ESTRO 38 Abstract book

S850 ESTRO 38

Hypofractionated proton therapy (HFPT) has not been extensively studied. The purpose of this study was to determine the toxicity profile and biochemical-clinical failure (BCF) rates for patients with localized prostate cancer treated with HFPT. Material and Methods Between 2010 and 2017, 184 men were enrolled on a prospective trial of 70Gy in 28 fractions of proton therapy for localized low- to intermediate-risk prostate cancer. Short-term androgen suppression (AS) was allowed, per institutional practice. Acute and late toxicity was evaluated using the Common Terminology Criteria for Adverse Events, version 4.0 (CTCAEv.4.0). Patient- reported urinary quality-of-life changes and erectile changes were captured using the International Prostate Symptom Score (IPSS) and the International Index of Erectile Function (IIEF-5) Questionnaire, respectively. Results Median follow-up was 49.2 months. Enrolled patients had low-risk [LR] (n=18), favorable intermediate-risk [FIR] (n=78), and unfavorable intermediate-risk [UIR] (n=88) disease. Twenty-six percent of patients received neoadjuvant and concurrent AS, with a median duration of 6 months. Four-year rates of BCF free survival were 95.7%, 94.4%, 98.7%, and 93.2% in the overall group and the LR, FIR, and UIR cohorts, respectively (p=.21). Overall survival at 4 years was 97.3%. All 5 deaths were unrelated to prostate cancer. Median IPSS before treatment and at 1 year after treatment were 6 and 7 for LR patients, 6 and 6 for FIR patients, and 8 and 7 for UIR patients. Median IIEF-5 before treatment and at 1 year after treatment were 21 and 18 for LR patients, 18 and 16.5 for FIR patients, and 15 and 10.5 for UIR patients. The incidence of acute CTCAE v4.0 grade 2 or higher gastrointestinal (GI) and urologic toxicities were 3.8% (7 events) and 12.5% (23 events), respectively. Only one acute grade 3 GI toxicity was reported. The estimated cumulative 4-year incidence of late grade 2 or higher urologic toxicity was 7.6% (14 events), with no grade 3 or 4 events reported. The estimated cumulative 4-year incidence of late grade 2 or higher GI toxicity was 13.6% (25 events), with one grade 3 event and no grade 4 events. The predominant late grade 2 GI toxicity was rectal bleeding, and late grade 2 urologic toxicity was urinary frequency, accounting for 79% and 57% events, respectively. All late grade 2 and 3 events were transient. There were no associations between risk group, anticoagulation use, or history of transurethral resection of the prostate and incidence of acute or late toxicity. Conclusion HFPT for the treatment of prostate cancer is associated with low rates of urologic and GI toxicity, and low rates of patient-reported urinary bother post-treatment. Further analyses are warranted to assess long-term toxicity and understand differences between proton and photon hypofractionated radiation therapy in the treatment of prostate cancer. EP-1575 Stereotactic Body Radiotherapy in bone oligometastatic prostate cancer patients F. Trippa 1 , F. Arcidiacono 1 , A. Di Marzo 1 , L. Draghini 1 , P. Anselmo 1 , S. Terenzi 1 , M. Casale 1 , S. Fabiani 1 , E. Maranzano 1 1 Radiotherapy Oncology Centre, Santa Maria Hospital, Terni, Italy Purpose or Objective The first line treatment of metastatic prostate cancer (PC) is androgen-deprivation therapy (ADT). A subgroup of patients (pts) with disease progression can present few lesions (i.e.,oligometastases).Stereotactic body radiotherapy (SBRT) delivers high ablative doses of radiotherapy while sparing adjacent tissues. We used SBRT

1 Department of Radiation Oncology, University Hospital, Tübingen, Germany ; 2 Section for Biomedical Physics Department of Radiation Oncology, University Hospital, Tübingen, Germany Purpose or Objective Prediction of genitourinary (GU) and gastrointestinal (GI) toxicity after prostate cancer intensity modulated radiotherapy (IMRT) based on organs at risk (OAR) prescriptions is limited. Therefore, we investigated the potential impact of bladder-, rectal- and target volumes on acute side-effects of patients treated in a prospective study. Material and Methods Twenty-eight patients received a dose-escalated IMRT of localized prostate cancer (257/2007B01). Radiotherapy was performed using the coverage probability concept to a total dose of 78Gy in 39 fractions. All patients underwent at least three planning CTs and were instructed according to a bladder-rectal filling protocol (aim: bladder volume of 250-500ml, empty rectum). Contouring of OARs and target volumes was performed in each CT and three OAR- volumes per patient were measured. Incidence of acute GU/GI-toxicities (RTOG) was correlated with bladder-, rectal- and PTV-volumes. The volumes of 28 patients were descriptively evaluated by SPSS and subdivided in 4 quartiles (i.e. 7 patients per quartile). Results Acute toxicity was predominately detected in patients with small bladder volume, large or small rectal volume and large target volume. GU-toxicity (G0/G1/G2: 5/15/8): Patients with the smallest bladder volume quartile (<188.41ml) represented with G2 (n=3) and G0/1 (n=1/3) toxicity. This equals 37.5% of G2-toxicities and 20% of G0- and G1-toxicities. The largest target volume quartile (>271.17ml) was connected to toxicity of grade G2 (4/7), G1 (2/7) and G0 (1/7) accounting for 50% (G2), 20% (G1) and 13% (G0) of GU- toxicities. Combination of adverse factors i.e. small bladder volume (<188.41ml) and large target volume (>271.17ml) was only present in 33% of patients experiencing G2 toxicity. GI-toxicity (G0/G1/G2: 6/16/6): Considering the lowest (<63.36ml) and highest quartile (>166.87ml) of rectal volume 66% of patients experiencing G2-toxicity were in these quartiles compared to G1 with 50% and G0 with 33%. The largest quartile of PTV (>271.17ml) was always connected to GI-toxicity of at least G1 (n=5) and higher (G2=2). Presence of even small rectal maxima exceeding 80Gy (rectal V80>0.26ml) were related to G2-toxicity (83%, n=5/6). Combination of adverse factors i.e. small/large rectal volume and rectal V80>0.26ml were associated with increased toxicity (G1=31%, G2=50%). Conclusion GU-toxicity was reduced in case of a minimal bladder volume ≥188.41ml and a target volume <271.17ml. GI- toxicity was less apparent in patients with intermediate rectal volumes between 63.36-166.87ml and absence of rectal hot spots >80Gy. Combination of these adverse factors was always connected to toxicity (no G0) and should therefore be avoided during treatment planning. These findings need to be substantiated in a larger cohort. EP-1574 Four-year outcomes of hypofractionated proton therapy for localized prostate cancer A. GrewaL 1 , C. Schonewold 1 , S. Both 2 , S. Lam 1 , S. Mazzoni 1 , J. Bekelman 1 , J. Christodouleas 1 , N. Vapiwala 1 1 University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA ; 2 University Medical Center of Groningen, Department of Radiation Oncology, Groningen, The Netherlands Purpose or Objective Moderately hypofractionated radiation therapy represents an effective treatment for localized prostate cancer.