Comparison of Ga-68 PSMA PET/CT and ^99mTC- PSMA SPECT/CT in the patients with diagnosed adenocarcinoma prostate with low S. PSA level: Early experience

Introduction: Ga68-PSMA PET/CT is considered a well establised technique in carcinoma prostate for detection of the primary tumor, metastases and recurrence. However, it is not universally available in low turn over departments. ^99mTc-PSMA SPECT/CT may be a cost-effective and logistically simple alternative for it. The study was aimed to compare these two techniques in patients with primary, metastatic and recurrent carcinoma prostate. Materials and Methods: Fifteen patients with a proven carcinoma prostate underwent both ^99mTc-PSMA-T4 SPECT/CT (POLATOM, Poland) and Ga-68 PSMA PET/CT (PSMA-11, ABX, Germany) within a week gap. The study was approved by ethical committee of our institute. We excluded patients with very high S. PSA (> 50 ng/dl). Ga-68 PSMA PET/CT was done 60 minutes after IV injection of 5 mCi of Ga-68 labelled PSMA. 20 mCi of ^99mTc-PSMA-T4 injected and whole-body imaging with SPECT/CT (thorax, abdomen and pelvis) were performed on Infinia Hawkeye 4 gamma camera. All PET and SPECT images were interpreted independently by two observers and the results recorded.

Results: The mean age of the patients was 67.3 years (46-82 years). Radical prostatectomy, orchidectomy, and chemoradiotherapy were done in 2, 3, and 5 patients respectively. Abnormal areas of the tracer uptake noted in all the patients. Four patients underwent scanning for primary staging and rest for recurrence. Mean PSA level was 8.1 ng/ml (0.08- 38) and Gleason score (6-10). 12/15 patients had S. PSA level < 10 ng/ml. The total number of detected lesions were 29 by PSMA PET/CT and 28 by SPECT/CT. The prostatic lesion, pelvic lymph nodes, retroperitoneal lymph nodes, bony and other lesions were noted (10 vs. 10, 6 vs. 5, 2 vs. 2, 6 vs. 6, and 5 vs. 5 in PET vs.SPECT). No patients with positive SPECT/CT had negative PET. Only one patient had a discordant pelvic lymph node, missed by SPECT. The smallest node detected by both radiotracer was ~1.0 x 1.5 cm in size (with SUVmax-5.5 on PSMA PET/CT) with S. PSA level 11 ng/ml. S PSA level of patient with missed lymph node on SPECT was 0.36 ng/ml with size of ~ 8 mm (with SUVmax-5.1 on PSMA PET/CT), however prostatic primary in this patient was seen on both scan and the patient was treatment naive.

Conclusions: ^99mTc-PSMA SPECT/CT could be acceptable and logistically simple alternative to Ga-68 PSMA PET/CT in patients of carcinoma prostate even with low S.PSA level. No significant difference was noted between two modalities for staging/ restaging in our early experience.