Correlation between molecular tumor volume evaluated with 68 Ga-PSMA PET/CT and prostatic specific antigen levels.

OBJECTIVE: To investigate the association between prostatic-specific antigen (PSA) levels and molecular tumor volume (MTV) measured in the 68 Ga-PSMA PET/CT, both done in a short period of time, in prostate cancer patients with biochemical failure.

METHODS: Eighty-four patients who underwent 68 Ga-PSMA PET/CT and measurement of PSA levels in the same week (trigger-PSA) were studied in this retrospective analysis. MTV was calculated from the sum of the metastatic lesions. To determine the association between trigger-PSA level and PET/CT findings, Spearman rank correlation was used.

RESULTS: The median MTV of metastatic bone disease (mBD) was significantly higher than in metastatic lymph-nodes (mLN) (139.5 versus 17.7; P<.05). Disease was limited to the prostate in 8 patients (9.5%), mLN in 21 patients (25%), mBD in 32 patients (38.1%) and the 3 sites (prostate, mLN, and mBD) in 17 patients (20.2%). In 6 patients (6.14%), 68 Ga-PSMA-PET/CT was not capable of detecting disease. The median trigger-PSA levels of patients with disease limited to the prostate (2.8ng/mL), mLN (6.8ng/mL), and for mBD (16.8ng/mL) was statically significant (P<.05). Positive patients had a mean trigger-PSA of 4.3ng/mL vs 1.5ng/mL in negative patients (P<.05). We established 3 threshold-points for trigger-PSA level detection rate:≤1ng/mL (47.3%), 1-4ng/mL (68.4%) and≥4ng/mL (96.7%). When trigger-PSA exceeded 4ng/mL, the MTV was higher (P<.001).

CONCLUSION: The correlation of MTV with trigger-PSA is demonstrated, which may have an impact on management. However, trigger-PSA levels were not capable of distinguishing between localized or distant disease. An accurate detection of disease can lead to a better therapeutic strategy.