Page 234 - Atlas of Small Animal CT and MRI

224 Atlas of Small Animal CT and MRI

carcinomas cannot be reliably distinguished by CT or and brain parenchyma invasion, indicating biologi­
MR imaging, although presence of drop metastases sug­ cally aggressive behavior. 44–46 On the basis of limited
gests CPC. 21,27,32–36 experience, it appears that histiocytic sarcoma tends
to present as an extradural or intradural/extraaxial
Lymphoma and hematopoietic neoplasms mass, although diffuse meningeal and intraaxial forms
also occur.
Lymphoma Characteristic CT features of histiocytic sarcoma have
Lymphoma is a relatively uncommon intracranial not been documented, but these tumors are T1 iso‐ to
neoplasm that generally occurs in younger patients hypointense and T2 iso‐ to hyperintense on unenhanced
(3–7 years) than other intracranial tumors and without MR images and can cause a mass effect accompanied by
any apparent breed predisposition. Intracranial regional or diffuse peritumoral edema.
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lymphoma may be primary or metastatic within the Contrast enhancement on both CT and MR images
CNS and of either B‐ or T‐cell type. The majority of CNS is moderate to marked and can be either uniform
lymphomas in dogs are due to metastasis of widely or heterogeneous. As observed with meningiomas,
disseminated disease. In people, primary CNS lymphoma the margins of extraaxial histiocytic sarcomas may
occurs most commonly in immunocompromised appear well defined on T2 and enhanced T1 images,
patients, is overwhelmingly B‐cell phenotype, and pre­ although the degree of contrast enhancement may
sents as a solitary invasive periventricular mass. There sometimes be less than that of meningiomas and have
are few published descriptions of MR features of intra­ a fine granular pattern (Figure 2.8.18). Dural tail
cranial lymphoma in dogs and cats, but the tumor can be signs have also been reported inconsistently with
either intraaxial or extraaxial and can be quite variable these tumors. 44
in appearance. Canine primary intracranial lymphoma
37
seems to frequently affect the thalamic/hypothalamic/ Metastatic neoplasms
sellar region, while metastatic lesions are disseminated
in the meninges, choroid plexus, multiple cranial nerves, In a series of 177 secondary intracranial neoplasms
and pituitary gland. Consistent with the variable mani­ in dogs, 29% were hemangiosarcomas and 12% were
37
festations of primary or metastatic lymphoma elsewhere, metastatic carcinomas. Lymphoma and pituitary
40
intracranial lymphoma may be mass‐like, diffuse, or tumors were included in this study, but if these neoplasms
multicentric. Diagnosis of intracranial lymphoma is are excluded, hemangiosarcoma and carcinomas account
sometimes possible, based on cerebrospinal fluid cytol­ for 50% and 20% of secondary (metastatic) intracranial
ogy, which may eliminate the need for brain biopsy. neoplasms, respectively. Metastatic tumors appear to
40
Most primary lesions have a mild to moderate mass preferentially distribute to the gray–white matter
effect. These tend to be iso‐ to hypoattenuating on interface, which may reflect the likelihood of tumor
unenhanced CT images, T1 iso‐ or hypointense, and emboli lodging in the cortical arterioles where they
variably T2 hyperintense with minimal to moderate narrow. (Figures 2.8.19, 2.8.20, 2.8.21)
4,5
peritumoral edema. 37 On MR images, metastatic hemangiosarcoma
Central nervous system lymphoma in dogs consistently often appears as multiple mass lesions, although
enhances after contrast medium administration on both metastasis should still be considered when a solitary
CT and MR images, but the degree of enhancement, lesion is found. Hemangiosarcomas typically have a
tumor margin definition, and contrast distribution is var­ marked mass effect, mixed signal intensity on T1 and
iable (Figure 2.8.16). 38–43 Marked meningeal enhancement T2 sequences and heterogeneous intensity with sus­
often occurs with the meningeal manifestation of the dis­ ceptibility effects on T2* images because of intratu­
order (Figure 2.8.17). This is easily seen on enhanced MR moral hemorrhage. Peritumoral edema may be
images and can be recognized on CT images, particularly marked. Contrast enhancement is variable and often
when there is significant pial enhancement. peripheral (Figure 2.8.19). Metastatic carcinoma also
appears as multiple variably contrast‐enhancing ill‐
Histiocytic tumors defined mass lesions with variable peritumoral
Central nervous system histiocytic sarcoma is an edema, but typically without the intratumoral hem­
uncommon neoplasm of the hematopoietic system orrhage characteristic of hemangiosarcoma
with cellular features of malignancy and meningeal (Figure 2.8.20).

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