Contribution of 68Ga-PSMA PET/CT to targeting volume delineation of prostate cancer treated with conformal radiation therapy: Which SUV threshold is appropriate?

Introduction: Prostate-specific membrane antigen (PSMA) has been demonstrated as a promising tool for specific imaging of prostate cancer (PCa) via positron emission tomography-computed tomography (PET/CT) scanning. Radiation treatment planning (RTP) based on 68Ga-PSMA PET/CT scanning can also lead to some decision modifications.  The specific goal of this comparative study is to show how 68Ga-PSMA PET/CT images can influence the target volume delineation (TVD) and normal tissue radiation dose for PCa RTP, and to compare gross tumor volumes (GTVs) delineated using various strategies for 68Ga-PSMA PET-based image segmentation techniques. Methods: This study consisted of eleven 68Ga-PSMA PET/CT images related to patients affected with locally advanced PCa. Four strategies also included manual segmentation techniques, a 2.5 standardized uptake value (SUV) cutoff (SUV=2.5), as well as a fixed threshold of 40% and 50% of the maximum signal intensity (SUV=%40 SUVmax and SUV=%50 SUVmax) for 68Ga-PSMA PET-based segmentation techniques to delineate GTVPET. Two treatment planning were accordingly generated for each patient based on manual GTVPET and CT-only. Results: The GTV was statistically and significantly smaller for PET/CT-derived volumes (9.39 vs. 77.98 cm3 for CT alone) (p<0.002). There was no significant difference in volumes of GTV2.5 and GTV40% with GTVman (p=0.11) although we observed a significant difference in volumes of GTV50% with GTVman (p=0.02). Mean bladder dose (MBD), V50 of rectum, and mean femoral dose (MFD) for PET/CT plans were significantly lower than CT-only (22.36 vs. 46.55 Gy; p=0.004), (33% vs. 67.82%; p=0.000), and (28.01 vs. 37.12Gy; p=0.013); respectively. Conclusion: The contribution of hybrid modalities of PSMA-PET/CT can be useful for detailed target volume planning and reduce radiation exposure to organs at risk. Using molecular images in RTP also demonstrates the biological volume of GTV so that it will not be left out of the field to cause recurrent tumor.