Use of whole-body MRI to evaluate residual tumour burden after initial systemic therapy in polymetastatic hormone-sensitive prostate cancer

OBJECTIVES: Treatment options for metastatic hormone-sensitive prostate cancer (mHSPC) have expanded, with evidence supporting prostate radiotherapy in low-volume disease. However, evidence for consolidative radiotherapy to residual sites in patients rendered oligometastatic after systemic therapy is lacking. Advanced imaging with WB-MRI and PSMA PET/CT enables more accurate assessment of tumour burden and response. METHODS: This feasibility study assessed response to initial systemic treatment with an ARPI or docetaxel in 29 patients with polymetastatic (>5 metastases) HSPC. PSA and WB-MRI response using MET-RADS-P criteria were recorded. RESULTS: 20/29 (69.0.%) of patients were down-risked to oligometastatic disease (≤5 sites) after a median of 5.5 (IQR 5-5.8) months, identified on WB-MRI. Median PSA was 0.2 ng/ml (IQR 0.1-0.7) in down-risked patients versus 1.5 ng/ml (IQR 1.1-84) in those not down-risked. Residual intraprostatic disease was seen in 55% (11/20) of down-risked patients. CONCLUSIONS: Most patients were down-risked on WB-MRI, supporting the existence of an induced oligometastatic state and providing rationale for the STAR TRAP randomised trial (ISRCTN16448082), which will assess the role of consolidative radiotherapy in this setting. ADVANCES IN KNOWLEDGE: WB-MRI correlates with PSA response and shows potential as an imaging biomarker in mHSPC in this small feasibility study.