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Oligometastatic characterization usually relies only on the number of radiologically detectable localizations. A deeper understanding of the burden of disease is needed to offer the best tailored treatment. The aim of this study is to evaluate the predictive value of volumetric parameters derived from prostate-specific membrane antigen/PET (PSMA-PET) in oligorecurrent PCa patients. The study is a spin-off analysis of the ongoing phase II randomized trial RADIOSA (NCT03940235).

Patients enrolled within the RADIOSA trial who underwent PET-PSMA at first biochemical recurrence were included. Two experienced nuclear medicine physicians retrospectively reviewed PSMA-PET images blinded to all histopathological and clinical data. PSMA-PET quantitative parameters PSMA tumor volume (PSMA-TV) and Maximum standardized uptake values (SUVmax) were collected. Patients were stratified according to these two parameters using the 3rd quartile of collected values as cut-off, and PSA at the first diagnosis, PSA at the first recurrence, and time between the first diagnosis and the first recurrence were compared. The site of oligometastases (bone or lymph nodal) was evaluated to assess any relation with the considered PSMA-PET quantitative parameters.

Among the 89 enrolled patients, a total of 29 met the inclusion criteria. Mean PSA at the first diagnosis and at the first recurrence were 10.2 ng/mL and 2.8 ng/mL, respectively. Median time between first diagnosis and first recurrence was 51.43 months. A total of 16 patients had lymph nodal metastases only in PSMA-PET, while 13 had at least one bone metastases. Considered variables across the different groups are shown in Table 1 and Figure 1.    

When stratified according to the PSMA-TV (cut-off= 3 cm3), mean PSA resulted lower in the group with smaller lesions, both at the first diagnosis and at the recurrence. The time between first diagnosis and recurrence resulted longer in this group, as well, indicating a smaller burden of disease. When stratified according to the maxSUV value (cut-off= 15.7), mean PSA resulted lower in the group with lower SUVmax at the first recurrence only. Time between first diagnosis and recurrence resulted longer in this group, as well. Grouping patients according to the metastatic site, those with at least one bone metastases had mean PSA higher at the first recurrence and a shorter time between first diagnosis and recurrence. Moreover, median PSMA-TV and median SUVmax values resulted in higher in this group.

Patients with high PSMA-TV, SUVmax, and bone lesions identified at PSMA-PET are more likely to have a worse oncological outcome, confirming the importance of an optimized patient selection to identify those who could reach more favourable outcome after metastases-directed therapy. These preliminary data suggest that the definition of functional volumes in PSMA-PET (PSMA-TV) might be of clinical interest for PCa patients staging and to evaluate the response to therapy.