Palliative radiotherapy of appendicular osteosarcoma in 95 dogs

Ninety-five dogs with either a presumptive (n = 24) or biopsy confirmed dia gnosis (n = 71) of osteosarcoma received palliative radiotherapy using Co-6 0 photons, parallel opposed beams were used with each dog receiving either 10 Gy on days 0,7 and 21 (n = 58) or 8 Gy on days 0 and 7 (n = 37). The 8 G y fractionation scheme was given with the intent of retreating upon relapse from pain relief. Only 9 of 37 (24%) dogs in the 8 Gy group returned for r etreatment. Forty-seven of the 95 dogs (49%) received concurrent or sequent ial chemotherapy, Seventy of the 95 dogs (74%) experienced pain relief foll owing treatment. In dogs experiencing pain relief the median duration of re sponse was 73 days. Numerous clinical variables were evaluated as predictor s of response. The only variable significantly related to achieving a respo nse was the use of chemotherapy. The following variables were significantly related to the duration of response: extent of bone lysis, chemotherapy us e, length of bone involved and tumor site (humerus). In a multivariate anal ysis (n = 73 dogs), after adjusting for chemotherapy use, extent of bone in volvement (p = 0.01) and tumor site (p = 0.02) retained statistical signifi cance, while degree of bone lysis did not (p = 0.11). No difference in resp onse incidence or duration was found between 3 fractions of 10 Gy vs. 2 fra ctions of 8 Gy. Administration of a low initial dose with the intent of ret reatment was not a successful strategy.