# Solitary Subcutaneous Nodular Lymphoid Lesions in Dogs: Histopathologic and Immunophenotypic Comparison of B-Cell Pseudolymphoma and Subcutaneous Panniculitis-like T-Cell Lymphoma

## TL;DR

This study compares two types of lymphoid lesions in dogs to highlight the importance of using histopathology and immunophenotyping for accurate diagnosis and treatment.

## Contribution

The study provides a detailed histopathologic and immunophenotypic comparison of two distinct canine subcutaneous lymphoid lesions.

## Key findings

- B-cell pseudolymphoma showed CD20+/PAX5+ B cells with strong BCL6 and no BCL2 expression.
- SPTCL exhibited CD3+ T-cell infiltrates with adipocyte rimming and minimal BCL2/BCL6 expression.
- Histopathology and immunophenotyping are essential to differentiate benign from malignant lesions in dogs.

## Abstract

Canine cutaneous lymphoproliferative disorders range from reactive hyperplasia to malignant lymphomas and can be challenging to differentiate without immunophenotypic characterization. Subcutaneous panniculitis-like T-cell lymphoma (SPTCL) and pseudolymphoma may present with similar clinical features but require distinct clinical management. This report describes two dogs presenting with subcutaneous nodules and compares histopathological and immunohistochemical findings between SPTCL and pseudolymphoma. The findings emphasize the critical role of histological and immunophenotypic analyses in differentiating benign from malignant subcutaneous lesions, enabling appropriate clinical management in canine patients.

This report describes two cases of solitary subcutaneous nodular lymphoid lesions in dogs. Case 1 involved a 6-year-old male Maltese and Case 2 a 5-year-old female Yorkshire Terrier. Both presented with firm, non-ulcerated dorsal subcutaneous nodules and were unresponsive to corticosteroids. Surgical excision was performed for diagnosis. Histopathology and immunohistochemistry revealed distinct patterns. Case 1 exhibited well-formed lymphoid follicles with CD20+/PAX5+ B cells and strong BCL6 but absent BCL2 expression, consistent with B-cell pseudolymphoma. Case 2 demonstrated diffuse CD3+ T-cell infiltrates with adipocyte rimming and minimal BCL2/BCL6 expression, diagnostic for SPTCL. Despite their similar clinical presentation, these two lesions were histopathologically and immunophenotypically distinct. These findings underscore the importance of histologic and immunophenotypic correlation in accurately distinguishing benign from malignant subcutaneous lymphoid proliferations in dogs.