Dosimetric Comparison of Intensity Modulated Radiotherapy (IMRT) and Intensity Modulated Proton Therapy (IMPT) for Novel Oral Tongue Avoidance Concept in Low-Risk Squamous Cell Carcinoma of the Oral Tongue

Abstract

<h3>Purpose/Objective(s)</h3> A subset of favorable oral tongue squamous cell carcinoma patients who undergo adequate surgical resection are at low risk of local recurrence to the primary site. Since irradiation of the primary site is the major cause of treatment morbidity during adjuvant radiotherapy, avoidance of this target while still treating the at-risk lymph nodes including the intervening lymphatics between the tongue and neck is an attractive potential treatment strategy. Prior to initiation of an investigator-initiated trial, we sought to quantify the dosimetric advantage of this strategy, hypothesizing intensity modulated proton therapy (IMPT) may further reduce dose to organs at risk (OAR) when avoiding the oral tongue compared to intensity modulated radiation therapy (IMRT). <h3>Materials/Methods</h3> Five consecutive patients with oral tongue cancer treated with postoperative radiation therapy from 8/2020 to 9/2021 were retrospectively reviewed. Novel Clinical Target Volume (CTV) contours were generated that excluded the oral tongue while maintaining coverage of at-risk lymph nodes. Comparison IMRT (X) and IMPT (PBT) plans were generated using both standard treatment volumes (Control) and oral tongue avoidance volumes (Study) (n=4 plans per patient). Dosimetric variables for critical OARs were compared using the paired t-test. <h3>Results</h3> Patients had T1-2 and N0-N2b disease with complete margin-negative partial or hemiglossectomy and bilateral neck dissections. The prescribed dose was 60 Gy in 30 fractions. D95% CTV coverage was similar between X and PBT plans for both Control and Study target volumes. Comparing Control to Study plans, both X (58.9 Gy vs. 38.3 Gy, p=0.007) and PBT (60.2 Gy vs. 26.1 Gy, p<0.001) decreased the mean dose to the oral cavity (OC). Similar results were demonstrated for mean pharyngeal constrictor dose with X (47.7 Gy vs. 44.6 Gy, p=0.003) and PBT (45.0 Gy vs. 34.9 Gy, p=0.001). There was no difference between Control and Study plans for larynx (p>0.19) or parotid (p>0.11) mean dose, or mandible max dose (p>0.59). Comparing the Study plans alone, PBT resulted in a significant reduction in mean OC dose (38.3 Gy vs. 26.1 Gy, p=0.007) and reduction in mean parotid dose (23.3 Gy vs.19.3 Gy, p=0.03) compared to X. For Control plans, there was no difference in OC dose using PBT compared to X but an improvement in mean parotid dose (26.6 Gy vs 19.7 Gy, p=0.02). <h3>Conclusion</h3> This study quantifies the dosimetric advantage of avoiding the oral tongue while still treating the at-risk intervening lymphatics and the bilateral necks during adjuvant radiotherapy for oral tongue cancer. The dosimetric difference between PBT and X was most prominent with an oral tongue avoidance strategy. Prospective studies are warranted to evaluate the clinical efficacy of oral tongue avoidance for favorable oral tongue cancer patients with indications for elective nodal irradiation.