Cool images and a nice differential table!

Meningeal null cell lymphoma causing diffuse pachymeningeal contrast enhancement in a dog

M. Madden, N. Israeliantz, A. Malbon, C. Piccinelli, K. Marioni-Henry, T. Schwarz, A. Suñol

Background
Diffuse pachymeningeal contrast enhancement on MRI is rare in dogs, and its etiology is not well understood. This case report documents a 2-year-old female neutered Pug with meningeal null cell lymphoma, an exceptionally rare type of lymphoma in veterinary medicine. Null cell lymphoma lacks typical B- or T-cell markers and constitutes less than 5% of canine lymphomas. This is the first confirmed report of meningeal null cell lymphoma in a dog.

Methods
A comprehensive diagnostic approach was undertaken:

-Neurological and Physical Exams: Multifocal neuroanatomical localization including brainstem and cerebellum was observed.

-MRI Findings: MRI revealed diffuse pachymeningeal thickening and strong homogeneous contrast enhancement primarily involving the tentorium cerebelli and falx cerebri.

-CSF Analysis: Cerebrospinal fluid (CSF) analysis showed marked neutrophilic pleocytosis and elevated total protein without infectious agents or atypical cells.

-Histopathology: Post-mortem analysis confirmed a densely cellular, infiltrative lymphoma with minimal parenchymal involvement. Immunohistochemistry and PCR showed a clonal B-cell receptor rearrangement, confirming a null cell phenotype.

Results
-Clinical Course: Initial immunosuppressive treatment provided temporary improvement. However, the disease progressed despite corticosteroids and cytosine arabinoside, leading to euthanasia five weeks post-presentation.

-Histopathological Findings: The dura mater was thickened with infiltration of large neoplastic cells and minimal subarachnoid/parenchymal extension. Immunohistochemistry showed negative markers for CD3, PAX5, CD79a, and CD30, confirming null cell lymphoma.

-Diagnosis: Meningeal null cell lymphoma was established as the underlying cause of pachymeningeal thickening.

Limitations
The case lacked ante-mortem staging and complete post-mortem analysis, leaving uncertainty about whether the lymphoma was primary or metastatic. Additionally, the rarity of null cell lymphoma limited comparative analysis.

Conclusions
This case highlights the diagnostic challenges of diffuse pachymeningeal enhancement in dogs, which may arise from diverse etiologies, including neoplasia. The findings emphasize the importance of thorough diagnostic evaluation, including dural biopsy, in cases of progressive pachymeningeal disease. Null cell lymphoma should be considered as a differential diagnosis for such presentations in dogs.