PET imaging for metastatic castration sensitive prostate cancer (mCSPC) for patients with PSA response after systemic therapy: A multicentric ambispective analysis.

37 Background: PET imaging is not recommended for surveillance of mCSPC but it is used by some physicians. Little is known about surveillance for metastatic prostate cancer using PET imaging. The aim of this study was to assess the correlation between PSA response and PET imaging for mCSPC. Methods: An ambispective, multicentric, observational study, recorded PET imaging for patient’s treated with a minimum of 6 months of systemic therapy with androgen deprivation therapy (ADT) +/- androgen receptor pathway inhibitor (ARPI) +/- docetaxel for mCSPC with PSA < 0.2 ng/ml, normal CT scan and non progressive disease in bone scan. All of them had baseline evaluation with standard imaging. Results: Between August 2022 and September 2024, 57 patients (pts) with mCSPC were included, in 4 centres in France. Baseline characteristics at the time of metastatic disease were: median age of 65 years (range: 45-84), ISUP ≥4 for 63%, median PSA 22.1 ng/ml (1.6-5000ng/ml) with T≥3 in 53%. The majority of pts had low volume disease 56%, with synchronous metastatic disease for 67% and ≥ 4 bone metastases in 44%. Only 5 pts had visceral metastases. Pts were treated, by ADT + ARPI in 81%, ADT + docetaxel in 7% and triplet in 12% of cases. Patients had prostate radiotherapy (RxT) in 39% and RxT to metastatic bone lesion in 19%. The 8 months PSA was <0.2 ng/ml in 89% of pts. PET imaging was choline in 86%, PSMA in 14%. Median time between treatment initiation and PET imaging was 17 months and delay between PSA < 0.2 and PET imaging was 12 months (range: 0-32). A complete response was observed in 46 patients (81%). Non-complete responders (CR) were more frequently high volume disease (66%) and synchronous metastasis (100%), treated by ADT + ARPI alone (77%) or ADT + docetaxel (11%). Non CR were: 1 neuroendocrine progression, 1 lung cancer and 9 partial responders (2/9 progressed later). Conclusions: In patients with mCSPC, treated by ADT with ARPI and or docetaxel and with PSA < 0.2 ng/ml, a complete response is observed in 81 % when using PET imaging. The information given by PET imaging could be and additional surrogate measure to PSA response for intensification or de intensification the systemic and local treatment for mCSPC.