Spontaneous pulmonary squamous cell carcinoma in an aging CD rat

Naturally occurring squamous cell carcinomas of lung of laboratory rodents are rare, and few cases have been reported and illustrated. A solid peripheral lung mass was observed grossly in a untreated 573-day-old male Sprague Dawley CD (Crl:CD [SD] BR) rat. Microscopically, the mass was composed of neoplastic squamous epithelial cells arranged in nests, cords, nodules, or solid sheets. Epithelial pearls were present. The mitotic index was high, and intercellular bridging (prickles) was noted. In some areas, neoplastic cells rimmed large cystic areas containing neutrophils, cell debris, and laminated keratin masses. Neoplastic cells had both expansive and invasive growth with entrapment of bronchioles and marked fibroplasia.