CT/MRI Society Case of the Month
Publication Date: 2008-04-30
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History
Two-year old, male-castrate, Long-haired Dachshund. Owners reported the dog had been abnormal in gait and mentation since a puppy. Two weeks prior to presentation there was an acute onset of abnormalities of the right front and rear limbs and circling to the left.
Physical exam: Negative menace on the right. Abnormal conscious proprioception (knuckles on right hind and delayed placement; negative wheelbarrow on the right front). Circling to the left.
Findings
A discretely marginated 3.8 X 2.7 X 3.3 cm mass is present in the left mid cranium. On all sequences the mass is resulting in severe compression of the left lateral ventricle and displacement of the left cerebral hemisphere. There is a focal discontinuity of the left parietal bone and extension of the mass laterally into the temporalis musculature.
T1 sagittal: The mass is hypointense to brain and isointense to muscle.
T2 transverse: The mass is hypointense to brain and iso to hyperintense to muscle.
T1 post contrast transverse and dorsal: There is mild to moderate, inhomogenous contrast enhancement resulting in the mass being hyperintense to both brain and muscle.
FLAIR dorsal: The mass is hypointense to brain and iso to hyperintense to muscle.
Ultrasound: A convex, hyperechoic, strongly acoustic shadowing area was noted and ultrasound guided aspirates and biopsy performed.
Diagnosis
Diagnosis: Large extra-axial mass with primary differential of neoplasia. Differentials included multilobular osteochondrosarcoma (MLO) or other types of extra-axial neoplasia (meningioma, hemangiopericytoma) considered less likely, based on hypointensity noted on T2 and FLAIR images. Mineralized granuloma was also considered less likely.
Histopathology: Fibroosseous neoplasm that has a lobular-like pattern. The sample was composed of osseous tissue with chondrocyte or osteocyte-like cells entrapped within the osseous tissue. An interconnecting network of collagenous connective tissue with a population of fibrocyte-like cells was present. The final diagnosis was MLO.
Outcome: Surgical excision was recommended but declined by the owner and the patient was lost to follow-up.
Discussion
Multilobular osteochondrosarcoma (MLO) is an uncommon neoplasm, generally arising from the flat bones of the skull in dogs. It is usually slow-growing and locally invasive. MLO typically occur in older, medium to large breed dogs, but have also been reported in younger, small breeds. Dogs with MLO usually present with progressive neurologic signs or a skull mass. Typical MR imaging findings in dogs with MLO have been reported in 3 dogs. In T1-weighted images the typical appearance is a mass that is hypointense to brain and hyperintense to CSF. In T2-weighted sequences the MLO is hypo to isointense to brain with smaller areas of hyperintensity. Contrast enhancement is variable with rim enhancement and areas of uniform to no enhancement reported. It is thought the low signal on T2 is due to the presence of calcification, osteoid, collagen, or low cellularity fibrous connective tissue. From studies in people it is also believed that the osteoid matrix does not enhance while the chondroid matrix enhances after contrast administration.
The MR imaging findings in our dog were similar to the previously described cases. Although MRI or CT guided aspirates/biopsy could have been performed, ultrasound provided the simplest, most cost-effective method in this case. The MLO was hyperechoic with strong distal acoustic shadowing, as would be expected with a mineralized mass. Thoracic radiographs had been performed and were normal prior to the MRI, but skull radiographs were not performed.
References
- Lipsitz D, Levitski RE, Berry WL. Magnetic resonance imaging features of multilobular osteochondrosarcoma in 3 dogs. Veterinary Radiology & Ultrasound, 2001:42(1); 14-19.
- Dernell WS, Straw RC, Cooper MF, Powers BE, LaRue SM, Withrow SJ. Multilobular osteochondrosarcoma in 39 dogs: 1979-1993. J Am Anim Hosp Assoc, 1998:34(1); 11-18.