Risk Factors for Pathologic Fracture in Dogs With Confirmed or Suspected Appendicular Osteosarcoma Treated With Low‐ or Standard‐Dose‐Rate Palliative External Beam Radiation Therapy

ABSTRACT

Pathologic fracture is a complication of bone cancer with probable increased risk following radiation therapy (RT). Low‐dose‐rate (LDR) RT can spare late‐responding normal tissues by allowing intrafraction DNA repair, which could also impact tumor response. This retrospective cohort study evaluated efficacy and fracture rate in canine patients ( n  = 184) with confirmed or suspected appendicular osteosarcoma that received 10 Gy × 2 palliative RT on consecutive days with LDR (cobalt‐60) or standard‐dose‐rate (SDR, linear accelerator). The overall fracture rate was 21.7%. Patients treated with SDR (600 cGy/min, n  = 46) and LDR (range 21.9–83.29 cGy/min, n  = 152) did not differ in pathologic fracture rates (22.4% vs. 17.4%, respectively, p  = 0.54) or median survival times (231 vs. 248.5 days, respectively, p  = 0.448). Pathologic fracture rates did not differ on the basis of weight (<35, 35–50, and >50 kg) or location (24% forelegs, 18% hindlegs, p  = 0.44). However, no dog ( n  = 7) weighing less than 20 kg developed a fracture. Radiographs prior to RT were subjectively assessed for bone integrity. Dogs with predominantly lytic lesions did not have a significantly higher pathologic fracture rate (21.2%–31.69% lytic/mixed vs. 10%–12.5% proliferative, p  = 0.054), suggesting that pretreatment radiographic appearance does not reflect fracture risk. Additionally, the presence of a pretreatment cortical bone defect was not associated with eventual fracture ( p  = 0.7). Adjunctive chemotherapy improved survival time significantly ( p  = 0.01). On the basis of our data, it is reasonable to compare response rates and survival outcomes in published studies using cobalt irradiation with reports using linear accelerators.