Malignant transformation of solitary spinal osteochondroma in two mature dogs

Canine osteochondroma is an uncommon bony tumor that arises in skeletally i mmature animals. Consequently, clinical signs typically occur in young dogs as a result of impingement of normal structures by the tumor. Radiographic ally, osteochondromas are benign in appearance. They are well circumscribed and cause no bony lysis nor periosteal proliferation. Osteochondromas may occur in two forms; solitary or multiple. Although histology and biologic b ehavior are identical, when in the multiple form the condition has been ter med multiple cartilaginous exostoses, Malignant transformation of multiple cartilaginous exostoses has been reported in three mature dogs. We report t wo dogs with malignant transformation of solitary spinal osteochondromas. B oth underwent transformation to osteosarcoma. Despite the benign radiograph ic appearance of osteochondromas and multiple cartilaginous exostoses, clin ical signs should alert the clinician to the possibility of malignant trans formation.