3754 - Proton Therapy for Oral Cavity Cancers: A Dosimetric Comparison of Patients Enrolled on Trial of Hypofractionation

J. M. Wilson¹, S. Shiraishi¹, M. Henze², M. Dorr³, K. O. Steinert², D. J. Ma⁴, M. A. Neben-Wittich¹, M. E. Gamez¹, D. M. Routman¹, S. Lee², O. Muller⁵, L. A. McGee⁶, D. Mundy¹, S. H. Patel⁶, and S. C. Lester¹; ¹Department of Radiation Oncology, Mayo Clinic, Rochester, MN, ²Mayo Clinic, Rochester, MN, ³Department of Quantitative Health Sciences and Alliance Statistics and Data Management Center, Mayo Clinic, Rochester, MN, ⁴Mayo Clinic Comprehensive Cancer Center, Rochester, MN, ⁵Mayo Clinic, Scottsdale, AZ, ⁶Department of Radiation Oncology, Mayo Clinic, Phoenix, AZ

Purpose/Objective(s): Data supporting proton therapy (PT) for oral cavity cancers (OCC) is lacking. As such, PT is rarely mentioned in publications, guidelines, and practice pattern surveys for OCCs. We examine differences in dosimetry for targets and organs at risk (OAR) by comparing prospectively generated clinical grade proton and VMAT plans for patients treated on a prospective trial of hypofractionation. Materials/

Methods: GMOR2171 (NCT05075980) is a prospective trial utilizing the headlight regimen (4350 and 4005 cGy in 15 fractions for the high and elective risk regions) to treat HPV-negative head and neck cancers with PT after surgery. The study mandated a backup VMAT plan including full QA in the event of a proton outage. VMAT plans were prescribed to a 3 mm isometric PTV while PT plans were planned for 3% uncertainty and 3 mm robustness. We examined dosimetric data for enrolled patients with oral cavity cancers to compare doses achieved with PT vs VMAT. Paired Wilcoxon Signed Rank tests were used to compare the Mean (cGY) of each structure at risk between proton and photon plans for oral cavity patients (N=42). Sign Tests and Signed Rank Tests were used in sub-analyses that compare how many patients met pre-determined trial thresholds for OARs plans (N=42).
Results: The adjuvant arm closed to full accrual in January 2024 at 75 patients, 42 of whom had OCC. PT demonstrated lower mean doses to all OARs except the right submandibular gland (Table 1). Pre-determined goal cutoffs of oral cavity mean =20 Gy (p-value 0.03), mandible V30 Gy=40% (<0.0001), constrictors mean =30 Gy (0.006), and larynx mean =20 Gy (0.0005) all had significantly more patients meeting the cutoff for proton plans when compared with photon plans.
Conclusion: Proton therapy demonstrated lower doses to multiple organs at risk for patients undergoing adjuvant radiotherapy for oral cavity cancer on a prospective trial. This finding challenges the assumption that patients with oral cavity cancers are unlikely to benefit from proton therapy. Abstract 3754 – Table 1

Structure	Number of Patients with Proton and Photon Data (Total = 42)	Median of Proton Mean [cGY]	Median of Photon Mean [cGY]	Median Percent Difference	Wilcoxon Signed Rank One-Sided P-Value
Oral Cavity	42	24.4	31.5	21.1	<.0001
Oral Cavity - CTV	35	18.5	27.1	33.1	<.0001
Mandible	42	24.4	31.6	24.3	<.0001
Parotid (R)	42	16.8	19.6	21.8	0.0002
Parotid (L)	42	14.0	19.0	21.1	<.0001
Sub Mandible (R)	15	40.0	41.3	1.9	0.1651
Sub Mandible (L)	22	36.7	39.6	5.3	0.0002
Lips	42	6.1	14.9	78.6	<.0001
Constrictors	42	24.6	29.3	17.9	<.0001
Larynx	41	18.9	23.6	26.6	<.0001
Esophagus	42	1.6	3.2	80.5	<.0001