‐up of 41 months (5‐75) late G2 GU and GI late toxicity was observed in 14.8% and 5.2% of patients respectively. Late G3 GU and GI toxicity occurred in 0% and 1% of patients respectively. No grade 4 or 5 events were recorded. Out of the 31 patients with G2 events, 21 had a resolution of their symptoms by the time the current analysis was