Clinical, Histopathological and Molecular Characterisation of Canine Epitheliotropic Cutaneous T‐Cell Lymphoma With Cytotoxic Interface Dermatitis: A Retrospective Case Series

Background Apoptotic keratinocytes have been described with canine epitheliotropic cutaneous T‐cell lymphoma (eCTCL) without further detailed characterisation of this feature. Hypothesis/Objectives This study aims to characterise confirmed eCTCL cases enriched with apoptotic keratinocytes as a novel cytotoxic variant of canine eCTCL. Animals Canine eCTCL cases from a veterinary pathology diagnostic laboratory database were searched from 2018 to 2024. Materials and Methods Haematoxylin and eosin‐stained slides were examined for evidence of lymphocytes, apoptotic keratinocytes with lymphocytic satellitosis, and epitheliotropism in the lower half of the epidermis and adnexal structures by a board‐certified veterinary pathologist. Immunohistochemical ( IHC ) staining for CD3 and CD20 was performed in addition to clinical follow‐up with response to treatment, and PCR for Antigen Receptor Rearrangement ( PARR ) assay (T and B cell). Results Six cases with representative features were identified. Various breeds were affected with a median age of 10 years at presentation. Skin lesions included generalised crusting, scaling, erythema and erosions/ulcerations; mucocutaneous junctions were involved in three of six dogs. Histopathological results confirmed an interface cytotoxic pattern eCTCL in all cases, marked by lymphocytic epitheliotropism and apoptotic keratinocytes. IHC staining demonstrated > 90% strong CD3 + T‐cell immunoreactivity in the epidermis and follicular epithelium in all cases. All six confirmed cases showed clonality for the T‐cell receptor gene using PARR analysis. Conclusions and Clinical Relevance This cytotoxic variant of canine eCTCL clinically and histologically can resemble other cutaneous diseases with cytotoxic dermatitis (e.g., hyperkeratotic erythema multiforme). IHC and clonality testing and monitoring response to treatment may be necessary for definitive diagnosis.