Outcome and Prognostic Factors of Dogs Treated for Infiltrative Lipoma Undergoing Radiation Therapy: A Retrospective Multi-Institutional Study of 29 Cases

Canine infiltrative lipomas are localised and invasive tumours that are commonly treated with surgery and/or radiation therapy (RT). There is limited efficacy data for treatment of infiltrative lipomas, and optimal fractionation strategies remain unclear. We retrospectively assessed the outcomes and prognostic factors in a cohort of dogs that underwent intensity modulated or three-dimensional conformal RT for infiltrative lipoma. Twenty-nine dogs were included from four academic institutions. The median total radiation dose prescribed and delivered was 51 Gy (range = 20-57 Gy). Dose per fraction ranged from 2.4 to 4.2 Gy, with a median of 3 Gy. Median progression-free survival (PFS) was 1483 days; the median overall survival (OS) was 1483 days. Disease progression was documented in four dogs (14%), all of which received less than 51 Gy (range = 20-50 Gy). Grade V adverse events (AEs) or secondary malignancies were recorded in six dogs (21%; two bones, one skin, one lungs, one urethra and one small intestine AE); assigned attribution was definite (n = 1), probable (n = 2), possible (n = 2) and unlikely (n = 1). Gross tumour volume (GTV) was prognostic for PFS (p < 0.01) while both GTV (p < 0.01) and total radiation dose (p < 0.01) were prognostic for OS (p < 0.01). The number of surgeries and tumour location were not associated with PFS or OS. These findings support the use of RT for long-term local control for unresectable or incompletely excised canine infiltrative lipomas. A higher total radiation dose may result in better long-term local disease control.