In a biopsy indicating a carcinoid tumor, what characteristic is unusual?

A carcinoid tumor is a type of neuroendocrine tumor that often presents with certain histopathological features. One hallmark of carcinoid tumors is their relatively uniform arrangement, often seen in nests or trabecular patterns. The cells typically exhibit scant cytoplasm, contributing to their distinctive appearance under the microscope. The well-defined borders of the nests, which typically are observed in carcinoid tumors, lend to their organized structure.

However, the characteristic that is deemed unusual among these tumors is the presence of frank malignancy. Carcinoid tumors are generally considered to be low-grade neoplasms, and a genuine frank malignancy with aggressive behavior is uncommon. Instead, many carcinoid tumors may remain indolent or exhibit only mild malignant potential, making overt malignant characteristics an atypical finding. Thus, the presence of frank malignancy is not associated with typical carcinoid tumors, setting it apart as an unusual characteristic.