Radiation-induced osteosarcoma in dogs after external beam or intraoperative radiation therapy.

This report describes radiation-induced osteosarcomas in two groups of dogs. One group was given radiation therapy for spontaneous tumors and the second group of normal adult beagle dogs was given experimental intraoperative radiation therapy. Secondary tumors developed between 1.7 to 5 years after irradiation. Three of 87 spontaneous tumor-bearing dogs or 3.4% of dogs treated for soft tissue sarcomas developed osteosarcoma within the field of irradiation. Twenty-two dogs or 25% of dogs treated for soft tissue sarcomas survived 20 months. This high incidence may be due to the use of fractions in excess of 3.5 Gy. These dogs received 10 fractions in 3 weeks with fractions ranging from 3.5 to 5.0 Gy. Tumor induction may be included in the late effects of irradiation which are worsened by the use of coarse fractionation. There appeared to be a dose relationship for tumors induced after single intraoperative radiation doses combined with fractionated external beam irradiation. Seven of 27 dogs given this treatment and surviving at least 4 years developed osteosarcomas in the field of irradiation. One of 26 dogs given intraoperative radiation alone developed a tumor between 4 and 5 years. The lower incidence after intraoperative radiation alone may have been due to the lower total dose. However, the sequence of a course of fractionated irradiation followed by a large single dose seemed to enhance carcinogenicity.