LAUSR

One radiotherapy (RT) protocol used for canine oral melanoma (OM) gives 36 Gy total, in six weekly or biweekly fractions (6 Gy x 6). This retrospective study characterizes oncologic outcomes for a relatively large group of dogs treated with this protocol and determines whether radiation dose intensity (weekly versus biweekly) affected either progression-free or overall survival (PFS and OS). Dogs were included if 6 Gy x 6 was used to treat grossly-evident OM, or if RT was used postoperatively in the subclinical disease setting. Kaplan-Meier statistics and Cox regression modeling were used to determine the predictive or prognostic value of mitotic count, bony lysis, WHO stage (I, II, III, or IV), using systemic anti-cancer therapies, tumour burden at the time of RT (macroscopic vs. subclinical), radiation dose intensity (weekly versus biweekly), and treatment planning type (manual versus computerized). The median PFS and OS times for all dogs (n = 101) were 171 and 232 days, respectively. On univariate analysis PFS and OS were significantly longer (p = <0.05) with subclinical tumour burden, WHO stages I or II, and weekly irradiation. On multivariable analysis, only tumor stage remained significant; therefore, cases were grouped by WHO stage (I/II versus III/IV). With low WHO stage (I/II), PFS and OS were longer when irradiating subclinical disease (PFS: risk ratio = 0.449, p = 0.032; OS: risk ratio = 0.422, p = 0.022); this was not true for high WHO stage (III/IV). When accounting for other factors, radiation dose intensity had no measurable impact on survival in either staging group.