NUT midline carcinomas in the thymic region.

NUT midline carcinomas (NMCs) are rare tumors described predominantly in the pediatric age group. We recently reported two cases of these tumors occurring in the thymic region. In order to establish the true incidence of these tumors, we examined a large series of thymic carcinomas for morphological features of NUT tumor and further assessed the expression of NUTM1 (also known as NUT) protein by immunohistochemistry. The histological review of slides from 110 cases of thymic carcinoma was undertaken to identify carcinomas with mixed undifferentiated and squamous features that are typically associated with NUT carcinomas. The presenting symptoms, morphological spectrum of tumors and outcome data of patients with these histologies are presented. Immunohistochemistry for NUTM1 was performed on 35 cases of thymic carcinoma with available blocks (3 with these histological features and 32 without these features) to exclude the possibility of midline carcinoma. Tumors from 10 patients had features of mixed small cell undifferentiated squamous cell carcinoma (M:F, 1.5:1; age range, 22-79). These patients predominantly presented with advanced disease and had respiratory-related symptoms or chest pain; four had paraneoplastic syndromes. The squamous component in all cases was well differentiated with little or no atypia. The undifferentiated component varied in cell size and lacked characteristic features of small cell carcinoma. All but one patients developed metastases or died within 3 years of diagnosis. NUTM1 expression was seen in two of three tumors with these histological features and in none of the 32 cases without. Mixed small cell undifferentiated carcinomas share histological and immunohistochemical similarity with NMCs and have aggressive clinical course. These tumors are not uncommon and should be considered in the differential diagnosis of carcinomas in the thymic region as novel therapies might be available.