ESTRO 37 Abstract book

S868

ESTRO 37

on the first day of treatment (baseline), in the middle, the last day (end) and after 40 or 50 days (1.5-months follow-up) after the end of SBRT. Samples were stored at room temperature until analyzed. Quantification of CPCs and CECs was performed by flow cytometry to detect and quantify different cells subsets and their kinetics at above mentioned times. The GraphPad software was used for the statistical analysis. Results CPCs were defined as CD34++/CD45dim/CD31+. Compared to baseline values, a significant reduction of total CPCs (1994.23 vs 1090.41 CPCs/µl; p=0.003) at the end of SBRT was observed. No significant differences were observed in the quantification of total CPCs at +40- 50 days after SBRT compared to baseline values (1994.23 vs 1931.02 CPCs/µl; p=0.67). CECs were defined as CD146+/31+/34+/CD45-. Tumor derived endothelial cells (TECs) were defined as CD45-/CD31++/CD276+. Compared to baseline values, no significant changes of total CECs and TECs numbers after SBRT were observed in any of the time-points evaluated. Conclusion In this study we have been able to quantify CPCs and CECs with a reproducible protocol in prostate cancer patients after SBRT. Preliminary results show a significant reduction in CPCs at the end of treatment that may be related to low doses of pelvis irradiation with IMRT technique. There were no significant changes in the total CECs and TECs during SBRT. Funding This study has been partially supported by grant GRS1106/A/15, from Gerencia Regional de Salud de Castilla y León (Spain). L. Marinelli 1 , C. Reverberi 1 , L. Nicosia 1 , S. Magrini 2 , S. Giacinti 2 , G. Poti 2 , G. Arrivi 2 , M. Osti 1 , V. De Sanctis 1 , C. Proietti 1 , A. Aschelter 2 , P. Marchetti 2 , M. Valeriani 1 1 Azienda Ospedaliera Sant' Andrea, Department of Radiation Oncology, Rome, Italy 2 Azienda Ospedaliera Sant' Andrea, Department of Oncology, Rome, Italy Purpose or Objective The effectiveness of abiraterone acetate has been demonstrated both in pre- and post-chemotherapy settings in mCRPC patients. Oligoprogression during abiraterone treatment might not represent a systemic strategy failure. Combining a local treatment, as radiotherapy, could permit to continue the systemic therapy and delay line shifts of curative strategies aiming to improve clinical outcomes. The purpose of our study is to demonstrate the efficacy of adding radiotherapy to abiraterone treatment in oligoprogressive mCRPC patients. Material and Methods From June 2014 to August 2017, 12 consecutive mCRPC patients treated with Abiraterone undergone curative/palliative radiotherapy for oligo-progression (1-3 sites). All patients presented increasing PSA demonstrated on 3 consecutive evaluations. Oligo- progression was confirmed with F-Choline PET-TC and with contrast enhanced MRI for patients scheduled for stereotactic radiotherapy. Four patients (33.3%) were treated with stereotactic radiotherapy (27 Gy in 3 fractions) and 8 (66.7%) with 3-D conformal palliative radiotherapy (20 Gy in 5 fractions or 30 Gy in 10 EP-1610 Oligoprogression during Abiraterone therapy treated with radiotherapy in mCRPC patients.

fractions). Fractionation was chosen based on the extension of lesion. All patients presented skeletal secondary localizations so bone was the unique site of metastasis. A total of 14 bone lesions were treated (2 patients presented 2 lesions). After radiotherapy all patients continued Abiraterone therapy. Results Median follow-up from the start of abiraterone was 23.7 months (range 10.5-35.7 months). Median actuarial overall survival (OS) was not reached. Actually 3 patients (25%) are dead with disease, 6 (50%) are alive with disease and 3 (25%) are alive without disease. Median duration of abiraterone treatment was 12.7 months (range 6.2-30.9 months). Median duration of Abiraterone after RT was 4.9 months (range 2.2-23.6 months). Median progression-free survival (PFS) calculated from the start of Abiraterone was 15.0 months (95%CI=1.5- 37.2). Progression-free survival calculated from the end of radiotherapy was 5.2 months (95%CI=3.9-6.5). Conclusion Our series demonstrated that radiotherapy might prolong Abiraterone treatment in mCRPC patients. More patients and comparative studies are mandatory to verify this result and to evaluate survival improvement. EP-1611 Decision support for rectum spacers: combining tumor control, rectum toxicity and genetic markers Y. Van Wijk 1 , B. Vanneste 1 , A. Jochems 1 , S. Walsh 1 , P. Lambin 1 1 Maastricht university, GROW - School for Oncology and Developmental Biology, Maastricht, The Netherlands Purpose or Objective Dose escalated radiotherapy for prostate cancer patients has revealed higher tumor control probabilities (TCP) [Zelefsky et al. Eur Urol. 2011] but consequently increases the normal tissue complication probability (NTCP) in the rectum. An implantable rectum spacer (IRS) reduces the high dose in the rectum, and could allow for more dose escalation, without increasing toxicity. An iso- toxic model was developed to calculate the amount of dose escalation possible for a given treatment plan with a chosen threshold for the NTCP. A virtual IRS (V-IRS) [van Wijk et al. Radiother Oncol. 2017], which used image deformation to obain CT images with IRS before IRS placement, was applied. The iso-toxic model in combination with the V-IRS could be used to support the decision to place an IRS in a given patient. Material and Methods This Proof of Concept study included 16 localized prostate cancer patients with an IRS. Treatment planning was performed on computed tomography (CT) images before and after IRS placement and on images with a V- IRS. For these plans the normal tissue complication probability (NTCP) was calculated using a Lyman-Kutcher- Burman model for Grade>=2 late rectal bleeding [Lui et al. Acta Oncol. 2010], which was expanded with single- nucleotide polymorphisms (SNP) for late rectal toxicity [Kerns et al. EBioMedicine 2010]. The model adjusted the number of dose fractions, recalculating the equivalent dose volume histogram and resulting NTCP, until the NTCP was under 5%. The resulting treatment plan was used to calculate the TCP using a published model [Walsh et al. Med Phys. 2012]. Results Figure 1 shows the median TCP and NTCP plotted against the number of fractions for the treatment plans without IRS (solid line) and with IRS (dashed). The median TCP for