Reduction of mandibular doses using noncoplanar VMAT radiotherapy in the adjuvant management of locally advanced oral cavity squamous cell cancers

This treatment planning study aims to evaluate the feasibility of ncVMAT for patients receiving OCSCC radiotherapy and whether ncVMAT will help to achieve optimal mandible dose constraints. About 10 cases of ORN were identified in patients with OCSCC who received postoperative (C)RT between November 2014 and November 2017. Scoping for optimal planning technique was performed by comparing original plans with 5 alternative solutions. A noncoplanar solution with one full arc with couch at zero and 2 half-arcs with the couch rotated 45° for each was deemed optimal (minimizing mandibular dose whilst maintaining parotid sparing and avoiding excess dose to optics). Plans were re-optimized using this technique, attempting to improve mandibular V50Gy and D30%. Mean improvement in mandibular doses were most pronounced in Dmean and V50Gy. Applying optimal ncVMAT technique led to reduced mandibular V50Gy, maintained parotid sparing and improved PTV coverage. Mean reduction in mandibular V50 was modest (5.6%), 3 cases (all with centralized high dose PTV with floor of mouth/oral tongue primaries) showed dramatic improvements in V50Gy with reductions of 13.6%, 12.2% and 23.3%. Mandibular sparing can be achieved by ncVMAT without increasing parotid doses but with marginal increases to optic structures. This improvement may confer to reduced frequency of ORN if examined in prospective randomized trial. Dosimetric benefit seems to be greatest in those with central floor of mouth or oral tongue primary cancers.