A retrospective study of 101 dogs with oral melanoma treated with a weekly or biweekly 6 Gy × 6 radiotherapy protocol

Abstract

One radiotherapy (RT) protocol used for canine oral melanoma (OM) gives 36 Gy total, in six weekly or biweekly fractions (6 Gy × 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 vs. biweekly) affected either progression-free or overall survival (PFS and OS). Dogs were included if 6 Gy × 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 modelling were used to determine the predictive or prognostic value of mitotic count, bony lysis, World Health Organization (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 vs. biweekly), and treatment planning type (manual vs. 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 = <.05) with subclinical tumour burden, WHO stages I or II, and weekly irradiation. On multivariable analysis, only tumour stage remained significant; therefore, cases were grouped by WHO stage (I/II vs. III/IV). With low WHO stage (I/II), PFS and OS were longer when irradiating subclinical disease (PFS: risk ratio = 0.449, p = .032; OS: risk ratio = 0.422, p = .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.

Keywords: acute radiation toxicity; oral mucositis; oral tumours; radiodermatitis; veterinary oncology.

FIGURE 1

Kaplan–Meier curve depicting progression‐free (grey line; 171 days, 95% confidence interval: 114–267 days) and overall survival times (black line; 232 days, 95% confidence interval: 171–415 days) for the entire cohort of 101 dogs. Tick marks represent the censored subjects

FIGURE 2

Kaplan–Meier curves depicting differences in oncologic outcome, as influenced by local tumour burden (subclinical vs. macroscopic) at the time of irradiation in dogs with either low (I/II) or high (III/IV) WHO stage disease OM. Dogs with WHO stage I or II disease and macroscopic tumour burden have shorter PF and OS as compared with dogs that have the same stage of disease but subclinical tumour burden at the time of RT (risk ratio [RR] for PFS: 0.449, p = .032; RR for OS: 0.422, p = .022). Tumour burden does not significantly impact either PF or OS times for dogs with stage III or IV disease. Tick marks represent the censored subjects