Role of ⁶⁸Ga-prostate-specific membrane antigen positron emission tomography/computed tomography for evaluation of response to docetaxel chemotherapy in metastatic castration-resistant prostate cancer

Glu-NH-CO-NH-Lys-(Ahx)-(Ga-68(HBED-CC)), abbreviated as ⁶⁸Ga-prostate-specific membrane antigen (PSMA), is a novel radiotracer undergoing evaluation for positron emission tomography/computed tomography (PET/CT) imaging of prostate carcinoma. Its major advantage is sensitive detection of lesions even at low prostate-specific antigen (PSA). The objective of this prospective study was to evaluate response to docetaxel chemotherapy (DCT) in metastatic castration-resistant prostate cancer (mCRPC) using ⁶⁸Ga-PSMA PET/CT. A total of 17 male patients (mean age - 62 ± 20) with mCRPC and rising PSA levels that had been planned for DCT were evaluated prospectively. All patients had undergone bilateral orchidectomy. The median PSA level was 88, range 4-1100 ng/mL, and mean Gleason score was 8.0 ± 1. Baseline ⁶⁸Ga-PSMA PET/CT was done in all patients followed by an interval PET/CT after 4 cycles DCT. Based on-response was graded as partial response (PR), stable disease (SD), and progressive disease (PD). In all cases, response was correlated to serum PSA levels. Abnormal tracer accumulation suggestive of metastases was seen in retroperitoneal lymph nodes in 5 (29%) patients, pelvic lymph nodes in 9 (53%), skeletal sites in 11 (65%), internal mammary and supraclavicular lymph nodes in 3 (18%), and liver in 1 (5%), suggesting PSMA expression in baseline scan. PSMA expression was also noted in the primary in prostate in 5 (29%) patients. After DCT, 4 patients had PR (23%), 6 had SD (35%), and 7 patients showed PD (42%) in posttherapy ⁶⁸Ga-PSMA scan. In all with PD, PSA levels were also found to rise with a new lesion on the Ga-68 PSMA PET/CT (28% rise in mean sPSA level). PR patients showed decline in PSA levels (up to 80% decreased in mean sPSA level). Thus, ⁶⁸Ga-PSMA PET/CT is a useful modality for evaluation of response to DCT in mCRPC patients, further these patients would also be suitable candidates for ¹⁷⁷Lu-PSMA therapy. (author)