18F-PSMA -1007 salivary gland dosimetry Comparison between different methods for dose calculation and assessment of inter - and intra -patient variability Daniele Pistone12 Silvano Gnesin3 Lucrezia Auditore12 Antonio Italiano24 Giuseppe

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18F-PSMA-1007 salivary gland dosimetry: Comparison between different methods for

dose calculation and assessment of inter- and intra-patient variability

Daniele Pistone1,2 §, Silvano Gnesin3 §, Lucrezia Auditore1,2, Antonio Italiano2,4, Giuseppe

Lucio Cascini5,6,7, Ernesto Amato1,2 *, Francesco Cicone5,6,7

1Department of Biomedical and Dental Sciences and of Morphologic and Functional Imaging

(BIOMORF), University of Messina, Messina, Italy

2National Institute for Nuclear Physics (INFN), Section of Catania, Catania, Italy

3Institute of Radiation Physics, Lausanne University Hospital and University of Lausanne,

Lausanne, Switzerland

4MIFT Department, University of Messina, Messina, Italy

5Department of Experimental and Clinical Medicine, “Magna Graecia” University of

Catanzaro, Catanzaro, Italy

6Neuroscience Research Centre, PET/MR Unit, “Magna Graecia” University of Catanzaro,

Catanzaro, Italy

7Nuclear Medicine Unit, University Hospital “Mater Domini”, Catanzaro, Italy

§ Equal contributors

*corresponding author: ernesto.amato@unime.it

Abstract

Objective: Dosimetry of salivary glands (SGs) is usually implemented adopting simplified

calculation approaches and approximated geometries. 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 recurrence 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 low-dose 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 (MC) simulation with GATE/GEANT4; 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 (max) 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 -14.5% and -

30.4%). Using MC, SM and EM, max were never below 25% (up to 113%). max values up to

702% were 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.

Keywords: PSMA, salivary glands, internal dosimetry, parotids, submandibular glands,

Monte Carlo, GATE, OLINDA/EXM, OpenDose, spherical model, ellipsoidal model

1. Introduction

Prostate-specific membrane antigen (PSMA) is a type II transmembrane glycoprotein having

neuropeptidase and folate hydrolase activity [Carter et al 1996, Pinto et al 1996], which is

expressed by prostate epithelial cell membrane as well as by other normal tissues, such as

salivary glands (SGs), proximal renal tubules, brain and intestine [Liu et al 1997].

Radiolabelled small molecule ligands of PSMA, such as PSMA-11, PSMA-617, PSMA-1007,

PSMA imaging and therapy (I&T), and others, are currently being used for theragnostic of

metastatic, castration-resistant prostate cancer (PC) [Eder et al 2012, Afshar-Oromieh et al

2015, Afshar-Oromieh et al 2016, Weineisen et al 2015, Giesel et al 2017, Sartor et al 2021].

The finding of a significant endothelial expression of PSMA by tumor neovasculature has

raised the interest on the use of PSMA-targeting radiolabeled probes for other malignancies in

addition to PC [Fragomeni et al 2018, Pozzessere et al 2019, Matsuda et al 2018, Tanjore

Ramanathan et al 2020]. With the increasing number of PSMA-targeted radionuclide therapies

performed worldwide, there is a growing interest on possible dose-limiting radiation-induced

toxicities to salivary and lacrimal glands [Heynickx et al 2021, Sjögreen Gleisner et al 2022].

Xerostomia is a well-documented side effect in patients receiving PSMA-targeted therapies

[Kratochwil et al 2016, Kratochwil et al 2017, Taïeb et al 2018, Heynickx et al 2021], and

some methods for the protection of salivary glands are being evaluated clinically [Paganelli et

al 2020, Belli et al 2020]. The most popular therapeutic PSMA ligand is the PSMA-617 [Sartor

et al 2021]. The theragnostic concept implies that the biodistribution of a therapeutic

radiopharmaceutical is reliably predicted by the preliminary use of a diagnostic companion.

Within a few hours after administration, this paradigm is valid for the fluorinated compound

18F-PSMA-1007, owing to its structural similarity to PSMA-617 [Giesel et al 2016]. Therefore,

in addition to its use in staging and detection of PC recurrence, 18F-PSMA-1007 can be

considered a well-suited diagnostic counterpart of PSMA-617, which may help inform the

selection of patients referred for PSMA-617 therapy.

The SGs include three pairs of glands, the major being the parotids and the submandibular

glands. However, standard dosimetry software only recently included this tissue as a

source/target region with a realistic geometry that includes separate bilateral parotids and

submandibular components, such as available in the ICRP-110 phantom [ICRP 2009] with

ICRP-89 organ masses [ICRP 2002]. Furthermore, in most previously published dosimetry

studies, absorbed dose (AD) estimates for SGs were based on quantitative imaging employing

S-values that consider only the AD to single or multiple spherical structures without

considering patient-specific gland composition, geometry and mass [Afshar-Oromieh et al

2016, Kratochwil et al 2016, Kratochwil et al 2017, Giesel et al 2017, Kratochwil et al 2018,

Rosar et al 2022].

The aim of the present work was twofold. First, to compare different methods for patient-

specific dosimetry of the SGs in patients undergoing 18F-PSMA-1007 positron-emission

tomography/computed tomography (PET/CT). Second, to assess the AD variation across

different patients and single SG components. The methods adopted for AD calculations were:

i) the direct Monte Carlo (MC) simulation with GATE/GEANT4 [Jan et al 2004, Sarrut et al

2014]; ii) the spherical model of OLINDA/EXM 2.1 [Stabin and Siegel 2018] using either

ICRP-89 standard masses [ICRP 2002] or patient-specific gland masses; iii) the ellipsoidal

model developed by Amato et al. [Amato et al 2014]; iv) the organ-level MIRD formalism with

OLINDA S-factors and OpenDose S-factors [Chauvin et al 2020].

2. Materials and Methods

2.1. Patient enrollment, PET/CT calibration, acquisition and reconstruction

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18F-PSMA-1007salivaryglanddosimetry:Comparisonbetweendifferentmethodsfordosecalculationandassessmentofinter-andintra-patientvariabilityDanielePistone1,2§,SilvanoGnesin3§,LucreziaAuditore1,2,AntonioItaliano2,4,GiuseppeLucioCascini5,6,7,ErnestoAmato1,2*,FrancescoCicone5,6,71DepartmentofBiomedicalandDe...

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18F-PSMA -1007 salivary gland dosimetry Comparison between different methods for dose calculation and assessment of inter - and intra -patient variability Daniele Pistone12 Silvano Gnesin3 Lucrezia Auditore12 Antonio Italiano24 Giuseppe

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