Percent tumor necrosis as a predictor of treatment response in canine osteosarcoma. The percent tumor necrosis was determined in 200 dogs with spontaneously occurring osteosarcoma. One hundred dogs had no treatment before amputation or death. One hundred other dogs were treated with either radiation therapy alone (n = 23), intraarterial (IA) cisplatin alone (n = 16), intravenous (IV) cisplatin alone (n = 6), radiation therapy plus IA cisplatin (n = 47), or radiation therapy plus IV cisplatin (n = 8). Eighty-nine of these 100 dogs had their tumors resected 3 weeks after the end of therapy (6 weeks after the initiation of therapy) and replaced with a cortical bone allograft. Dogs with preoperative treatment were evaluated for local tumor control and time to metastasis. The mean percent tumor necrosis in untreated osteosarcoma was 26.8%. The mean percent tumor necrosis for dogs receiving radiation only, IA cisplatin only, and IV cisplatin only was 81.6%, 49.1% and 23.8%, respectively. The mean percent tumor necrosis for dogs receiving radiation therapy plus IA cisplatin or radiation therapy plus IV cisplatin was 83.7% and 78.2%, respectively. There was no significant difference between percent tumor necrosis in untreated osteosarcoma compared with those receiving IV cisplatin, but there was a significant increase in percent tumor necrosis with all other treatments. A mathematic model for the effect of cisplatin and radiation dose was developed using multiple regression analysis. The radiation dose calculated to cause at least 80% tumor necrosis was 42.2 Gy (95% confidence interval [CI], 38.0 to 47.6 Gy) when radiation was given alone and 28.1 Gy (95% CI, 21.3 to 36.6 Gy) when radiation was combined with IA cisplatin. Areas of viable tumor tended to be most frequent adjacent to the articular cartilage and in the joint capsule. Percent tumor necrosis was strongly predictive for local tumor control; 28 of 32 dogs with greater than 80% tumor necrosis had local control, and only eight of 29 dogs with less than 79% tumor necrosis had local control (P = 0.0047). There was no correlation between percent tumor necrosis and time to metastasis. 