Outcomes and prognostic factors in dogs with subcutaneous and intramuscular hemangiosarcoma treated with hypofractionated radiation therapy and chemotherapy

IntroductionSubcutaneous (SC) and intramuscular (IM) hemangiosarcoma (HSA) in dogs are aggressive malignancies with high rates of recurrence and metastasis. Surgery is the primary treatment, but incomplete excision is common, and the role of radiation therapy (RT) remains unclear.MethodsThis single-institution retrospective study evaluated outcomes and toxicities in dogs with SC and IM HSA treated with hypofractionated RT using either 24 Gy in three 8-Gy fractions (days 0, 7, and 21) or 30 Gy in five 6-Gy fractions every other day, with or without surgery and chemotherapy.ResultsFifty-three dogs were included: 27/53 (51%) SC HSA and 26/53 (49%) IM HSA; 35/53 (66%) had macroscopic disease at RT. Overall, 47/53 (89%) received chemotherapy, of which 42/47 (89%) received doxorubicin; 35/42 (83%) received it concurrently with the first RT fraction. Among 30 evaluable dogs, the RT objective response rate (ORR) was 93% (37% complete, 56% partial). Local progression or recurrence (LPR) occurred in 11/36 (30%) dogs after a median of 128 days, predominantly with macroscopic disease (10/11). Median time to progression (TTP) was not reached (95% CI, 621 days–not reached). In an exploratory multivariable Cox analysis, metastasis was associated with increased hazard of LPR (HR, 14.95; 95% CI, 2.84–78.72; p = 0.0014), whereas surgery was associated with a lower hazard, but not significantly (HR, 0.15; 95% CI, 0.018–1.24; p = 0.079). Overall median survival time (MST) was 306 days (95% CI, 284–875). On univariable analysis, anemia, absence of surgery, macroscopic disease at RT, metastasis, and tumor size at diagnosis >6 cm were associated with shorter survival. Dogs undergoing surgery prior to RT had longer survival (MST, 2059 vs. 287 days; p = 0.007). In multivariable analysis, anemia remained independently associated with increased hazard of death (HR, 3.20; 95% CI, 1.38–7.45; p = 0.0069), whereas metastasis, surgery, and tumor size were not independently associated with survival. Chemotherapy was not associated with survival. Radiation-associated adverse events were generally mild.ConclusionHypofractionated RT, as part of multimodal treatment in most dogs, was associated with a high ORR and may provide durable local control in dogs with non-visceral HSA.