Pulmonary adenocarcinoma with metastasis to skeletal muscle in a cat

A 14-year-old, neutered male cat was presented with a 2-week history of lethargy, decreased appetite, cough, weight loss, and an abnormal gait. Examination revealed 4 subcutaneous masses involving the proximal part of the hind and forelimbs, varying from 1/2 cm to 3 cm in diameter, firm in texture, and attached to or within the underlying soft tissue. The cat was also found to have a heart murmur and thin body condition. On radiographs, the masses were soft tissue in density and the underlying bones were normal in appearance. Radiographs of the chest revealed a 2.5-cm soft tissue density mass in the left middle lung lobe.

A minimum data base (complete blood [cell] count [CBC], biochemical profile, and urinalysis) revealed a mildly increased urea of 17.2 mmol/L (reference range, 4.0 to 10.7 mmol/L); a urine specific gravity of 1.033; a mild nonregenerative anemia with a red blood cell count of 4.6 × 10¹²/L (reference range, 5.0 to 10 × 10¹²/L); a moderate mature neutrophilia, 20.2 × 10⁹/L (reference range, 2.5 to 12.5 × 10⁹/L); and a moderate lymphopenia, 0.8 × 10⁹/L (reference range, 1.5 to 7.0 × 10⁹/L).

Cytologic examination of the leg masses revealed large clusters of cells (> 50 cells) with high nuclear to cytoplasmic (N/C) ratio, marked anisokaryosis, and basophilic and granular, occasionally vacuolated cytoplasm. There was a second population consisting of a moderate number of discrete cells with spindle or flame shapes, high N/C ratio, moderate anisokaryosis, and basophilic cytoplasm.

Due to the poor prognosis and the rapid deterioration of the cat’s condition, the owner elected for euthanasia. Necropsy revealed multiple, 0.5 to 3 cm in diameter, firm encapsulated masses in the skeletal muscles of the proximal part of the right hind, left hind, and left forelimbs, and a 2 to 3-cm diameter pulmonary mass in the left middle lung lobe. One of the skeletal muscle masses, located in the left mid-part of the semimembranosus muscle, was 3 cm in diameter and slightly larger than the mass in the lung.

Histopathologic examination revealed a tumor in the lung forming multifocal poorly demarcated nodules up to 1 cm in diameter. These consisted of neoplastic cells arranged in acini or tubules. In several areas, the cells clustered around bronchioles and large blood vessels, but only rarely invaded the lumen of the bronchioles (Figure 1). The neoplastic cells were epithelial, with indistinct borders, a modest amount of eosinophilic cytoplasm, round nuclei with dispersed chromatin and prominent nucleoli, 2-fold variation in nuclear size, occasional binucleate cells, and a high mitotic rate at 2 per high-power (400 ×) field. Ciliated cells were not apparent. The tumors in muscle were histologically similar but showed more prominent variation in nuclear size and a more extensive scirrhous reaction (Figure 2). Cells in one section formed solid sheets or cords rather than acini or tubules. The final diagnosis was pulmonary adenocarcinoma with metastasis to muscle.

Figure 1.

Lung, cat. Neoplastic cells aggregate within pulmonary alveoli, and invade into a bronchus (white arrow) and the surrounding lung tissue (black arrows). Hematoxylin and eosin. Bar = 1000 μm.

Figure 2.

Skeletal muscle, cat. Neoplastic epithelial cells with prominent atypia form clusters and rare acini (white arrow) within the skeletal muscle, invading and displacing the myofibers (black arrows). Hematoxylin and eosin. Bar = 50 μm.

Primary lung tumors are less common than metastatic lung lesions in dogs and cats, especially compared with their frequency in humans (1). Adenocarcinomas account for 70% to 80% of primary pulmonary neoplasia in dogs and cats (1); less common carcinomas include squamous cell carcinomas and adenosquamous carcinomas (2–4). Fibrosarcomas, osteosarcomas, chondrosarcomas, hemangiosarcomas, lymphomatoid granulomatosis (angioinvasive T cell lymphoma), histiocytic sarcoma, and adenomas are other primary neoplasms that have been documented in canine and feline lungs (2–4).

Metastasis of primary lung tumors in dogs and cats can occur to other areas of the lungs, or to the long bones, liver, spleen, pancreas, kidneys, adrenal glands, heart, brain, esophagus, abdominal or mediastinal lymph nodes, eyes (3,5–12), or digits in cats (4,7,13,14). A literature search for mention of metastasis of pulmonary carcinoma to muscle yielded only one reference (14).

Most dogs and cats with pulmonary neoplasia are middle-aged or geriatric (2,3). Presenting signs can include cough, exercise intolerance, dyspnea, lameness (from metastasis or hypertrophic pulmonary osteopathy, the latter more common in dogs than in cats), weight loss, anorexia, and lethargy (2,3,8). Diagnosis is usually established based on radiographs, cytologic examination, and histopathologic examination.

Cats and dogs that present with firm soft tissue masses should have metastatic pulmonary neoplasia added to the list of differential diagnoses.