The histopathology of NC is characteristic, but not diagnostic. The most common appearance is that of a poorly differentiated carcinoma with focal, abrupt squamous differentiation (Fig. 7.2). In contrast to many other poorly differentiated carcinomas, which consist of highly pleomorphic large cells, NC cells are usually medium sized, round, and often monomorphic in appearance (Fig. 7.3). Overt areas of squamous differentiation are seen in approximately half of cases [8] but may not always be present, particularly in small biopsies. A peculiar feature seen often in NC is a brisk neutrophilic infiltrate not associated with necrosis (Fig. 7.4). Well documented NC with unequivocal features of adenocarcinoma have not been seen. The histopathologic features of NC overlap with those of several other poorly differentiated cancers, including poorly differentiated squamous cell carcinoma, sinonasal undifferentiated carcinoma [5], Ewing sarcoma, Epstein-Barr virus-associated nasopharyngeal carcinoma, thymic carcinoma, neuroblastoma, pancreatoblastoma [12], and even primary salivary gland carcinoma (Fig. 7.5) [26]. NC is commonly misdiagnosed; it has even been mistaken for acute leukemia (due to the occasional expression of CD34) [2]. Also contributing to the failure to diagnose NC is poor awareness of the disease among clinicians and pathologists, its rarity, and until recently, the absence of widely available diagnostic tests. To counter the lack of awareness of NC among pathologists and oncologists, an Internet-based international NC registry (http://​www.​NCRegistry.​org) has been established. This registry provides access to (a) pathologic review, (b) updated information about the disease, (c) treatment guideline suggestions, (d) a repository for clinical data, and (e) educational information for physicians and patients about this disease.

NUT carcinoma has a cytopathologic appearance that is nonspecific and mimics other primitive small round cell tumors or basaloid neoplasms [27–30]. Samples are highly cellular with monotonous, primitive-appearing small-to-mid-sized cells distributed most often singly and only occasionally in groups (Fig. 7.6). Vacuolated cytoplasm, corresponding with intracytoplasmic glycogen, is frequently seen, creating artifactual separation of cells and imparting a “fried-egg”-like appearance (Figs. 7.7 and 7.8) [27, 28, 31]. Chromatin ranges from open and pale, to hyperchromatic and neuroendocrine-like (Figs. 7.9 and 7.10) [32]. Mitoses, necrotic debris, and crush artifact are common [28, 33]. Overall, the cytologic characteristics overlap considerably with other poorly to undifferentiated carcinomas.

NCs often present at advanced stages and therefore are not frequently amenable to surgical resection. NC typically grows as a large mass extending into hilar structures or along the pleura and chest wall (Fig. 7.11) [17]. NC has a fleshy, tan-white cut surface with or without prominent geographic necrosis.

NUT carcinoma typically presents as sheets and nests of undifferentiated cells with a monomorphic appearance. The nuclei have irregular contours and granular to open chromatin (Figs. 7.9 and 7.10). The characteristic presence of abrupt foci of keratinization (Fig. 7.2) could be absent in small biopsy specimens [11, 16, 34]. Tumor cells infiltrate and expand the interstitium and may appear contiguous with bronchial epithelium (Fig. 7.12) [17]. Prominent geographic necrosis might be present (Fig. 7.13).

Tumor cells may be associated with a reactive pneumocyte proliferation, leading to diagnostic consideration of an adenosquamous carcinoma.

The diagnosis of NC depends on the demonstration of NUT (aka NUTM1) gene rearrangement or mis-expression. Although cytogenetic demonstration of a t(15;19) by karyotyping is sufficient for a presumptive diagnosis of NC, most diagnoses are based on formalin-fixed, paraffin-embedded (FFPE) tissues because few suspected carcinomas are submitted for karyotyping. To fill this diagnostic need, one of the authors developed a fluorescence in situ hybridization (FISH) assay that uses dual-color, break-apart bacterial artificial chromosome probes flanking the NUT and BRD4 loci (Fig. 7.14) [35–37]. This assay can be used on virtually any specimen preparation, including frozen tissue, acetic acid-fixed cytogenetic preparations, thin (4–5 μm) sections of FFPE tissues or disaggregated cells, and air-dried or ethanol-fixed slides. Because this approach works on archival, formalin-fixed tissue, retrospective analysis can be performed on samples that are decades old [2].