Comparative Analysis of ⁶⁸Ga-Prostate-Specific Membrane Antigen Positron Emission Tomography/Computed Tomography and Multiparametric Magnetic Resonance Imaging for Gross Tumor Volume Delineation in Radiation Therapy Planning of Prostate Ca

<b>Purpose : </b>Magnetic resonance imaging (MRI) with multiparametric assessment is a cornerstone in radiation therapy planning for primary prostate cancer (PCa), offering the potential for focal dose escalation to dominant intraprostatic lesions to enhance PCa management. The prostate-specific membrane antigen (PSMA), which is frequently overexpressed in PCa, has led to the development of ⁶⁸Ga-labeled PSMA inhibitors for positron emission tomography (PET)/computed tomography (CT), showing promising results in PCa detection. This study aimed to conduct a comparative analysis of ⁶⁸Ga-PSMA PET/CT and multiparametric MRI (mpMRI) in delineating the gross tumor volume (GTV) in PCa. <b>Methods and Materials :</b> A retrospective analysis was performed on 25 PCa patients who underwent ⁶⁸Ga-PSMA PET/CT and mpMRI at 2 distinct centers. The GTVs were delineated on MRI (GTV-MRI) by 2 nuclear medicine physicians and 1 radiation oncologist independently, while the GTVs based on PET/CT (GTV-PET) were outlined by 2 nuclear medicine physicians. The laterality (left, right, and bilateral) prostate lobes on mpMRI and PET/CT was assessed. To account for registration uncertainties, both GTV-PET and GTV-MRI were expanded isotropically by 5 mm to form the planning target volume (PTV), and the overlap between specific PTVs and their corresponding GTVs was quantified. <b>Results : </b>The average ± SD GTV-MRI and GTV-PET were 5.468 ± 11.6 cm³ and 11.136 ± 14.3 cm³, respectively, with the GTV-PET being significantly larger than the GTV-MRI (<i>P</i> = .003). GTV-MRI exhibited an intersection with GTV-PET of 3.5 ± 6.0 cm³. The PTV derived from PSMA PET/CT encompassed 62% ± 27% of the GTV-MRI, with 44.4% of patients having the PTV covering 100% of the GTV-MRI. Conversely, the PTV based on GTV-MRI covered 50% ± 31% (mean ± SD) of the GTV-PET. <b>Conclusions </b>: ⁶⁸Ga-PSMA PET/CT and mpMRI demonstrated consistent outcomes in 47% of patients (40%-54% of lesions). Notably, the GTV-PET was larger than the GTV-MRI, indicating a potential role for ⁶⁸Ga-PSMA PET/CT in radiation therapy planning for targeted radiation delivery to PCa.