ESTRO 36 Abstract Book

S719 ESTRO 36 2017 _______________________________________________________________________________________________

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Particle therapy offers the opportunity to significantly reduce the NTCP, but require decision support systems, using multi-factorial prediction models and ultimately including cost-effectiveness analyses to choose the optimal treatment modality and justify the accompanying increased costs. EP-1356 SBRT benefit in oligometastatic prostate cancer patients detected by [18F]fluoromethylcholine PET/CT E. Bouman-Wammes 1 , J.M. Van Dodewaard- de Jong 1 , M. Dahele 2 , M.C.F. Cysouw 3 , O.S. Hoekstra 3 , A.H.M. Piet 2 , A.J.M. Van den Eertwegh 1 , H.M.W. Verheul 1 , D.E. Oprea- Lager 3 , V.M. H. 4 1 VU University Medical Center, Medical Oncology, Amsterdam, The Netherlands 2 VU University Medical Center, Radiotherapy, Amsterdam, The Netherlands 3 VU University Medical Center, Nuclear Medicine, Amsterdam, The Netherlands 4 VU University Medical Center, Urology, Amsterdam, The Netherlands Purpose or Objective For patients with oligometastatic recurrence of prostate cancer, stereotactic body radiation therapy (SBRT) represents an attractive treatment option as it is generally safe without major side effects. The aim of this study is to investigate the impact of SBRT in postponing the start of androgen deprivation therapy (ADT), and assessthe pattern of recurrence post SBRT Material and Methods Forty-three patients treated with SBRT for oligometastatic recurrence of prostate cancer were included. Also, a control group of 20 patients not treated with SBRT was identified from other hospitals. Data was retrospectively collected and analyzed. Results A post-SBRT PSA response was seen in 29/43 patients (67.4%), with undetectable PSA in 6/43 patients (14.0%). The median ADT free survival (ADT-FS), defined as time between the start of SBRT and start of ADT, was 15.6 months (95% CI 11.7-19.5) for the whole group, and 25.7 months (95% CI 9.0-42.4) for patients with an initial PSA response. Seven patients were treated with a second course of SBRT because of oligometastatic disease recurrence; the ADT- FS in this group was 32.1 months (95% CI 7.8-56.5). We compared the data of SBRT-treated patients with a group of 20 patients, managed in another hospital, by watchfull waiting followed by ADT. Compared to the control group, ADT-FS (from the date of first diagnosis of metastasis until start of SBRT) was significantly longer for SBRT treated patients with 17.3 months (95% CI 13.7-20.9) versus 4.19 months (95% CI 0.0-9.0), p<0.001. Once ADT had been started, the subsequent PFS during ADT treatment was comparable between both groups (median 31.5 months for SBRT-treated patients, versus 26.9 months for the control group, p=0.54). This results in a significanty longer period between the diagnosis of oligometastatic disease and development of castration resistant prostate cancer (see figure). Seventeen patients had a [ 18 F]fluoromethylcholine PET/CT performed because of a rising PSA after the first course of SBRT. In 15 patients the rise in PSA could be attributed to lesions which were outside the high-dose SBRT volume, in 2 patients no cause was found, no local failures were identified on these scans. One patient had progressive disease in a previously non-suspicious 3mm lymph node adjacent to the irradiated node. Another patient had persistent disease in a partially irradiated lymph node adjacent to the index node treated at first SBRT. Seven patients (16.2%) had some form of toxicity recorded in their medical chart: 2 of the patients with bone

Material and Methods Twenty-five consecutive patients with T3-T4 prostatic cancer were included for high-dose radiotherapy. Proton and Carbon-ion treatment planning was performed by the ROCOCO trial partners following a strict clinical protocol, prescribing 78 Gy (biologically equivalent dose) to the target and compared pair-wise to generated IMRT treatment plans. All 75 plans were analysed centrally to compare the dose in the rectum (the main organ at risk) and the NTCP for late rectal bleeding. A validated multi-factorial NTCP model used the mean dose (Gy) in the rectum and the percentage of the rectum receiving more than 75 Gy (V75) as dosimetric predictors. It also included a set of clinical predictors such as haemorrhoids and hormonal therapy. Results An overview of the results is shown in table 1.

The mean dose in the rectum resulting from the IMRT, proton and carbon-ion plans was 42.9 Gy [range: 30.8-47.2 Gy], 30.8 Gy [range: 23-45.2 Gy] and 18.9 Gy [range: 11.8- 33.3 Gy] respectively. The V75 for the IMRT, proton and carbon-ion was 3.2% [range: 0.3-10.8 %], 2.1% [range: 0.5- 4.3 %] and 1.9% [range: 0.6-3.9 %] respectively. Both proton and carbon-ion plans showed improvement with respect to the IMRT plans in both dosimetric parameters, and for carbon-ions it showed an improvement when compared to protons. The NTCP predicted for the IMRT, proton and carbon-ion plans was 8.7% [range: 6-14.5 %], 6.7% [range: 5-9.2 %] and 5.7% [range: 4.7-7.2 %] respectively. On average these treatments didn’t show large improvements in NTCP, however, individuals with significant improvement were identified. One patient showed that proton therapy would lower the NTCP with 5.3%, and 4 patients showed that carbon-ion therapy would lower the NTCP with 7.3, 5.2 and twice with 4.1%. Conclusion