The main distinguishing histologic feature of atypical carcinoids is increased mitotic activity (2-10 mitoses per 10HPF or 2 sq.mm) and/or the presence of necrosis. The necrosis is usually punctate (as shown in the center of this image) but may be more extensive. These findings may be focal and require thorough examination of the tumor. Mitotic activity should be assessed in the fields with viable tumor and greatest number of mitotic figures. Ideally, several 2 sq.mm fields should be counted and the results averaged. Atypical carcinoids make up between 10%-30% of all pulmonary carcinoids. They are slightly more common in smokers unlike typical carcinoids which show no association with smoking. Mutational inactivation of TP53 and RB1 genes is seen in about 20% of atypical carcinoids and in less than 5% of typical carcinoids. Atypical carcinoids are more likely to metastasize to regional lymph nodes (70% of cases) and distant sites as compared to typical carcinoids (<5% of cases) and have a 5-yr survival rate of about 60%.