The Role of Histologic Grading and Ki-67 Index in Predicting Outcomes in Pulmonary Carcinoid Tumors

Pulmonary carcinoid tumors are relatively uncommon and have an indolent clinical course. The role of histologic grading and cell proliferation as measured by a Ki-67 index in predicting long-term recurrence in carcinoid tumors of the lung is not defined. We report the largest single-institution study of carcinoid tumors and correlate histologic grade and Ki-67 index with clinical outcome. We reviewed all surgical lung resection cases from 1995 to 2016 with a diagnosis of primary carcinoid tumor. We collected clinicopathologic parameters, including tumor size, nodal status, histologic pattern, presence of lymphovascular invasion, mitotic count, %Ki-67 positive cells (Ki-67 index) using a digital algorithm, time to tumor recurrence, and staged these tumors based on the 8th edition of TNM Staging. The final cohort consists of 176 carcinoid tumor cases with complete data: 165 (94%) were typical carcinoids and 11 (6%) were atypical carcinoids. The Ki-67 index is significantly increased in atypical versus typical carcinoids and in higher stage disease. Only the Ki-67 index and not the histologic patterns or lymphovascular invasion status was a significant predictor of tumor recurrence on multivariate analysis among all pulmonary carcinoid tumors and within typical carcinoid tumors alone. A Ki-67 index cutoff of 5% offered the optimal combination of sensitivity and specificity in predicting long-term recurrence based on the receiver operating characteristic curve. In addition, stratifying pulmonary carcinoid tumors based on a 3-tier histologic grading system (grade 1: typical carcinoids with Ki-67 index ≤5%, grade 2: typical carcinoids with Ki-67 index >5%, and grade 3: atypical carcinoids regardless of Ki-67 index) significantly correlated with likelihood of tumor recurrence. Finally, we propose an integrated staging system unique to pulmonary carcinoid tumors by keeping the original TNM stage for grade 1 tumors, but upstaging grade 2 tumors to stage II, and grade 3 tumors to stage III.