Ga-PSMA PET/CT compared with MRI/CT and diffusion-weighted MRI for primary lymph node staging prior to definitive radiotherapy in prostate cancer: a prospective diagnostic test accuracy study.

Background: The aim was to compare the diagnostic accuracy of Ga-PSMA PET/CT with conventional cross-sectional imaging and diffusion-weighted MRI (DW-MRI) for detecting lymph node metastasis (LNM) to stage prostate cancer patients. Twenty consecutive, newly- diagnosed prostate cancer patients were prospectively enrolled and underwent Ga-PSMA-11 PET/CT, anatomical MRI or contrast-enhanced CT, and DW-MRI prior to laparoscopic, template-based, extended lymph node dissection. Histopathological findings served as the reference test.

Results: Histopathology showed LNM in 13 of 20 patients (19 high-risk, 1 intermediate risk). Five patients had metastasis-suspected lymph nodes on Ga-PSMA PET/CT. Patient-based analysis showed that the sensitivity and specificity for detecting LNM were 39% and 100% with Ga-PSMA PET/CT, 8% and 100% with MRI/CT, and 36% and 83% with DW-MRI, respectively. The positive and negative predictive values were 100% and 49% with Ga-PSMA PET/C, 100% and 37% with MRI/CT, and 80% and 42% with DW-MRI. Of 573 dissected lymph nodes, 33 were LNM from 26 regions. True-positive LNM on Ga-PSMA PET/CT was 9-11 mm in diameter, whereas false-negative LNM had a median diameter of 4 mm, with only 3 of 30 lymph nodes being larger than 10 mm. LNM were positive for PSMA by immunostaining.

Conclusions: The sensitivity of Ga-PSMA PET/CT was notably better than that of MRI/CT and comparable to that of DW-MRI. Some false positive findings with DW-MRI reduced its specificity and positive predictive value compared with those of Ga-PSMA PET/CT and MRI/CT.