Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

622 Small Animal Radiolo g y and Ultrasono graphy
ABNORMAL FINDINGS
C A LVA R I U M
Neoplasia. Primary bone tumors of the calvarium are uncommon and appear to be
equally distributed among the cranial vault, facial bones, and mandibles. 45,46 Most lesions
of the facial bones and mandible are osteolytic or mixed osteolytic and osteoblastic
in nature. 46-48 A notable exception is the osteoma, which is usually densely osteoblastic
(Fig. 5-12). 49,50 The most common tumors are fibrosarcoma, melanoma, squamous cell
carcinoma, and osteosarcoma in dogs and squamous cell carcinoma in cats. 51-54
Multilobular osteochondrosarcoma is a uncommon tumor that is seen most often in
older, medium- to large-breed dogs, arising from the aponeurosis adjacent to the calvarium.
Several names have been applied to it over time, including calcifying aponeurotic
fibroma, juvenile aponeurotic fibroma, cartilage analogue of fibromatosis, ossifying
fibroma, multilobular osteoma, multilobular chondroma, multilobular osteosarcoma, and
chondroma rodens. 55 This lesion is well defined and densely calcified in a lobulated pattern.
56 Varying degrees of destruction of the adjacent bone may be present. The tumor has
also been reported to involve the frontal bone, mandible, maxilla, and orbit. 56-59
Multiple myeloma is a specific tumor that usually becomes apparent as multifocal lytic
areas in the calvarium that lack sclerotic borders. However, in dogs and cats with myeloma,
it is more common to see these changes involving the appendicular skeleton or spine.
In cats, meningioma may cause a thickening of the calvarium adjacent to the tumor
(Fig. 5-13). Dilation of the meningeal arteries may cause enlarged vascular impressions on
the surface of the bone. The calvarial thickening has not been reported in dogs with meningioma.
Meningioma also may produce destruction of the calvarium (Fig. 5-14). Other
reported changes include mineralization of the mass and increased size of the middle
meningeal artery. 60
Tumors of the base of the skull are uncommon and usually are primary bone tumors
(Fig. 5-15). These may be lytic or productive and are often difficult to perceive. Lesions
involving the osseous bullae usually are not primary bone tumors but more often arise
from the lining of the bulla. Squamous cell carcinomas and ceruminous gland carcinomas
may affect the bullae and may completely destroy the osseous bulla, petrous temporal
bone, and other bony structures that surround the external and middle ear. 61,62 A stippled
pattern of soft-tissue mineralization often is seen in conjunction with squamous cell carcinoma.
This helps discriminate between destruction of the bulla secondary to tumor and
the changes that may be present secondary to previous bulla osteotomy.
Fig. 5-12 A 1-year-old male mixed breed dog had a hard
palpable mass involving the right parietal area. The modified
occipital view revealed a bone-dense mass that extends
both laterally (open white arrow) and medially (open black
arrow) from the right parietal bone. The pattern of calcification
is dense and laminar. Differential diagnoses include
osteoma, osteosarcoma, chondrosarcoma, other primary
bone tumor, and metastatic tumor. Diagnosis: Osteoma.