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5011 Background: Emerging data suggest metastasis-directed therapy (MDT) improves outcomes in patients with oligometastatic castration-sensitive prostate cancer (omCSPC). Prostate-specific membrane antigen positron emission tomography (PSMA-PET/CT) can detect occult metastatic disease and has been proposed as a biomarker for treatment response. Herein we identify and validate a PSMA-PET biomarker for clinical outcomes following MDT in omCSPC. Methods: This was an international multi-institutional retrospective study of two completely independent cohorts of men with omCSPC, defined as ≤3 lesions, treated with metastasis-directed stereotactic ablative radiation therapy (SABR) who underwent PSMA-PET/CT prior to and 3-6 months after treatment. Pre- and post-SABR PSMA-PET/CT standardized uptake value (SUV) was measured for all lesions and PSMA response defined discretely using a cutpoint of ≥ 30% decrease in SUVmax. PSMA-PET response was correlated with lesion local control (LLC), radiographic progression-free survival (rPFS) defined using conventional and PET imaging, and metastasis-free survival (MFS) defined by conventional imaging alone. Results: A total of 131 patients with 261 treated metastases were included in the analysis, with median follow-up of 29 months (IQR 18.5-41.3). Following SABR, 78.4% of lesions experienced a partial or complete PSMA response. Multivariable analysis demonstrated SUV response significantly associated with improved LLC (HR = 9.97, 95%CI 3.92-25.4; p < 0.01). Patients with PSMA response in all lesions experienced significantly better rPFS (HR = 0.49, 95%CI 0.26-0.92; p = 0.03) compared to their counterparts and this maintained significance within both the discovery (p < 0.01) and validation (p = 0.01) cohorts. Within the discovery cohort, patients with PSMA response in all lesions also experienced significantly improved MFS (HR = 0.24, 95%CI 0.07-0.85; p = 0.03); analysis of the independent validation cohort is ongoing. Conclusions: Following SABR, PSMA-PET response is a robust and externally validated radiographic biomarker for rPFS and appears to be associated with MFS pending validation. This approach holds promise for guiding clinical management of omCSPC.

308 Background: The aim of this study is to evaluate the prognostic implications of neutrophil-to-lymphocyte ratio (NLR) and platelet-to-lymphocyte ratio (PLR) at baseline and after 4 and 12 weeks of treatment with abiraterone in patients with metastatic castration-resistant prostate cancer (mCRPC). Methods: We retrospectively evaluated 102 mCRPC treated with abiraterone either pre- or post-chemotherapy between November 2012 and October 2017 in three institutions. We calculated NLR and PLR at baseline and at 4 and 12 weeks after initiation of abiraterone, and we evaluated prostate-specific antigen (PSA) response every 4 weeks. Fifty patients (49%) were treated with abiraterone post-docetaxel, and 52 patients (51%) received abiraterone pre-chemotherapy. Based on receiver operating characteristic analysis, patients were stratified as low NLR ( < 3.1) or high NLR (≥ 3.1), and low PLR ( < 163) or high PLR (≥ 163). The cutoff for anemia was < 12g/dL. Results: Median follow-up times for patients overall and for those who survived were 24.0 months (range, 0.3 – 54.9 months) and 25.5 months (range, 2.8 – 54.9 months), respectively. The median time of abiraterone treatment was 8.1 months (range, 2.4 – 40.1 months). The median overall survival (OS) was 20.8 months (interquartile range: 17.3–24.4). In univariate analysis, NLR, PLR, PSA response, and low hemoglobin (Hgb) were found significantly predictive of OS and progression-free survival (PFS). In multivariate analysis, declines in NLR and PSA of ≥ 90% emerged as significant independent predictors of OS and PFS. High-NLR patients who remained high or who returned to low NLR after 4 and 12 weeks showed significantly worse OS than patients with low baseline NLR. Patients with baseline Hgb > 12 g/dL had significantly longer median OS compared with patients with Hgb ≤ 12 g/dL; however, the significance of Hgb was lost at 12 weeks. Conclusions: NLR and PSA response to abiraterone was a significant predictor of OS and PFS in mCRPC patients treated with abiraterone delivered either pre- or post-chemotherapy. Furthermore, persistent increase in NLR during abiraterone has prognostic value for OS in patients with mCRPC.

68 Background: The aim was to assess the intensity of 68Ga-PSMA uptake in the primary tumour in patients locally advanced prostate cancer treated with definitive radiotherapy. Also correlation between the maximum standardized uptake value (SUVmax) of primary tumour and the Gleason score (GS) or prostate-specific antigen (PSA) level was assessed. Methods: The data of 201 biopsy-proven prostated cancer patients were retrospectively analysed. Median age of entire cohort was 68 years (range; 45 – 85 years). PET/CT images were analysed visually and semiquantitatively by measuring the SUVmax. The SUVmax of the primary tumour was assessed in relation to both PSA level and GS. Results: Median age of entire cohort was 68 years (ranged 45 – 85 years). Median PSA value at diagnosis was 20.3 ng/mL (ranged 2.1 – 301.0 ng/mL). Forty-two patients (23.9%) was in intermediate risk group, and 159 patients (79.1%) had high risk disease. Seventy two patients (36.8%) had regional lymph node metastasis. Median SUVmax of primary tumor and metastatic lymph nodes were 13.0 (ranged 1.3 - 84.3) and 12.6 (ranged 3.6 – 64.5), respectively. Patients with GS 7 or lower had significantly lower SUVmax compared to patients with GS > 7 (12.1±8.4 vs. 20.9±16.4; p < 0.001). However, there was no significant difference in SUVmax of tumors with GS of 3+4 and 4+3 (14.8±8.6 vs. 16.3±12.7; p = 0.6). Patients with PSA ≥10.0 ng/mL exhibited significantly higher uptake than those with PSA levels < 10.0 ng/mL (12.1±8.4 vs. 20.9±16.4; p < 0.001). SUVmax of intermediate risk patients was significantly lower than that of high risk patients (12.7±11.1 vs. 19.8±16.8; p = 0.01). In 72 patients with lymph node metastasis SUVmax was significantly higher compared to 129 patients without lymph node metastasis (24.9±21.3 vs. 14.7±10.6; p < 0.001). Conclusions: Tumours with GS 7 or lower, patients with PSAvalues ≤10 ng/mL, intermediate risk patients and patients without regional lymph node metastasis showed significantly lower 68Ga-PSMA uptake. 68Ga-PSMA-PET/CT should be preferentially applied for primary staging of prostate cancer in patients with GS > 7, PSA levels ≥10 ng/ml, high risk patients and patients with regional lymph node metastasis.

316 Background: To evaluate the potential benefit of curative radiotherapy (RT) to primary tumor in metastatic castration resistant prostate cancer (mCRPC) patients treated with abiraterone. Methods: The clinical parameters of 106 mCRPC patients treated with abiraterone in the either pre- or post-chemotherapy setting were retrospectively evaluated. All patients had PSA progression or radiographic progression suitable for metastasis according to the RECIST guidelines with or without PSA progression. Patients were either oligometastatic (≤5 metastases) at diagnosis or became oligometastatic after systemic treatment were analyzed. Local RT to primary tumor and pelvic lymphatics was delivered in 44 patients (41%), while 62 patients (59%) did not have RT to primary tumor. Results: Median follow-up times for patients overall and for those who survived were 14.2 months (range, 2.3–54.9 months) and median OS for entire cohort was 21.9 months. The median OS was significantly higher in patients treated with local RT to primary tumor compared to patients without local RT (p = 0.04). Local RT to prostate and pelvic lymphatics significantly diminished local recurrence rate ( p = 0.003). In univariate analysis, a decline in PSA levels ≥50% of baseline was significant prognostic factor for both OS and PFS in entire cohort and in patients receiving prostate RT. In multivariate analysis, PSA response ≥50 of baseline obtained ≥3 weeks after abiraterone therapy is the only significant prognostic factor for better OS and PFS. Additionally, local RT to prostate is nearly significant prognosticator for improved OS. Patients treated with primary RT to prostate had significantly less progression under abiraterone and longer abiraterone period compared to patients without local prostate RT. Conclusions: Local prostate RT significantly improves OS and local control but not PFS, in mCRPC treated with abiraterone. Patients treated with primary RT to prostate had significantly less progression under abiraterone and longer abiraterone period compared to patients without local prostate RT.

213 Background: Metastases-directed therapy by metastasectomy or radiotherapy (RT) might delay disease progression and postpone systemic treatment in patients with oligometastatic prostate cancer. Here, we evaluated retrospectively the efficacy and toxicity of 68Ga prostate-specific membrane antigen (PSMA) PET-CT guided radiotherapy (RT) in the treatment of oligometastatic prostate cancer. Methods: A total of 23 prostate cancer patients with biochemical relapse, of which 13 castration-sensitive and 10 castration-resistant, were treated with intensity-modulated and image-guided RT (IMRT-IGRT) on ≤ 3 metastases detected by 68Ga PSMA PET-CT. Androgen deprivation therapy was continued in castration-resistant patients. Local control (LC), progression-free survival (PFS) and overall survival (OS) were estimated with the Kaplan-Meier method. Results: A total of 38 metastases were treated. Involved sites were pelvic bone (n = 16), pelvic lymph nodes (n = 11), para-aortic lymph nodes (n = 6), ribs (n = 3) and vertebral body (n = 2). The median PSA prior to RT was 1.06 ng/ml (range 0.10 – 29.0 ng/ml). A median dose of 43.5 Gy (range, 30-64 Gy) was delivered by IMRT-IGRT in 12-27 fractions. At a median follow-up of 7 months (range, 2-17 months), 19 patients (83%) are in remission. Four patients (17%) developed distant recurrence. The actuarial 1-year LC, PFS and OS rates were 100%, 51% (95% CI 8-83%) and 100%. Castration-sensitive patients displayed a statistically significantly superior PFS on univariate analysis as compared to castration-resistant patients (1-year PFS 67% vs 0%, p < 0.01). One patient experienced grade 2 acute gastro-intestinal toxicity. No grade 3 or more toxic events were observed. Conclusions: By providing optimal LC, low toxicity and a promising PFS in castration-sensitive patients, the current retrospective study illustrated that 68Ga PSMA PET-CT guided RT may be an attractive treatment option in patients with oligometastatic prostate cancer. Validation by randomized trials is eagerly awaited.