Osteoblastoma in the Radius of a Pony

An osteoblastoma in the distal radial metaphysis of a 4’h-year-old pony, apparently the first in the veterinary literature, is described. In man osteoblastoma (syn. benign osteoblastoma, giant osteoid osteoma) is a rare, solitary, benign, bone- and osteoid-producing tumor which is rich in osteoblasts, and which most often involves vertebrae and long bones of the extremities in adolescents and young adults [ 1, 4, 51. This tumor has not been positively identified in animals [ 1-31. Osteoblastic, apparently benign lesions in the vertebrae with some resemblance to osteoblastoma, have been referred to in only two cats [2]. A 4Ih-year-old female pony was presented with moderate lameness of four months duration in the left forelimb. Preliminary examination of the limb revealed enlargement of the medial metaphyseal area of the distal radius. This area was hard and warm on palpation, and the pony showed signs of pain. Radiographically, a dorso-palmar view (fig. 1 ) revealed a large, well-defined, expansive, osteolytic lesion extending from the medial distal metaphysis into the epiphysis; the subchondral bone plate appeared to be intact, however. The lesion extended far outside the site of the original cortex and was bordered by a thin uninterrupted layer of bone (“ballooning out”). The axial aspect of the lesion was bordered by a well-defined sclerotic zone. Within the radiolucent mass, thin bone septa were present. A biopsy of the lesion showed a vascular connective tissue stroma with a deposition of osteoid and primitive bone. A diagnosis of a benign lesion, probably osteoblastoma or fibrous dysplasia, was made. Surgical treatment consisted of complete resection of the lesion. In some areas the underlying bone was removed with the tumor, in others the mass could be peeled easily from the bone, which was then rigorously curetted. The immediate post-operative course was uneventful. The resected specimen measured 7 X 5 x 4 cm and was oval, well-circumscribed, and surrounded by an expanded shell of cortical bone. The cut surface was gritty, uniform yellowish-brown, firm on palpation, and could be cut with a knife. The histological pattern was characterized by a maze of widely situated osteoid and primitive trabeculae, which were poorly oriented and irregular in size and shape (fig. 2). The von Kossa stain showed that most, but not all, trabeculae were semicalcified. With the reticulin stain and under polarized light, all the bone appeared woven rather than lamellar. The stroma was rich in osteo-fibroblastic cells, with a few mitoses, and highly vascular. Mainly thin-walled vascular spaces and some small and large cystic vessels were present, and scattered extravasation of blood was seen. The trabeculae were not demarcated sharply, and were outlined by accumulation of osteo-fibroblastic stroma cells, with the appearance of active osteoblasts in many sites. Numerous multinucleated giant cells, probably osteoclasts, were present through all sections, in both bone tissue and free in the stroma (fig. 3). The clinical, radiographical, and histological features are of a benign lesion and are similar to that described in the literature for osteoblastoma in man [ 1, 4, 51. To our knowledge no unequivocal cases have been documented in animals. Our diagnosis was confirmed by