PD54-03 SPECT IMAGING WITH TC99PSMA FOR PROSTATE CANCER AND ITʼS USE IN SALVAGE SURGERY

Abstract

INTRODUCTION AND OBJECTIVE: PSMA-PET with either Ga68 or F18 has revolutionised the imaging of prostate cancer. SPECT imaging with Tc99-PSMA became commercially available later than PSMA-PET and uptake has been slower. Tc99-PSMA SPECT offers a much cheaper option than PSMA-PET, provides access to PSMA imaging where PET scanners are not available and allows PSMA radio guided surgery. Reliable identification of small or atypically placed pelvic nodes can be difficult at salvage surgery for recurrent prostate cancer. Tc99 injected into the penis is used for radio guided sentinel node dissection for penile cancer. In a similar, but chemically more challenging way, Tc99 bound to PSMA injected intravenously allows the possibility of radio-guided salvage node dissection to identify nodes with metastatic prostate cancer. The hope is that this will improve the likelihood of successful salvage nodal dissection, perhaps especially for small lesions. We describe our initial experience of this novel technique. METHODS: A prospective database of all patients undergoing Tc99 PSMA-SPECT either for staging, investigation of PSA recurrence or prior to salvage radio guided surgery at our regional hospital was kept. Munchen University PSMA I&S product was initially used. On average it required 90 minutes to create the Tc99-PSMA ligand complex. Following synthesis, a standard whole body SPECT was performed. Due to availability issues this was replaced by the Izotop PSMA ligand. This required only 20 minutes’ preparation time. There was no obvious difference in quality of imaging obtained between kits used. Stability of product and ligand to radiopharmaceutical binding has been excellent. Radio guided surgery is performed by injecting Tc99-PSMA prior to surgery. A SPECT image is then created localising the known (metastatic) disease. At surgery a hand held gamma probe is used to locate the "hot" node. After removal gamma readings are performed on the resected node to confirm presence of Tc99. RESULTS: 40 patients have undergone Tc99-PSMA SPECT imaging. Initial patients were chosen to ensure high probability of metastases to confirm suitability of SPECT imaging as a tool to image early prostate cancer metastases. SPECT imaging has identified metastatic disease including atypical sites in both patients presenting with initial disease and with PSA recurrence. 3 patients have undergone radio guided salvage surgery. The initial patient had salvage surgery with a handheld gamma probe to assist identification of the affected node on the same day as the injection. Significant background radioactivity in the urine limited the applicability of the technique. The gamma probe could however confirm the presence of Tc-PSMA in the resected node once removed from the body. The subsequent 2 patients underwent injection of Tc99-PSMA the day prior to surgery. This enabled a much more targeted approach with less background radioactivity and successful identification of the affected nodes in situ. Size of metastases was 2.5 mm, 3 and 4mm. In one patient successful node dissection for a 2.5 mm and a 4 mm seperate node was performed using radio guidance after a previous failed open salvage node dissection, which itself was post RALP with node dissection and post salvage radiotherapy. Nodes were deep to scar tissue and would not have been located without the gamma probe. CONCLUSIONS: Tc99-PSMA SPECT provides affordable PSMA imaging as an alternative to PET imaging. Both the Munich and Izotop product have been stable and produced high quality PSMA based images and allowed identification of atypical prostate cancer metastases. The Izotop product has a markedly quicker production time without loss of efficacy. In addition Tc99PSMA has allowed us to perform successful radio guided salvage node dissection in patients with small and usually atypically placed nodal metastases in recurrent disease. It is anticipated that this can also be used in men who present with small nodal oligo metastases at diagnosis and wish to have this treated. Source of Funding: The Waikato Urological research Trust