Discussion: 

Neuroendocrine tumors represent 25% of primary lung neoplasms [1]. Carcinoid tumors consisting predominantly of spindle cells are uncommon. The spindle cell carcinoid constitutes approximately 5% of all pulmonary carcinoids [2]. Spindle cell carcinoid tumors are well-demarcated, located predominantly in the periphery of the lung. These tumors are usually asymptomatic but can show a clinical behavior ranging from extremely indolent to more aggressive than non-spindle cell carcinoid tumors [3].

Features that facilitate the cytologic diagnosis of a spindle cell carcinoid includes dispersed and loosely cohesive clusters of intact monotonous neoplastic cells with fusiform or elongated shapes, sharply circumscribed cell borders and scant to moderate amount of cytoplasm. The nucleus is oval or elongated, with finely granular chromatin, and sometimes the presence of a small nucleolus. Nuclear molding is usually not noted. There is no necrosis and mitotic activity is scanty [2]. Immunohistochemical stains for neuroendocrine differentiation, including chromogranin, synaptophysin, CD56 and keratin, are positive.

Differential Diagnosis:

The cytologic differential diagnosis of spindle cell carcinoid is wide and includes benign and malignant neoplasms, as well as non-neoplastic lesions. Here is a brief description of some of the differential diagnoses.

Solitary fibrous tumor
Solitary fibrous tumor is a rare tumor of serosal membranes, most frequently localized on the pleura. In a pleural location, it may be confused clinically with a localized mesothelioma or a peripheral lung tumor. It is often pedunculated and therefore easily resectable. The cytopathological features include isolated spindle cells, stretched fibromyxoid material, thin collagen fibers, bare nuclei, and fibrous tissue fragments. This entity tends to be vimentin positive, CD34 positive, cytokeratins negative and a variable actin reactivity [4].

Leiomyoma
It is a benign smooth muscle tumor with wide gross and morphologic spectrum. The cytopathological features include multiple clusters of uniform spindle cells with cigar shaped, bland nuclei. Usually, there is no significant nuclear atypia, hyperchromasia or mitosis [4].
 

Mesothelioma
It is a malignant tumor arising in mesothelial-lined surfaces. The cytopathological features are loosely cohesive sheets of malignant spindle cells or few atypical spindle cells and fragments of collagen strands. Immunohistochemistry shows that the malignant cells tend to be vimentin and cytokeratin positive, while CD34 is negative [4].

Granulomatous lesion
A granuloma is a compact aggregate of histiocytes. Epithelioid histiocytes have indistinct cell borders and elongated, sole-shaped nuclei, as opposed to the well-defined cell borders and round, oval, or kidney bean–shaped nuclei of ordinary histiocytes. These are nonspecific lesions by themselves, but always an infectious etiology (fungi, mycobacteria) needs to be excluded. In the absence of an identifiable etiologic agent, sarcoidosis should also be considered in the differential [5].