Concomitant diffuse pulmonary carcinoma and chronic interstitial pneumonia in a cat with hypertrophic cardiomyopathy: unusual pathological findings and differential diagnosis.

Introduction: Bronchioloalveolar carcinoma (BAC) may occasionally occur in a diffuse form involving most or all of the lung parenchyma. This form may mimic other, more common diffuse interstitial lung diseases in both humans and animals. Materials and Methods: A 12-year-old spayed female European shorthair cat presented with severe dyspnoea. Echocardiography revealed hypertrophic cardiomyopathy and pleural effusion. The cat died form acute decompensated heart failure. At necropsy, lungs appeared diffusely congested and firm, with presence of multifocal to coalescing, irregular grayish areas, and sparse hemorrhages. Lung tissue samples were routinely processed for histology and immunohistochemistry for pan-cytokeratin, vimentin, CAM 5.2, MAC387, CD3, Ki-67. Results: Lung histopathology revealed a diffuse neoplastic proliferation characterized by irregular alveolar growth with multifocal micropapillary formations. Tumour cells were large, predominantly cuboidal in shape, with marked nuclear pleomorphism, prominent nucleoli, multifocal binucleated elements, high Ki-67 proliferative index and intense positivity for pan-cytokeratin and CAM 5.2. Metastases to tracheobronchial lymph nodes or other organs were not observed. Tumour growth was obscured by simultaneous lesions related to chronic congestion and interstitial pneumonia, characterized by numerous, intra-alveolar MAC387+ macrophages with multifocal erytrophagocytosis and segmental foci of bronchioloalveolar hyperplasia. Conclusions: Histological tumour pattern characterized by neoplastic cells budding from alveolar surface was consistent with a diagnosis of diffuse BAC with unusual presence of large, pleomorphic tumour cells. Differential diagnosis included large cell carcinoma, which is usually characterized by rosettes or solid clusters of cells occupying alveolar lumen. Extensive cytokeratin immunostaining was also helpful in the differentiation from histiocytic proliferative diseases.