Dosimetric benefits of customised mouth-bite for head neck cancer patients undergoing modern proton therapy – An audit

Abstract

Background and aims

Proton therapy (PRT) for Head Neck Cancer (HNC), in view of the Bragg peak, spares critical structures like oral mucosa better than IMRT. In PRT, mouth-bites, besides immobilising and separating mucosal surfaces, may also negate the end-of-range effect. We retrospectively analysed the details and dosimetric impact of mouth-bites in PRT for HNC.

Materials and methods

The data of consecutive HNC patients treated with IMPT from May 2020 to August 2022 were studied retrospectively. Details of the mouth-bite used, compliance and resultant mucosal separation were noted. Further analysis, restricted to previously unirradiated patients, comprised volumetric dosimetric data pertaining to the mouth-bite and distal mucosal surfaces. High LET zones, corresponding to 6–12 keV/micron, for mouth-bite doses above 30 Gy, were recalculated from existing plans.

Results

A mouth-bite was used in 69 of 80 consecutively treated patients, ranging from 8 to 42 mm in thickness, and 12 to 52 mm in the resultant mucosal sparing. In 42 patients in whom the mouth-bite V 32 Gy was > 0, median Dmean, absolute V32, V39, V50 and V60 GyE (Gray Equivalent) of the mouth bite was 35.65 GyE (Range: 2.65 – 60 GyE), 10 cc (Range: 0.1 – 32 cc), 7.6 cc (Range: 0.1 – 30.8 cc), 5.7 cc (Range: 0.2 – 29.2 cc) and 1.45 cc (Range: 0.2 – 18.1 cc) respectively, all significantly more than the spared adjacent mucosal surface. In absence of a mouth-bite, the spared mucosa would have at least partially received the high dose received by the mouth-bite. High LET zones were noted in 12 of 48 mouth-bites.

Conclusion

In PRT for HNC, mouth-bites play a vital role in improving the sparing of mucosa outside the target.