History
A 5-year-old male neutered Labrador Retriever was presented to an academic referral institution for further evaluation of chronic, progressive right hind limb lameness. Approximately 4 months prior to presentation, the dog developed a mild intermittent right hind limb lameness after minimal trauma (person fell on him). Due to failure to improve, he was evaluated by the referring veterinarian approximately 2 months after the trauma. Sedated radiographs (craniocaudal stilfe joints, lateral right stifle) reveal a large radiolucent lesion associated with the medial aspect of the right distal femur. The lameness continued to progress, despite NSAID (carprofen, 4.2 mg/kg, by mouth, q 24 h for a total of 6 weeks) and gabapentin (dosage not reported) administration, and referral was recommended.
On presentation, the patient was bright, alert, and responsive with normal vital signs and an unremarkable general physical exam. An orthopedic exam revealed an ambulatory toe-touching to intermittently non–weight-bearing right hind limb lameness. There was marked gluteal and quadriceps muscle atrophy of the right hind limb with no pain elicited on long bone palpation. Palpation of the right stifle joint revealed mild stifle joint effusion; however, there was no evidence of tibial thrust or cranial drawer in either stifle. A complete blood count and biochemistry profile revealed no clinically significant findings.
The previously acquired radiographs were assessed (Figure 1).
Craniocaudal stifle joints (A) and right lateral stifle (B) radiographs of a 5-year-old male neutered Labrador Retriever presenting for chronic, progressive right hind limb lameness. Radiographs were acquired by the referring veterinarian 2 months prior to presentation.
Citation: Journal of the American Veterinary Medical Association 262, 2; 10.2460/javma.23.06.0345
Diagnostic Imaging Findings and Interpretation
A large, well-circumscribed radiolucency is present within the right medial femoral condyle extending to the subchondral bone (Figure 2). There is a moderate increase in soft tissue opacity within the cranial and caudal joint space of the right stifle. Based on the radiographic appearance of the lesion, prioritized differential diagnoses included a benign process (bone cyst, other) versus neoplasia (chondrosarcoma, osteosarcoma, soft tissue sarcoma, synovial cell sarcoma, other). Three-view thoracic radiographic views and abdominal ultrasound imaging are normal. Noncontrast CT revealed medium-sized solitary expansile geographic bone lysis with articular involvement of the right medial femoral condyle, mild locally extensive periosteal reaction of the right medial fabella, mild right stifle joint effusion and moderate right pelvic limb muscle atrophy (Figure 3). Following administration of iohexol (697 mg/kg, IV), the region of lysis contains fluid to soft tissue attenuating material with large numbers of small diameter strongly contrast enhancing internal blood vessels (Figure 4). The contrast enhancement images are displayed in a soft tissue window (average, 40 HU; range, 15 to 70 HU). Based on the CT appearance of the lesion, a neoplastic process was prioritized; however, a benign process, such as a bone cyst, could not be excluded (Supplementary Video S1; Supplementary Video S2).
Craniocaudal stifle joints (A) and right lateral stifle (B) radiographs of the dog described in Figure 1, revealing a large, well-circumscribed radiolucency within the right medial femoral condyle extending to the subchondral bone and distal medial metaphysis (yellow arrows). There was mild to moderate increased soft tissue opacity within the cranial and caudal joint space of the right stifle. Prioritized differential diagnoses included a benign process (bone cyst) versus neoplasia (chondrosarcoma, osteosarcoma, soft tissue sarcoma, synovial cell sarcoma).
Citation: Journal of the American Veterinary Medical Association 262, 2; 10.2460/javma.23.06.0345
Transverse plane (A), frontal plane (B), and sagittal plane (C) CT images of the dog described in Figure 1, revealing medium-sized solitary expansile geographic bone lysis with articular involvement of the right medial femoral condyle. All images were obtained in bone window (window width, 4,500 HU; window level, 1,100 HU) with a 0.5-mm slice thickness. Medial (A and B) or cranial (C) is toward the left of the image.
Citation: Journal of the American Veterinary Medical Association 262, 2; 10.2460/javma.23.06.0345
Transverse plane (A), frontal plane (B), and sagittal plane (C) CT images of the dog described in Figure 1 immediately after iohexol administration revealing fluid to soft tissue–attenuating material (average, 40 HU; range, 15 to 70 HU) within the area of bony lysis of the right medial femoral condyle with innumerable small-diameter, strongly contrast-enhancing internal blood vessels. All images were obtained in soft tissue window (window width, 320 HU; window level, 30 HU) with a 0.5-mm slice thickness. Medial (A and B) or cranial (C) is toward the left of the image.
Citation: Journal of the American Veterinary Medical Association 262, 2; 10.2460/javma.23.06.0345
Treatment and Outcome
Due to the significant contrast enhancement of the lesion and concern for bleeding risk associated with biopsy, the owners elected to forgo a biopsy in lieu of proceeding with a right coxofemoral disarticulation amputation. The amputation was performed in routine fashion and the dog recovered uneventfully. The dog was hospitalized in the intermediate nursing care unit for approximately 24 hours following his procedure for supportive care and pain management. The following day he was ambulating well on 3 legs and was comfortable on incisional palpation. He was discharged for further care at home.
The entire right hind limb was submitted for histopathology, which reveals a well demarcated, unencapsulated, mildly infiltrative, densely cellular intraosseous mass. The mass consists of neoplastic chondrocytes within lacunae embedded in an abundant, amorphous, amphophilic, chondroid matrix, consistent with a chondrosarcoma. The mass is within the marrow cavity of the femur without extension into the articular surface. All bony and soft tissue margins are free of neoplastic cells, consistent with complete excision. Based on a proposed grading scheme for canine appendicular chondrosarcomas assessing matrix production, architecture, degree of cellular pleomorphism, cellular density, presence of necrosis, and mitotic activity, the neoplasm is classified as a grade 2 out of 3.1 The owners were advised to pursue screening thoracic radiographs every 3 months for a year, then every 4 to 6 months thereafter to monitor for metastasis.
Comments
Chondrosarcoma is the second most common appendicular primary bone tumor in dogs, accounting for up to 10% of primary bone neoplasms.2 When compared with appendicular osteosarcoma, appendicular chondrosarcomas typically grow slower and are associated with a significantly lower rate of metastasis.2 Metastatic rates can range from 18% to 60%, with the lungs being the most common site; however, metastasis has also been reported in the pleura, mediastinum, pericardium, lymph nodes, bone, liver, kidneys, adrenal glands and eyes.1 In a retrospective study conducted by the Veterinary Society of Surgical Oncology in 2009,1 the pulmonary metastatic rate at the time of diagnosis was 28%. The dogs in this study that were treated with amputation alone had a mean survival time (MST) of 979 days, with a significant association being identified between histologic grade and MST (MST of 6.0, 2.7, and 0.9 years with neoplasm grades of I, II, and II, respectively).1 Other studies have reported an MST for dogs treated with surgery alone for skeletal chondrosarcoma to be between 163 to 2,618 days.1 The most common long bone affected by appendicular chondrosarcoma is the tibia, followed by the femur, then the humerus.1 Radiographically, the appearance of appendicular chondrosarcoma may vary greatly, with the most common early changes being cortical bone destruction and secondary periosteal reaction. Appendicular osteochondromas can be highly osteolytic and tend to affect the architecture of the metaphyseal cortex and medullary cavity.1,2
Because of the well-circumscribed nature of the lesion radiographically, a bone cyst was initially prioritized. Bone cysts in dogs are benign lesions that can be further classified as cystic (simple, unicameral), aneurysmal, or subchondral.3–5 Simple bone cysts are fluid-filled cavities lined by fibrous connective tissue and can be monostotic or polyostotic. The cause for simple bone cysts is unknown, but metaphyseal venous obstruction with secondary osteolysis has been proposed.5 Aneurysmal bone cysts are uncommon and are characterized as benign expansile osteolytic lesions containing vascular sinusoids.5 Again, the etiology is unknown, but a secondary insult to the underlying bone leading to arteriovenous shunting has been proposed.3,4 Subchondral cysts are those that are located adjacent to a synovial membrane insertion, can open into a joint space, and are often associated with osteochondrosis. Only 1 case of a subchondral bone cyst has been reported in a dog.5 Clinically, bone cysts may be subclinical, cause lameness, cause a painful, firm localized area of swelling or can precipitate pathologic fractures.4 Radiographs typically reveal an expansile, locally aggressive lucent defect with no periosteal reaction, as was the case with this patient.5
This case illustrates the importance of a complete differential list and how advanced imaging may reorder a prioritized list of differentials. Given the severity of nonresponsive lameness appreciated in this case, as well as the extent of the bone lesion observed on radiographs, amputation without biopsy or further imaging is justified. Post amputation, the lesion must be submitted for histopathology review to obtain a definitive diagnosis.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org
Acknowledgments
The authors would like to thank Nicholas Walsh, DVM, and Ian Porter, DVM, DACVR, for their contributions to this case.
Disclosures
At the time of submission, Jacqueline M. Chevalier served as a student Associate Editor for the Journal of the American Veterinary Medical Association (JAVMA). She declares that she had no role in the editorial direction of this manuscript. The authors have nothing additional to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
References
- 1.↑
Farese JP, Kirpensteijn J, Kik M, et al. Biologic behavior and clinical outcome of 25 dogs with canine appendicular chondrosarcoma treated by amputation: a Veterinary Society of Surgical Oncology retrospective study. Vet Surg. 2009;38(8):914-919. doi:10.1111/j.1532-950X.2009.00606.x
- 3.↑
Schrader SC, Burk RL, Liu SK. Bone cysts in two dogs and a review of similar cystic bone lesions in the dog. J Am Vet Med Assoc. 1983;182(5):490-495.
- 4.↑
Nojiri A, Akiyoshi H, Ohashi F, et al. Treatment of a unicameral bone cyst in a dog using a customized titanium device. J Vet Med Sci. 2015;77(1):127-131. doi:10.1292/jvms.13-0548
- 5.↑
Johnston SATK. Miscellaneous orthopedic conditions: bone cysts. In: Veterinary Surgery: Small Animal Expert Consult. Elsevier; 2017:1314-1315.
