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

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.