ESTRO 38 Abstract book

S832 ESTRO 38

longer follow-up and the patterns of PSA changes were different according to the risk groups. EP-1543 Early Results of a Phase 2 Multicentre Study of Linac-based Stereotactic Boost for Prostate Cancer D. Pryor 1 , M. Sidhom 2 , S. Arumugam 2 , J. Bucci 3 , J. Smart 4 , M. Grand 2 , P. Greer 4 , S. Keats 2 , L. Wilton 4 , M. O'Neill 4 , J. Martin 4 1 Princess Alexandra Hospital, Radiation Oncology, Brisbane, Australia ; 2 Liverpool Hospital, South Western Sydney Radiation Oncology, Sydney, Australia ; 3 St George Hospital, Radiation Oncology, Sydney, Australia ; 4 Calvary Mater Newcastle, Radiation Oncology, Newcastle- NSW, Australia Purpose or Objective To report early toxicity and PSA kinetics following a novel, linac-based, stereotactic radiotherapy (SBRT) boost for a prospective multicentre phase 2 study (PROMETHEUS ACTRN12615000223538). Material and Methods Patients were treated with linac-based SBRT, 19-20 Gy in 2 fractions delivered one week apart, followed by conventionally fractionated IMRT/VMAT (46 Gy in 23 or 36 Gy in 12 fractions). MRI fusion for RT planning was mandated, as was rectal displacement during SBRT. Toxicity was prospectively graded using the Common Terminology Criteria for Adverse Events version 4.0 (CTCAE v4). Results Between 3/2014 and 10/2018, 134 patients (76% intermediate, 24% high-risk) with a median age of 70 years (range 53–85) have been treated across 5 centres. Short course (≤6 m) ADT was used in 37%, long course in 17%. Rectal displacement method was SpaceOAR in 59% and Rectafix in 41%. 42 and 92 patients were treated at the 19 Gy and 20 Gy dose levels respectively. Median follow-up is 24 months. Acute G2 GI and GU toxicity occurred in 3% and 24% with no cases of acute G3 toxicity. Late ≥G2 GI toxicity was 3%, 1% and 0% at 12, 24 and 36 months respectively. Only one G3 GI toxicity occurred (18 months post-RT). Late ≥G2 GU toxicity at 6,12,18, 24 and 36 months was 1%, 11%, 12%, 7% and 6% respectively with only two G3 events (18 months post RT – figure 1). For patients not receiving ADT the median PSA value pre-treatment was 7.8 ug/L (range 1.1–20) and at 12, 24, 36 and 48 months post-treatment was 0.86, 0.36, 0.2 and 0.07 ug/L (figure 2).

for patients without previous RT (>90%) and new curative options in half of patients after SRT who usually would otherwise receive palliative ADT. Cost efficacy is further substantiated by the new measure NNI. Therefore, 68 Ga- PSMA PET/CT staging should become the standard method for staging of high risk prostate cancer patients with BCR, also after initial salvage therapy. EP-1542 Long-term results and PSA kinetics after robotic SBRT for prostate cancer Y. Park 1 , H.J. Park 2 , W.I. Jang 3 , B.K. Jeong 4 , H. Kim 5 , A.R. Chang 1 1 Soonchunhyang University Seoul Hospital, Radiation Oncology, Seoul, Korea Republic of ; 2 Hanyang University College of Medicine, Radiation Oncology, Seoul, Korea Republic of ; 3 Korea Institute of Radiological and Medical Sciences, Radiation Oncology, Seoul, Korea Republic of ; 4 Gyeongsang National University School of Medicine, Radiation Oncology, Jinju, Korea Republic of ; 5 Inha University Hospital, Radiation Oncology, Incheon, Korea Republic of Purpose or Objective To evaluate the treatment outcome and prostate-specific antigen (PSA) change after stereotactic body radiotherapy (SBRT) for localized prostate cancer. Material and Methods Patients with localized prostate cancer treated with SBRT at three academic hospitals were enrolled. Treatment was delivered using Cyberknife with dose range from 35 to 37.5 Gy in 5 fractions. Biochemical failure (BCF) was assessed with Pheonix definition and toxicities were scored with Radiation Therapy Oncology Group (RTOG) toxicity criteria. The PSA kinetics were analyzed in patients who received no androgen deprivation therapy (ADT) and showed no recurrence. Results Of the total 88 patients, 14 patients (15.9%) received ADT. After median follow-up of 63.8 months, the 5-year BCF free survival (BCFFS) was 94.7%. Two patients experienced late grade ≥3 GI toxicities (2.2%). The median nadir PSA was 0.12 ng/mL and the median time to nadir was 44.8 months. Patients who reached nadir before 24 months showed poorer BCFFS than the others. The rate of PSA decline was maximum in the first year after treatment and gradually decreased with time. The pattern of PSA change was significantly different according to the risk groups ( p =0.011) with the slope of -0.139, -0.161 and -0.253 ng/mL/month in low-, intermediate- and high-risk groups, respectively (Figure)

Conclusion SBRT for localized prostate cancer showed favorable efficacy with minimal toxicities. The time to PSA nadir was significantly associated with treatment outcome. PSA revealed rapid initial decline and slower decrease with