“Low grade” mucoepidermoid carcinoma of the bronchus with “high grade” biological behavior

A case of mucoepidermoid carcinoma of the bronchus with low grade histologic features exhibited an aggressive biological behavior. All gross, microscopic, and ultrastructural features were those of a morphologically low grade mucoepidermoid carcinoma. Grossly, the lesion was an exophytic endobronchial mass; microscopically, it contained sheets of monomorphic cells with well‐formed mucus glands, no mitoses, and no necrosis; and ultrastructurally, it consisted of numerous goblet cells and mitochondria‐ and glycogen‐rich cells with well‐formed microvilli. However, the patient developed widespread metastases to skin, subcutaneous tissue, femur, vertebral column, and pericardium and died two months after the primary endobronchial tumor was excised. The metastases, like the primary tumor, had the appearance of “low grade” mucoepidermoid carcinoma. Explants of the original tumor grown in nude mice for three months showed persistence of the low‐grade morphologic pattern. Although metastases are known to occur frequently from high‐grade lesions, widespread metastases from histologically low‐grade lesions have not previously been reported. Low‐grade histologic features of mucoepidermoid carcinoma are not necessarily then indicative of low‐grade biological behavior.