Page 121 - New epidemiological and PSMA-expression based paradigms in salivary gland tumors
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PSMA PET/CT: as a tool to assess and guide salivary gland irradiation
that is sufficient for interpretation of intra-gland variations. This finding could be elucidated further with prospective research that includes voxel-based evaluations of the dose-effect relation.
In case B, a relationship between clinical symptoms and the distribution of damage in specific gland locations on PSMA PET/CT is suggested. This helps to clarify previously unexplained toxicity in a patient where the major salivary glands were spared and indeed appear to be otherwise unaffected.
The remarkable findings in case C suggest functional compensation by hypertrophy of minor glands of minor glands in the mucosa of the lips on both sides, to explain the residual salivation capacity after complete loss of PSMA-ligand binding in major salivary glands after RT. This finding suggests the relevance of gland locations that are currently not considered in RT planning, especially in case of damage to the major salivary glands, and provides more insight in the functional compensation capacity of minor mucosal gland locations.
The spatial resolution of PET images (generally in the order of 2-5mm FWHM in
the head-neck area) allows evaluation of inhomogeneous loss of ligand binding 7 within larger salivary glands. In combination with the high contrast resolution
of PET, more and smaller gland locations can be evaluated than with common
methods such as planar scintigraphy or functional magnetic resonance imaging
(fMRI) [9]. As a result, PET can for the first time allow quantitative assessment
of glandular damage using gland-based or voxel-based approaches. As stated
before, the expression of PSMA is reported to be located in the secretory glandular
(acinar) cells [8]. Although the function of the epitope in the salivary glands is
unknown, the level of PSMA-expression per acinar gland is thought to be stable.
Therefore, the observed reduction in local PSMA-ligand binding is thought
to indicate loss of secretory cells in the affected glands, but this remains to be
confirmed in upcoming studies [10]. This method for quantification of cell loss
would potentially be more objective and specific than patient-reported outcomes,
e.g. independent from factors such as patient experience, medication use, examiner
experience etc., and more convenient and potentially better reproducible than
salivary gland cannulation or scintigraphy during stimulated salivation. If the
suggested relationship can be confirmed, PSMA PET/CT may contribute to the development of new and more accurate dose-effect relations and RT planning
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