Outcomes Following Proton Radiotherapy for Pediatric Salivary Gland Tumors

Abstract

<h3>Purpose/Objective(s)</h3> In contrast to adult salivary gland tumors, our institutional guidelines in children utilize lower radiation doses, smaller target margins, and proton therapy to minimize radiation to the developing skull base anatomy. Furthermore, we do not routinely recommend prophylactic nodal irradiation or elective neck dissection. Previous series of children treated with adjuvant radiation therapy report 5-year local control ∼85%. Herein we report treatment outcomes using our pediatric approach. <h3>Materials/Methods</h3> Twenty-seven consecutive pediatric patients were treated with proton therapy between 2006 and 2020 for mucoepidermoid carcinoma (n= 13), acinic cell carcinoma (n= 5), adenoid cystic carcinoma (n= 4), pleomorphic adenoma (n= 2), myoepithelial carcinoma (n= 1), adenocarcinoma (n= 1), and sialoblastoma (n= 1) at our institution. The 8 patients with radiographic lymphadenopathy (cN1) underwent neck dissection prior to presentation (7/8 selective dissections involving <4 nodal levels). The most common indications for adjuvant radiotherapy were microscopic positive or close (<1 mm) margins (n=15), perineural invasion (PNI; n=9), high-grade histology (n=6), and gross residual disease (n=4). Radiotherapy was delivered in sequential plans using 2 clinical target volumes (CTV1, CTV2). Gross tumor volume (GTV=CTV2) included the residual tumor and tumor bed. CTV1 received 50.4 Gy and was defined as GTV + 1 cm and the at-risk neck if indicated. The dose to CTV2 depended on histology, grade, and margin status. The median total dose was 64.8 Gy (range, 61.2-70.2 Gy). Only 3 patients underwent prophylactic nodal irradiation. Toxicity data was graded based on CTCAE v3 or v4 using patient-reported and physician-reported outcomes according to our prospective outcome registry. <h3>Results</h3> The median follow-up was 5.5 years. Overall survival was 100% and local control was 96% with a single local recurrence seen in a patient with gross unresectable disease. There were no nodal recurrences. In terms of acute toxicity, 3 patients experienced mucositis requiring opioid pain medication and 2 patients developed grade 3 radiation dermatitis. The most serious late toxicity involved the ear or auditory system including 4 patients requiring hearing aids, 1 with chronic otitis media, 1 with osteonecrosis of the mastoid bone, and 1 with external ear canal stenosis. Four additional patients reported variable hearing loss not requiring hearing aids. Other serious late toxicity included 1 patient with persistent trismus, 1 with growth hormone deficiency, and 1 with excessive dental caries along the ipsilateral maxilla. No secondary malignancies nor toxicities grade ≥ 4 were seen. <h3>Conclusion</h3> An approach of conservative neck management and moderate-dose proton radiotherapy delivered to small volumes resulted in excellent disease control and limited toxicity in children with common salivary gland tumors at elevated risk of local recurrence.