Introduction: Neutron radiotherapy is an attractive option for the treatment of salivary gland cancers, which are considered to be relatively radio-resistant, as it inflicts a higher proportion of irreparable double-stranded DNA damage within tumor cells compared to photon radiation. A prior randomized study has reported improved locoregional control of neutron over photon therapy for patients with unresectable salivary gland malignancy. Additional retrospective data shows good control rates in the post-operative adjuvant setting, but there has yet to be a large, multi-institutional study comparing photons versus neutrons in post-operative patients.
Objectives: Determine the overall survival (OS) of patients undergoing neutron radiotherapy in comparison with patients treated with photon therapy for high-grade salivary malignancies using the National Cancer Database (NCDB). Compare OS in patients treated with primary radiation.
Methods: The NCDB was queried for patients with high-grade parotid and submandibular malignancies, and 1,972 patients were identified, from 2004-2014. Of these, 1,682 underwent adjuvant therapy with photons and 55 underwent therapy with neutrons. Overall survival was compared between these groups at 1, 3, and 5 years by Cox regression. Subgroup analysis was performed for patients with locally advanced disease and positive microscopic margins at resection. A separate analysis was performed comparing the 235 patients within the NDCB who did not undergo surgery prior to radiotherapy.
Results: There was no statistically significant difference in OS for patients receiving either neutron or photon radiotherapy at 1, 3, and 5 years in the adjuvant setting. This was also true of subgroups with T4 disease and microscopic positive margins. When patients who did not receive surgery were analyzed, there was a statistically significant difference in OS favoring neutron therapy at 1 year (HR 0.198, CI 0.049 - 0.801), but no differences were found at 3 and 5 years.
Conclusions: Within the subset of post-operative patients with high grade salivary gland carcinoma undergoing photon versus neutron radiotherapy, there was no association with changes in OS. However, there was a statistically significant advantage in OS at 1 year among those who do not receive surgery. This finding is consistent with prior work suggesting that the primary advantage of neutron therapy may be local control in unresectable disease. Further prospective studies would be needed to confirm these conclusions.