Page 122 - New epidemiological and PSMA-expression based paradigms in salivary gland tumors

120
Chapter 7
objectives, for the first time with opportunities to derive these per gland subtype and including previously unevaluable gland locations. However, this will require evaluation of potentially interfering factors such as the role of renewed PSMA expression, related to (long term) regeneration of irradiated salivary glands from stem cells.
There are other PET-tracers that accumulate in salivary gland tissues to allow visualisation, with examples including F-18-FDG (metabolic activity), F-18- choline (cell membrane synthesis) or Gallium-68-citrate (non-specific cell activity). However, PSMA-ligands have some unique advantages for this specific application. The uptake in healthy glands is very high, and together with a virtually negative background in all surrounding normal organs this simplifies signal contouring and quantification [9]. But most importantly, histopathological evaluation has proven that PSMA-ligand binds the most relevant cell population, the acinar secretory cells. The distribution of less specific tracers in normal salivary glands is currently not as well understood and is likely to involve other cell types, which brings a risk on signal contamination due to e.g. infiltration with inflammatory cells, and this makes them less suited for measuring salivary gland damage. Tracers for scintigraphy and single photon emission computed tomography (SPECT) imaging like Tc-99m-pertechnetate have a well-understood uptake mechanism, but suffer from low spatial resolution and poor quantification, and they rely on poorly reproducible stimulated salivation to yield a measurable signal. These factors have led to the identification of PSMA PET/CT as a new and promising instrument to quantify specific cell damage in salivary glands, with a better understood biological relation, better spatial resolution, and lower patient burden than currently available alternatives. In conclusion, PSMA PET/CT has the potential to further elucidate the effect of radiation to salivary and seromucous glands. These findings support further exploration of PSMA PET/CT to guide and evaluate RT, with the ultimate aim to reduce salivary gland toxicity.
Grants: Dutch Cancer Society-grant; number: 10606/2016-2. The Dutch Cancer Society had no role in decisions regarding the study design, the collection, analysis and interpretation of data, in the writing of the report, nor in the decision to submit the article for publication.