The Effect of ADT on PSMA-PET.

Positron emission tomography has been commonly and successfully used, in combination with CT and MRI devices, in the staging of intermediate or high risk prostate cancer. Proper staging is essential to guide the treatment options, which usually are radical prostatectomy or radiotherapy in localized prostate cancer or hormonal treatment in patients with metastasized disease, patients with hormonal relapse after radical radiotherapy or as an adjuvant treatment together with radiotherapy. The use of PET imaging increases the sensitivity in the evaluation of lymph node involvement, as almost 80% of metastatic lymph nodes in prostate cancer are smaller than the threshold size usually measured with CT or MRI. Nowadays new specific receptor targeted PET tracers in prostate cancer imaging has been introduced. One of the most used is 68Ga-PSMA that evaluates the expression of prostate-specific membrane antigen. This tracer has been rapidly taken into account for its better sensitivity and specificity in prostate cancer staging compared to the lipid metabolism tracers, like 11C/18F labeled fluorocholine or 11C-acetate. In the recent literature it has been demonstrated for the first time on humans that PSMA expression, imaged with 68Ga-PSMA-11-PET, has increased in a patient with metastatic prostate cancer after androgen deprivation therapy (ADT). These findings suggest that the use of hormonal therapy can affect the expression of PSMA and our hypothesis is that ADT therapy could increase the sensitivity of 68Ga-PSMA PET to detect nodal or distant metastasis in patients with prostate cancer. This prospective study consists in two phases. In phase A, 5 patients with newly diagnosed high risk prostate cancer with PSMA-positive nodal or distant metastasis screened by 68Ga-PSMA-11 PET/MRI, are given androgen deprivation therapy (ADT) with GNRH antagonist. After ADT therapy initiation, 68Ga-PSMA-11 PET/MRI is repeated at 7, 14 and 30 days to determine the highest PSMA expression based on SUVmax measurement. In phase B, 20 high risk prostate cancer patients determined to undergo radical prostatectomy are screened with 68Ga-PSMA-11 PET/MRI, and then given GNRH antagonist therapy. 68Ga-PSMA-11 PET/MRI is repeated at the time of maximum PSMA expression based on phase A results. The patients then undergo radical prostatectomy and lymphadenectomy and imaging findings are matched with histological data.