ESTRO 38 Abstract book

S821 ESTRO 38

RapidArc technique. For right and left femurs, the RapidArc technique showes better results. Results RapidArc MU values are lower than MU values of Elekta- VMAT, so treatment duration was shorter in RapidArc. Clinically both VMAT-techniques for each machine were equally effective in producing acceptable plans. Conclusion As a result it can be said that these two techniques produce equivalent results by practice. EP-1519 PSMA-PET/CT for guidance and response assessment of SABR for prostate cancer oligometastases P. Dirix 1 , C. Mercier 1 , C. Billiet 1 , P. Vermeulen 2 , S. Oeyen 2 , S. Van Laere 2 , P. Huget 1 , D. Verellen 1 1 Iridium Cancer Network, Radiation Oncology, Antwerp, Belgium ; 2 Oncologisch Centrum GZA, Translation Cancer Research Unit, Antwerp, Belgium Purpose or Objective Prostate-specific membrane antigen (PSMA) PET/CT is increasingly used to detect oligometastases in biochemical recurrent prostate cancer and subsequently guide metastasis-directed treatment such as SABR. However, limited prospective evidence is available. Material and Methods All patients were included in a prospective phase I dose- escalation trial on SABR for non-spine bone and lymph node oligometastases (NCT03486431). According to protocol, a new PMSA-PET/CT was performed at 6 months after treatment. Metabolic response was evaluated according to PERCIST 1.0 criteria. PSA progression was defined as a PSA increase of ≥ 25% and ≥ 2 ng/mL if PSA was ≥ 2 ng/mL at baseline, or a PSA increase of ≥ 25% if PSA was < 2 ng/mL. PSA response was defined as a decline from baseline of ≥ 80%, while complete response was defined as a decline from baseline of ≥ 90%. Toxicity was graded using Common Terminology Criteria for Adverse Event version 4.0. Results From July 2017 to March 2018, 15 consecutive patients had 1 (n = 12) or 2 (n = 3) metastases detected on PSMA- PET/CT and were prospectively included in the trial. They then received SABR to a total of 18 oligometastases. Four patients had non-spine bone only, 8 had node only, and 3 had mixed disease. The median age was 67 (52-78) years. Four patients received concomitant androgen-deprivation therapy (ADT) for 6 months. Eight patients were included in the first cohort (5x 7.0 Gy) and 4 in the second cohort (3x 10.0 Gy). SABR was feasible and delivered as planned in all cases. At 6 months after SABR, 5 patients had a complete PSA response, while 7 patients showed PSA progression. Already 8 patients had new metastatic lesions visible on the second PSMA-PET/CT. Interestingly, only 5 of those patients had clear PSA progression at that time. Regarding local control, there was residual PSMA uptake in 6 patients (7 lesions in total). Remarkably, 3 of those 6 patients had a complete PSA response at the same time. All 4 patients who received concomitant ADT presented a complete metabolic response on PSMA-PET/CT as well as a complete PSA response. Median follow-up was 11 (7 – 14) months. No grade ≥ 3 toxicity was observed. Progression-free survival at 1 year was 100% for patients who had received ADT versus 21% for patients who did not (p = 0.032). In total, 6 of the 11 patients not on ADT had started ADT a the last follow-up (1-year ADT-free survival of 32%). A further 10 patients were included the second cohort, whose PSMA follow-up will be available at ESTRO. Conclusion Repeated imaging with PSMA-PET/CT could be incorporated into prospective clinical trials on SABR for prostate cancer oligometastases to evaluate the true

v3.0). Patterns of prostate-specific antigen (PSA) response were analyzed. Biochemical failure was defined as Phoenix definition. Results The initial median PSA was 9.4 and 40.8 ng/ml for the low/intermediate-risk and high-risk patients, respectively. After SBRT treatment, median PSA at 60 months level was 0.16 and 0.07 ng/ml for the low/intermediate-risk and high-risk patients, respectively. In the low/intermediate-risk patients, there were four patients with biochemical failures within a median follow-up of 58 months. For the high-risk patients, seven patients with biochemical failure were disclosed in the median 46-month follow up. The estimated 10-year biochemical failure-free survival was 95.1% and 90.2% for low/intermediate-risk, and high-risk patients, respectively. In the low/intermediate-risk patients, late Grade 3 GU and GI toxicities were seen in 2.5% and 0%, respectively. In the high-risk patients, the incidence rate of late Grade 2 GU and GI toxicity were 5% and 2%. There was no late Grade 3 and GI toxicity in the high-risk patients. Most of acute toxicity effects in all the patients resolved within three to six months of treatment This is a 10-year experience of SBRT for localized prostate cancer in a single institution. SBRT with or without whole pelvic irradiation for localized prostate cancer is feasible with minimal toxicity and excellent biochemical failure- free survival. Continued accrual and follow-up would be necessary to confirm this long-term biochemical control and the late toxicity profiles. EP-1518 A dosimetric comparison of treatment plans by using aaa/mc with vmat technique for prostate patients P. Boydak 1 , K. Temizyurek 2 1 istanbul Aydin University, Medical Physics, Istanbul, Turkey ; 2 istanbul Aydin University, Medicine, Istanbul, Turkey Purpose or Objective In this study, it was aimed to compare the performance of two different algorithms on the VMAT technique. Twenty patients with prostate cancer were selected for comparison. Tomography images were scanned with a 3mm slices. Target volume PTV and at-risk organs (OAR) as bladder, rectum, right -left femur heads were defined. Doses of target volumes were prescribed as 70 Gy in 35 fractions for all treatment plans. In Varian technique patient plans are prepared using 6 MV energy, AAA algorithm, and double arc VMAT technique with the Eclipse Treatment Planning System (TPS), then in Electa for same patients, treatment plans using 6 mV energy, the Monte Carlo algorithm and VMAT technique are made with the Monaco TPS. Dose distributions were obtained for each patient. These two different algorithms were compared in terms of critical organ dose values, target volume maximum dose values, HI, CI and MU values of PTV. Material and Methods Plans were normalized to cover 95% of the target volume. CI and HI values in Varian Rapidarc are 1.01±0.02 (1), 0.08±0,01 (0.08), respectively. For Elekta-VMAT CI and HI values are 0.96±0.02 (0,95) and 0.1±0.02 (0.09), respectively. Consequently RapidArc gave better result for CI and HI values. The monitor unit value of RapidArc 761,35 ± 94,29 was observed to be lower than that of Elekta-VMAT monitor unit 922,15 ± 93,4. In terms of bladder and rectum doses Elekta-VMAT technique give lower results than results of RapidArc technique. There was no significiant difference (p>0.05) in the calculation of PTV Dmax and bladder 50% for each machine. When the critical organ doses were considered, the Elekta-VMAT technique has statistically significant lower values for bladder and rectum doses than those values of the completion. Conclusion