Evaluating the Impact of Elective Nodal Irradiation for Dogs With Oral Malignant Melanoma Undergoing Hypofractionated Radiotherapy.

Hypofractionated radiotherapy (hRT) is often used to treat dogs with oral malignant melanoma (OMM); however, there is no consensus as to whether clinically uninvolved regional lymph nodes should be prophylactically irradiated. The objective of this retrospective study is to compare outcomes for dogs with OMM treated with hRT+/- elective nodal irradiation (ENI). Dogs with nonmetastatic OMM undergoing hRT+/- ENI with a prescription of ≥ 30 Gy were included. Survival statistics were evaluated with Kaplan-Meier curves and log-rank testing. Univariable and multivariable Cox proportional hazard models were used to assess how survival was impacted by the use of ENI, WHO T-stage, mitotic count, RT technique, and use of Oncept melanoma vaccine. Data from four institutions and 100 dogs (80 with ENI and 20 without) were included. In the ENI group, nodal and distant metastases were documented in 4 and 30 dogs, respectively. In the non-ENI group, nodal and distant metastases were documented in 6 and 4 dogs, respectively. There was no significant difference in the 1-year nodal or distant progression-free intervals (p = 0.174, and 0.563, respectively). The only variable maintaining significance on multivariable analysis was T-stage (overall progression-free survival, HR 1.393, p = 0.006; overall survival time, HR 1.426, p = 0.005; distant progression-free interval, HR 1.521, p = 0.033). ENI did not measurably alter the oncologic outcomes in this study population. Results should be interpreted cautiously given the lack of standardised staging/restaging and the heterogenous nature of this clinical population. Future investigations are needed to clarify the role of ENI in the treatment of canine OMM.