Phase 1 RIALTO Trial: Radiotherapy for Metastatic Prostate Cancer With Incomplete Hormone Response

The RIALTO trial challenges decades of oncology dogma that treating the prostate becomes irrelevant once cancer spreads. For patients with de novo metastasized prostate cancer who show incomplete PSA-response (PSA >0.2 ng/ml after six months of combined ADT+ARPI), the prostate may function as a persistent tumor reservoir continuously seeding metastases. Local radiotherapy to the prostate could interrupt this cycle, reducing overall tumor burden and improving systemic control. This trial targets the troubling subgroup where maximal systemic therapy fails to achieve complete control, approximately 20-30% of metastatic patients who harbor hormone-resistant clones with persistent metabolic activity and higher progression risk.

The oligoresidual concept is critical here. Patients with fewer than 16 bone or lymph node metastases without visceral involvement represent a therapeutic window where local ablative therapy might meaningfully alter disease trajectory. Previous STOMP and ORIOLE trials demonstrated that treating oligometastases delays progression and extends the hormone-sensitive period. RIALTO extends this by asking what happens if we also treat the source and a bigger number of metastases. The trial leverages PSMA-PET/CT’s unprecedented sensitivity to detect small bone lesions, sub-CT-resolution lymph nodes, and exact tumor locations, enabling image-guided radiotherapy (SBRT or IMRT) to the prostate and every PET-positive metastasis rather than indiscriminate radiation.

RIALTO’s primary question is precise: does radiotherapy to the prostate and PSMA-PET-positive metastases lead to radiological response one year after treatment, assessed via PSMA-PET/CT comparing pre- and post-radiotherapy scans? This measures actual tumor disappearance or metabolic shutdown, not just PSA decline. Quality of life assessments at every 3-month visit for 36 months determine whether aggressive local therapy provides meaningful patient benefit beyond radiological endpoints. The trial will recruit 27 patients, representing a pilot/feasibility study that would justify larger international trials if successful.

If validated, this establishes a new paradigm where metastatic prostate cancer with incomplete hormone response requires both maximal systemic therapy and aggressive local ablation. Implications include new standard of care for this subgroup, earlier radiotherapy integration in metastatic settings, PSA >0.2 ng/ml as trigger for intensified local therapy, mandatory PSMA-PET-guided treatment, and potential downstream survival benefits.

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