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

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 [18F]-PSMA-1007 injection, and ii) to assess the AD
variation across patients and single SG components.
Approach: Five patients with prostate cancer underwent sequential PET/CT acquisitions of
the head and neck, 0.5, 2 and 4 hours after [18F]-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 (δmax) and across patients (∆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%). δmax values were never below 25%. The highest δmax (=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 ∆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.