Carcinoid Tumor of the Lung: An Audit of Small Specimen Reports and Correlation with Resection

Summary: Introduction: Carcinoid tumor (CT) of the lung is a neuroendocrine, well-differentiated neoplasm characterized by a nested/organoid architecture and representing approximately 5% of all pulmonary neuroendocrine neoplasms. These tumors are further subclassified into typical carcinoid (TC) and atypical carcinoid (AC) according to their mitotic activity and the presence or absence of necrosis. TC has fewer than two mitoses per 2 mm² without evidence of necrosis, whereas AC is characterized by two to ten mitoses per 2 mm² and/or necrosis. The accurate distinction between TC and AC relies on assessing histologic criteria on resection specimens and has significant prognostic implications. Materials and Methods: For quality assurance purposes, a retrospective analysis was conducted to evaluate the accuracy of CT diagnoses obtained from small specimens (cytology and biopsy samples) by reviewing cases diagnosed between 2016 and 2024 with subsequent tumor resection. Epic Beaker LIS was utilized to audit our institution's reporting methods for CTs and the concordance rate for CT subclassification on small biopsy specimens (based on Ki-67 and mitotic rate). Results: Forty-two cases (n = 42) of CT meeting the search criteria were identified (small sample + resection). The diagnoses in small samples were as follows: TC n=37 (88.1%); AC n=3 (7.1 %); suspicious for neuroendocrine tumor n=1 (2.4%) (surgical follow-up AC), atypical epithelioid cells n=1 (2.4%) (surgical follow-up TC). Subclassification was reported in 40 (95%) cases in small samples. There was a discrepancy in 3 (7.5%) cases when subclassification was given. Two of these TCs were classified as AC on resection. One additional discrepant case (AC) was diagnosed in a small sample and classified as TC on resection. All small samples with discrepancies were reviewed, and the diagnosis remained unchanged, indicating sampling error. The patient with AC in the small sample has been followed as having AC since the small sample result was confirmed. When considering cases where the resection led to a change in the patient's prognosis (TC to AC), the discrepancy rate was 5%. Conclusions: Subclassification of CTs of the lung in atypical carcinoid tumors using small samples is a reliable method of subclassification. TC has a 5.5% discrepancy rate. Despite the relatively low discrepancy rate, given the potential clinical impact on these patients, it is reasonable to write a comment regarding the final classification of the resection specimen.