Secondary tumors are those tumors that have shown metastatic dissemination from a primary neoplasm elsewhere in the body. Invasion of the adjacent skeleton by a soft tissue tumor is not considered a secondary bone tumor and has been discussed in Chapter 76. Direct invasion by extension of a tumor into the adjacent osseous structures is far more common in the dog than are primary bone tumors. In the dog, skeletal metastasis is rare, whereas in humans it is extremely common. The majority of case reports of secondary bone tumors in the dog are of single cases. Thus the true incidence of secondary bone tumors is difficult to estimate. The most detailed investigation of this subject was done by Goedegebuure.(2) He found that of 141 dogs with metastatic tumors, 17% showed skeletal metastasis compared with 77% with pulmonary metastasis. All dogs with skeletal metastasis showed metastasis to other internal organs as well.
Clinical signs do not always accompany metastasis.(2) In 24 dogs with postmortem evidence of metastatic disease, 15 failed to show clinical signs. When noted, clinical signs are similar to those found with primary tumors of bone and include lameness, swelling, pain, loss of function of the affected area, and muscle atrophy. Radiographically the lesions are indistinguishable from primary bone sarcomas and fungal infections of bone.(3,6) Most cases show marked periosteal new-bone formation. A positive diagnosis of metastatic bone tumor requires biopsy of the affected site or sites and histopathologic examination of the tissue.
The majority of secondary bone tumors are epithelial in origin; fewer mesenchymal tumors show skeletal metastasis The average age of affected dogs is 8.7 years, with a range of 3 to 17 years.(2) No breed or sex predilection can be found. In one study the number of bones showing metastasis ranged from l to 13, with the largest number of cases showing involvement of only 1 or 2 bones.(2) The humerus, femur, and vertebrae have the highest incidence of metastatic bone disease. In long bones, the metastases are found primarily in the cancellous bone of the proximal epiphysis and metaphysis and rarely involve the diaphysis and cortical bone. This is thought to be due to the presence of well-vascularized red bone marrow in these sites.
On gross examination the metastatic lesion may be only barely visible to the naked eye or may fill the medullary cavity. Lesions vary from pale white to dark red. Tumor may extend into the cortex and destroy cortical bone as well as elicit severe periosteal new-bone proliferation and marked swelling of the soft tissues. Occasionally the periosteal new-bone formation may be severe with minimal destruction of medullary bone. Often, however, bone destruction will not be found until late in the disease process. These tumors show extensive proliferation of tumor tissue in the intertrabecular spaces, with little destruction or lysis of the trabecular bone.
The tumors that most commonly show skeletal metastasis are mammary carcinomas, primary lung carcinomas, and prostatic carcinomas. (1,5,6) One study also shows an increased incidence of skeletal metastasis in adrenal and hepatic carcinomas. Hemangiosarcoma is the mesenchymal tumor most commonly showing skeletal metastasis. (1)
Lumbar vertebral and pelvic metastasis is seen in a large proportion of prostatic carcinomas.(4) Many of these dogs present clinically with difficulty in urination and defecation. It has been postulated that straining to urinate and defecate raises the intra-abdominal pressure and causes the vena cava to temporarily collapse, thereby shunting the venous flow into the vertebral venous plexus. The veins of the vertebral plexus are valueless, thus allowing venous blood containing tumor emboli to flow in a retrograde fashion and seed the vertebral bodies and pelvis with tumor cells. In those cases in which intra-abdominal pressure is not increased, hematogenous or lymphogenous spread of the tumor is more likely to occur.(6)
1. Brodey RS, Reid CF, Sauer RM: Metastatic bone neoplasms in the dog. J Am Vet Med Assoc 148:29, 1966
2. Goedegebuure SA: Secondary bone tumours in the dog. vet Pathol 16:520, 1979
3. Kas NP, van der Heul RO, Misdorp W: Metastatic bone neoplasms in dogs, cats and a lion. Zentralbl Veterinarmed (A) 17:909, 1970
4. Leav I, Ling GV: Adenocarcinoma of the canine prostate. Cancer 22: 1329, 1968
5. Misdorp W. Den Herder BA: Bone metastasis in mammary cancer. Br J Cancer 20:496, 1966
6. Morgan JP: Radiology in Veterinary Orthopedics, p 154. Philadelphia, Lea & Febiger, 1972