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

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.