Bilateral Aural Masses with Malignant Transformation to Sebaceous Adenocarcinoma and Osseous Invasion in a German Shepherd Dog: A Case Report

Background : Sebaceous gland tumors are among the most common cutaneous neoplasms in dogs and are generally benign. However, malignant transformation into sebaceous adenocarcinoma is rare and may result in local invasion or metastasis. Cytological examination plays a crucial role in the initial evaluation of canine cutaneous and ear canal masses, facilitating differentiation between benign and malignant lesions. Aims : This case report aimed to describe an unusual presentation of bilateral malignant aural sebaceous tumors with osseous invasion in a German Shepherd dog, emphasizing the diagnostic value of cytology in identifying malignant transformation within otherwise benign-appearing sebaceous lesions. Methods : A four-year-old intact male German Shepherd dog was presented with bilateral obstructive ear canal masses. Fine-needle aspiration (FNA) cytology was performed on both lesions, and skull radiographs were obtained to assess potential bone involvement. Cytological smears were evaluated for cellular morphology, population diversity, and evidence of malignancy. Results : Cytology revealed two distinct cellular populations: (1) large clusters of mature sebaceous epithelial cells consistent with a benign sebaceous component and (2) smaller, cohesive clusters of basaloid reserve cells showing marked cytological atypia characterized by anisokaryosis, high nuclear-to-cytoplasmic ratio, and multiple prominent nucleoli. These findings were compatible with sebaceous adenoma exhibiting foci of malignant transformation. Radiographic examination demonstrated osteolytic changes of the temporal bones, supporting the presence of invasive behavior. The owner declined further diagnostic and therapeutic interventions. Conclusion : This case highlights the importance of comprehensive cytological evaluation in sebaceous gland tumors to detect malignant components that may be overlooked in mixed lesions. The bilateral presentation and evidence of bone invasion underscore the potential for locally aggressive behavior and emphasize the need for histopathological confirmation when cytological findings suggest malignancy.