Prostate-specific membrane antigen positron emission tomography/computed tomography as a potential tool to assess and guide salivary gland irradiation

Dose response modelling of secretory cell loss in salivary glands using PSMA PET

BACKGROUND AND PURPOSE

Xerostomia remains a common side effect of radiotherapy (RT) for patients with head and neck (H&N) cancer despite advancements in treatment planning and delivery. Secretory salivary gland cells express the prostate-specific membrane antigen (PSMA), and show significant uptake on PET scans using ⁶⁸Ga/¹⁸F-PSMA-ligands. We aimed to objectively quantify the dose-response of salivary glands to RT using PSMA PET.

METHODS AND MATERIALS

28H&N cancer patients received RT with 70 Gy in 35 fractions over 7 weeks. PSMA PET/CT was acquired at baseline (BL), during treatment (DT) and at 1-&6-months post-treatment (PT_1M/PT_6M). Dose, BL- PT_1M- and PT_6M-SUV were extracted for every voxel inside each parotid (PG) and submandibular (SMG) gland. The PT_1M/6M data was analysed using a generalised linear mixed effects model.Patient-reported xerostomia and DT-PSMA loss was also analysed.

RESULTS

Dose had a relative effect on BL SUV. For a population average gland (BL-SUV of 10), every 1 Gy increment, decreased the PT_1M/PT_6M-SUV by 1.6 %/1.6 % for PGs and by 0.9 %/1.8 % for SMGs. TD₅₀ of the population curves was 26.5/31.3 Gy for PGs, and 22.9/27.8 Gy for SMGs at PT_1M /PT_6M. PSMA loss correlated well with patient-reported xerostomia at DT/PT_1M (Spearman's ρ = -0.64, -0.50).

CONCLUSION

A strong relationship was demonstrated between radiation dose and loss of secretory cells in salivary glands derived using PSMA PET/CT. The population curve could potentially be used as a dose planning objective, by maximising the predicted post-treatment SUV. BL scans could be used to further tailor this to individual patients.

Discovery of new salivary gland - A substantial histological analysis

Regardless of the COVID-19 pandemic in 2020, discovering a new salivary gland turned into all the rage among the medical fraternity. The significance of the disclosure has been correlated with its clinical relevance in radiotherapy of oropharyngeal carcinoma. However, there are views against this new revelation, owing to the lack of substantial evidence. We have endeavoured to illuminate Tubarial glands with potential shreds of evidence.

The increasing potential of nuclear medicine imaging for the evaluation and reduction of normal tissue toxicity from radiation treatments

Radiation therapy is an effective treatment modality for a variety of cancers. Despite several advances in delivery techniques, its main drawback remains the deposition of dose in normal tissues which can result in toxicity. Common practices of evaluating toxicity, using questionnaires and grading systems, provide little underlying information beyond subjective scores, and this can limit further optimization of treatment strategies. Nuclear medicine imaging techniques can be utilised to directly measure regional baseline function and function loss from internal/external radiation therapy within normal tissues in an in vivo setting with high spatial resolution. This can be correlated with dose delivered by radiotherapy techniques to establish objective dose-effect relationships, and can also be used in the treatment planning step to spare normal tissues more efficiently. Toxicity in radionuclide therapy typically occurs due to undesired off-target uptake in normal tissues. Molecular imaging using diagnostic analogues of therapeutic radionuclides can be used to test various interventional protective strategies that can potentially reduce this normal tissue uptake without compromising tumour uptake. We provide an overview of the existing literature on these applications of nuclear medicine imaging in diverse normal tissue types utilising various tracers, and discuss its future potential.