2717 - Nasopharyngeal Cancer Treated with Proton Beam Radiotherapy: Patterns of Disease Control and Toxicity with Extended Follow-Up

Purpose/Objective(s): Nasopharyngeal cancers (NPC) are uncommon in the United States and often present with locally advanced disease. While NPC is highly sensitive to chemotherapy and radiation, treatment-related toxicities can be significant given the disease location. Proton beam radiotherapy (PBT) has the potential to reduce dose to normal tissues, however, long-term outcomes and toxicity data on NPC patients treated with PBT are scarce. The purpose of our study is to report outcomes, recurrences, acute and late toxicities, and dose-volume histogram data for a large cohort of NPC patients treated with PBT.

Materials/Methods: Under a prospective institutional review board-approved outcomes study protocol, we reviewed the charts of adult patients (> 18 y/o) diagnosed with NPC between 2008 - 2023. Forty-two patients were included (mean age 53), 50% were Epstein-Barr virus positive and 31% were p16 positive. At diagnosis, 50% were T4 and 29% were T3. Fifty-seven percent received PBT only, while 43% received mixed PBT and intensity modulated radiation therapy plans. The primary clinical target volume received a median dose of 7000 cGy/RBE (range, 6120 – 7440). Ninety-five percent of patients received concurrent platinum-based chemotherapy and 62% received induction chemotherapy.

Results: Median follow-up was 4.6 years (range, 0.4-16.2) for all patients, and 5.3 years (range, 0.9-16.2) for living patients. Five-year rates of local control, locoregional control (LRC), distant metastasis-free survival, disease-free survival, cause-specific survival, and overall survival were 92%, 89%, 89%, 80%, 92%, and 75%, respectively. Seven patients had disease progression events, three of which were local recurrences that were all within the primary dose level. Three of the four patients with non-local recurrences were successfully treated with salvage therapy. LRC was worse for T4 patients (76%) compared to T1-3 patients (100%). 55% (n=23) of patients developed a grade 3 toxicity and 2% (n=1) developed a grade 4 toxicity. Thirty-six percent (n=15) developed a late toxicity, the most common of which was hearing loss (n=7). No treatment-related brain necrosis or grade 3+ ocular toxicities occurred.

Conclusion: This study represents one of the largest long-term analyses of locally advanced NPC patients treated with PBT in North America, demonstrating excellent LRC without increased marginal recurrences. While it is possible to spare normal tissue with PBT, serious treatment-related toxicities remain a concern for these patients. These findings support the use of PBT in NPC, but further research is needed to better understand the benefits and cost-effectiveness.