18F-PSMA-1007 salivary gland dosimetry: comparison between different methods for dose calculation and assessment of inter- and intra-patient variability

Objective: Simplified calculation approaches and geometries are usually adopted for salivary glands (SGs) dosimetry. Our aims were (i) to compare different dosimetry methods to calculate SGs absorbed doses (ADs) following [F-18]-PSMA-1007 injection, and (ii) to assess the AD variation across patients and single SG components. Approach: Five patients with prostate cancer underwent sequential positron-emission tomography/computed tomography (PET/CT) acquisitions of the head and neck, 0.5, 2 and 4 h after [F-18]-PSMA-1007 injection. Parotid and submandibular glands were segmented on CT to derive SGs volumes and masses, while PET images were used to derive Time-Integrated Activity Coefficients. Average ADs to single SG components or total SG (tSG) were calculated with the following methods: (i) direct Monte Carlo simulation with GATE/GEANT4 considering radioactivity in the entire PET/CT field-of-view (MC) or in the SGs only (MCsgo); (ii) spherical model (SM) of OLINDA/EXM 2.1, adopting either patient-specific or standard ICRP89 organ masses (SMstd); (iii) ellipsoidal model (EM); (iv) MIRD approach with organ S-factors from OLINDA/EXM 2.1 and OpenDose collaboration, with or without contribution from cross irradiation originating outside the SGs. The maximum percent AD difference across SG components (delta (max)) and across patients (Delta(max)) were calculated. Main results: Compared to MC, ADs to single SG components were significantly underestimated by all methods (average relative differences ranging between -11.9% and -30.5%). delta (max) values were never below 25%. The highest delta (max) (=702%) was obtained with SMstd. Concerning tSG, results within 10% of the MC were obtained only if cross-irradiation from the remainder of the body or from the remainder of the head was accounted for. The Delta(max) ranged between 58% and 78% across patients. Significance: Simple geometrical models for SG dosimetry considerably underestimated ADs compared to MC, particularly if neglecting cross-irradiation from neighboring regions. Specific masses of single SG components should always be considered given their large intra- and inter-patient variability.