Dosimetric Effect of Sparing the Primary Site for Oropharyngeal Squamous Cell Carcinoma After Transoral Laser Microsurgery

Abstract

Purpose/Objective(s): To quantify the effect of sparing the primary site after transoral laser microsurgery (TLM) for oropharyngeal squamous cell carcinoma (OPSCC). Materials/Methods: We reviewed five patients who were treated with TLM and post-operative radiation. All patients were staged T1 of the base of tongue with 1 patient being N1, 3 being N2 and 1 N3. 1 patient’s tumor extended bilaterally, 3 were lateralized to the right, and 1 lateralized to the left. All patients were treated using 9-field, step-and-shoot, intensity modulated radiation therapy (IMRT) to the primary site and at risk neck(s). The prescribed dose to the high risk (HR) and standard risk (SR) planning target volumes (PTVs) was 60 Gy and 46 Gy at 2 Gy/fx, respectively. We re-planned these patients with the purpose of sparing the primary site. PTV-SR and PTV-HR were adjusted to exclude the primary site from the treatment volumes. Re-plans were performed such that the primary site was maximally spared while maintaining similar PTV coverage and normal tissue avoidance as the initial, standard treated plan. Results: Dosimetric results are shown in the table. Both plans were able to adequately (at least 95% of the target volume receives 100% of the prescribed dose) cover the PTVs and similarly spare normal tissues (spinal cord, brainstem, contralateral parotid, ipsilateral cochlea, and larynx). The mean dose to the primary site was reduced by 33% (19 Gy) and the mean dose to the oral cavity was reduced by 50% (24 Gy). The conformity index was worse when the primary site was spared. Conclusions: Reducing the mean dose delivered to oropharynx after TLM by approximately 20 Gy is feasible using 9-field, step and shoot, IMRT. Sparing the primary site post TLM may yield reductions in toxicity.