“Scooby” was a 10 year old male castrated labrador retriever who presented to the Oncology service at Hope Veterinary Specialists for a large firm mass arising from the left side of his face. Initial aspirates of the mass obtained at the referring hospital were suggestive of osteosarcoma.
In order to stage the tumor as well as determine whether or not it would be amenable to surgical resection, a CT scan was performed of the entire head, using a bone algorithm and calculating retroactive reconstructions in detail and soft tissue windows. Intravenous contrast (Iohexol) was also given and the scan repeated using the same protocol.
A large well-defined mineralized mass measuring ~4.4 x 6.0 x 7.1cm was found underneath the left zygomatic arch. The mass was arising from a stalk-like projection coming out of a shallow ovoid region of geographic lysis along the junction between the zygomatic process of the maxillary bone. No periosteal reaction was present along the involved or adjacent bones, nor was there any proliferative changes beyond the mass itself. A CT of the thorax demonstrated no evidence of pulmonary metastatic disease.
The owners opted to pursue a more conservative, local surgical resection of the mass involving removal of the caudoventral aspect of the maxillary bone including M1 and M2 and the rostroventral aspect of the zygomatic arch, while preserving the orbit (see figure 3). This was followed up with chemotherapy in the form of low-dose Cytoxan.
After a little over 9 months, the owners noticed recurrence of the soft tissue swelling along the left side of Scooby’s face. A follow-up CT scan of the skull revealed a large multi-lobulated mass with patchy regions of contrast-enhancement and several fluid-filled pockets, but no mineralization. An irregular periosteal reaction was present along the ventral aspect of the zygomatic arch, but no additional lysis (see figure 4). The owners elected not to pursue additional surgery or radiation therapy.
Multilobular osteochondrosarcomas (MLO’s) are locally-invasive neoplasms that typically arise from the flat bones of the skull and tend to occur in older medium and large breed dogs, although they have been reported in younger and small breed dogs, and have also been reported in cats, humans, a horse and a ferret. MLO’s exhibit characteristics of both benign and malignant tumors– they are relatively slow-growing and well-defined, but often cause extensive regional destruction and osteolysis, with metastases possible. They most commonly arise from the top or caudal aspects of the cranial vault, in which cases they often exhibit a finely stippled, granular pattern of mineralization and frequently invade into the cranial vault itself, resulting in various neurologic abnormalities such as seizures. They less commonly arise from the mandible, zygomatic bone and the zygomatic arch and other more rostral areas of the skull– in such cases they tend to exhibit a more coarse, multilobular pattern of mineralization.
The CT and MRI characteristics of a small number of MLO cases have been described, with the tumors on MRI generally being hypointense to brain tissue on T1W, T2W and proton-density sequences, with variable degrees and patterns of contrast enhancement after administration of gadolinium IV. In addition to the different patterns of mineralization, MLO’s arising from the zygomatic arch, the mandible and other more rostral structures of the skull do not typically exhibit much if any significant contrast enhancement on CT (eg Iohexol), whereas those arising from the cranial vault exhibit variable degrees of contrast enhancement. Whether or not the behavior and/or histopathology varies between the two regions of tumor growth remains to be seen.
Submitted by Hadley Bagshaw, VMD, DACVR
Hathcock JT, Newton JC. Computed Tomographic Characteristics of Multilobular Tumor of Bone Involving the Cranium in 7 Dogs and Zygomatic Arch in 2 Dogs. Vet Rad & US 2000, vol. 41 (3): 214-217.