Answer: Carcinoid tumor

Histology: This tumor appears to be closely related to the bronchus and bronchial cartilages. The tumor shows varying growth patterns consisting of areas in which the tumor grows in small nests and cords, and other areas in which the tumor shows a more spindled appearance. The tumor cells are cytologically rather bland in appearance, lacking significant cytologic atypia or mitotic activity. There is no necrosis associated with this tumor and mitotic activity is absent. In addition, the cells have a significant amount of pale pink-to-clear cytoplasm and do not have the cytologic atypia of a large cell undifferentiated carcinoma or a small cell undifferentiated carcinoma.

Discussion: This is clearly an epithelial tumor that appears to be closely associated with the bronchus. The tumor lacks the cytologic atypia that would be associated with a carcinoma of either a large cell or small cell undifferentiated type. In general, bronchial carcinoids show the histochemical features of carcinoid tumors arising from the foregut and would stain with keratin, seratonin, NSE, chromogranin, synaptophysin, etc. In addition, sometimes these tumors may also stain with other antibodies associated with neuroendocrine differentiation such as somatostatin. From a histologic standpoint, this tumor shows the usual nesting pattern of growth which is seen in carcinoid tumors elsewhere. For practical purposes, the pulmonary well differentiated neuroendocrine tumors are divided into three major categories: central, peripheral, and atypical. The case illustrated here is the most common type, a central carcinoid tumor. They often form a polypoid mass within a major bronchus and because of this location and high vascularity of these tumors, patients with central carcinoids may present with hemoptysis or pulmonary infection due to blockage of the bronchial airway. In general, this tumor tends to be a tumor of adults, with equal frequency in males and females. In general, these tumors are covered by bronchial mucosa. The cells show the usual features typical of carcinoid tumors elsewhere so that the cells grow in compact nests, ribbons, or in a diffuse pattern. Sometimes the stroma may be hyalinized and occasionally focally calcified. Although spindling of the tumor cells is more often seen in peripheral carcinoids as in the case of this tumor, it can also be seen in the central bronchial carcinoids. It is important also to distinguish between carcinoid tumors and atypical carcinoids. The atypical carcinoid in general has the same overall structure and immunohistochemical features of a carcinoid but it exhibits atypical features such as increased mitotic activity, foci of necrosis and hyperchromasia. The atypical carcinoids behave in a more aggressive fashion than typical carcinoids with lymph node metastases in up to 70% of cases in contrast to the incidence in typical carcinoids which is at about the level of 5%. Therefore, the treatment for atypical carcinoid is more aggressive. For purposes of discussion we will not in this space be able to cover the so-called large cell neuroendocrine carcinomas or small cell carcinoma.