Lung tumors are histologically heterogeneous, but classification of lung carcinoma has prognostic impact and increasingly, specific molecular correlates.
To update the gross, microscopic, and molecular pathology of unusual lung carcinomas to assure accurate classification. In entities with mixed histology, the recognition of specific features or rare patterns is critical to diagnosis. These diagnoses can identify tumors with aggressive clinical behavior, and diagnostic pitfalls can therefore result in underdiagnosis of these already rare entities. Incorrect classification of more indolent tumors into the more aggressive categories can also occur. In the area of molecular pathology, these unusual tumors have a specific spectrum of molecular alterations.
PubMed searches for lung and sarcomatoid carcinoma, pleomorphic carcinoma, blastoma, carcinosarcoma, and adenosquamous and mucoepidermoid carcinoma were undertaken and this information was integrated with clinical experience of the author.
These uncommon carcinomas have specific clinicopathologic features, and attention to their gross and microscopic pathology leads to classification with important associated molecular findings.
The classification of uncommon tumors with a mixed histology can be challenging, considering that lung carcinomas, especially adenocarcinoma, are histologically heterogeneous. As a result, the criteria for these tumors must be clearly defined to avoid overinclusion of tumors that may not have the clinicopathologic features of the bona fide examples. This becomes especially relevant when molecular associations overlay the classification, most notably when potential therapeutic implications are in play.
PULMONARY SARCOMATOID CARCINOMA
This category represents a set of rare carcinomas that are grouped together by the presence of a spindle or giant cell component. An examination of Surveillance, Epidemiology, and End Results data support the impression of an uncommon class of tumor, about 0.5% to 0.8% of lung carcinomas.1,2 Most patients are white with a 3:2 male to female ratio. These tumors have been reported to be smoking associated.3
Pulmonary sarcomatoid carcinomas (PSCs) are most often large tumors with central necrosis (Figure 1, A) and hemorrhage. This can result in a cavity that can mimic abscess formation. They can be either central or peripheral, but central location is more common. The areas of sarcomatoid histology can correspond to grossly apparent solid, soft, and “fish-flesh”–appearing areas (Figure 1, B).
Before discussing definitive classification, it is of note that the identification of this tumor type on small samples is a challenge. The finding of a spindle or giant cell component on a small sample needs to be reported, and the epithelial component also mentioned. Therefore, for example, adenocarcinoma with giant cell carcinoma would be a descriptive small-sample diagnosis that relays the important information without committing to a more discrete category. The greater concern is that a small sample may not contain a component essential to the identification of a sarcomatoid carcinoma (eg, giant cells, spindle cells, or heterologous element), resulting in underdetection of PSC.
The International Association for the Study of Lung Cancer/World Health Organization classification4 includes 5 histologic categories of PSC. These are defined by different combinations of epithelial and nonepithelial patterns, some of which are specific, rare patterns.
The first, pleomorphic carcinoma, is characterized by a carcinoma with epithelial histology—adenocarcinoma, squamous carcinoma, or large cell carcinoma—combined with spindle cell or giant cell component (Figure 2, A and B). The definition includes a minimum 10% component for the epithelial and nonepithelial component. The purpose of this definition is to prevent inclusion of tumors with scattered enlarged cells or focal spindled cells, as well as to allow for the classification of spindle or giant cell carcinoma when the morphologically epithelial component is focal. At this end of the spectrum, the 2 next categories of PSC—predominantly spindle and/or giant cell—are defined as carcinomas in which these patterns are dominant. In these 2 groups, extensive sampling may demonstrate definitive carcinomatous components that support the classification as a carcinoma but are sufficiently focal as to be excluded from the pleomorphic carcinoma group.
For pleomorphic carcinoma, the frequency of different histologic types among the epithelial component is worthy of mention. In a series of 78 cases, Fishback et al5 described the most common pattern as adenocarcinoma, followed by large cell and squamous carcinoma. Other studies have confirmed a preponderance of large cell carcinoma and adenocarcinoma.6,7 Small cell carcinoma with spindle cell or giant cell component appears to be particularly rare. Overall, these epithelial patterns may be relevant in the finding of driver mutations but do not seem to impact survival.
The nonepithelial component of pleomorphic carcinoma is more frequently spindled than giant cell, although many cases will have a mixture of both. The spindle cell pattern is by definition not heterologous—that is, recognizable cartilaginous, osteogenic, myogenic, or vascular differentiation is absent. The proliferations resemble fibroblastic and myofibroblastic cells (Figure 2, C) and their cellularity is often moderate to high. Cytologic atypia can be relatively low, but nuclear enlargement exceeds what is present in reactive desmoplasia. Therefore, the combination of cellular bundles, nuclear enlargement, and cellularity leads to the recognition of a morphologically spindled carcinomatous component. As previously noted, small foci, when in doubt, are not sufficient for the diagnosis. When cases are difficult to resolve, immunohistochemistry for cytokeratin in the spindle component can be very helpful in distinguishing desmoplasia from spindle cell carcinoma elements. Also useful are areas of transition from the epithelial components that are strongly cytokeratin positive to morphologically spindled cells that progressively lose cytokeratin reactivity.8,9
Giant cell areas are also morphologically distinct. There can be sheets of enlarged cells with lack of cohesion, but they are often intermixed with spindled cells. The giant cells are large, as their name implies, and frequently have multilobated nuclei as well as multinucleation. An important finding is the presence of emperipolesis; phagocytic activity of these cells can include red cells and inflammatory cells (Figure 2, D). This description is distinct from marked pleomorphism amidst otherwise carcinomatous nests, a finding regularly encountered in high-grade squamous carcinoma and adenocarcinoma and not sufficient for a giant cell designation.
The fourth category of sarcomatoid carcinoma is carcinosarcoma. In this entity, there is a combination of a carcinoma and heterologous sarcomatous element, such as cartilage, bone, or skeletal muscle.10 The location, size, and male predominance are similar to those described in pleomorphic carcinoma. In a series of 66 cases, squamous histology was described more often than in series of pleomorphic carcinomas, which were characterized by adenocarcinoma or large cell patterns. For the heterologous element, in order of frequency, rhabdomyosarcomatous followed by osteosarcomatous and chondrosarcomatous elements (Figure 2, E) are seen.11
The fifth category, that of pulmonary blastoma, is the rarest. It is specifically a biphasic tumor in which the epithelial component is fetal-type adenocarcinoma (Figure 2, F) with a mesenchymal component that is heterologous and/or immature (blastema-like).11,12 Fetal-type adenocarcinoma is characterized by glands lined by elongated cells with subnuclear and often supranuclear vacuoles; they are described as resembling developing airways in the pseudoglandular phase. In contrast to other tumors in this group, blastoma can be seen in the pediatric population, but is distinct from pleuropulmonary blastoma, a mesenchymal tumor of infancy. Interestingly, while pulmonary blastomas with well-differentiated fetal adenocarcinoma as the epithelial component are a clearly defined group,13 tumors with a high-grade carcinomatous component are distinct from it. This morphologic difference has a molecular correlate as well. Mutations in β-catenin that result in protein persistence and nuclear localization are characteristic of well-differentiated fetal adenocarcinoma and pulmonary blastoma with a well-differentiated epithelial component.14 Additionally, squamous morule formation is associated with β-catenin mutation.15 Immunohistochemistry for β-catenin shows nuclear immunoreactivity in the setting of β-catenin mutation and can be used in support of the diagnosis. In contrast, high-grade cases are not typically β-catenin mutated and show membranous or cytoplasmic β-catenin reactivity.
Overall, PSCs are known for their aggressive behavior. Most patients present with high-stage disease with 72% at stage 3 or 4. A higher risk of death is reported when compared to other non–small cell lung cancers.16 While surgical treatment is currently the main effective therapy,17 recurrences occur within 7 to 12 months even after R0 resection. Survival after recurrence is often very short. Vascular invasion has been reported as an especially poor prognostic feature.18 While chemotherapy has been attempted, response rates are low (<20%) and progression is seen with most first-line therapies.19 Second-line therapy is not generally effective.
The finding of epidermal growth factor receptor (EGFR) mutations in PSC, mostly pleomorphic carcinomas, varies in different series, some finding a very low rate20,21 and others showing rates up to 28%.22,23 However, despite reported EGFR mutations in these tumors, EGFR tyrosine kinase inhibitor (TKI) response rates are relatively low and not durable, when compared to lung adenocarcinoma.24,25 Rare examples of EGFR-mutated carcinosarcomas are reported.26 KRAS mutations, however, are frequently encountered in PSC, from 20% to 38% of cases, notably in tumors with an adenocarcinoma component.20,21,27 To date, targeting of oncogenic KRAS has not been successful. Other targetable mutations, such as ALK translocations, are rare but encountered,28,29 as are BRAF V600E mutations.2 ALK and MET amplifications are also seen.30 It is also notable that sarcomatoid transformation has been reported in ALK resistance post treatment for ALK-translocation adenocarcinoma.31
While MET immunohistochemistry and MET copy number increase is seen in lung carcinoma including PSC, targeting this pathway with antibodies such as onartuzumab (METMAB) or TKIs such as tivantinib has not had positive outcomes even after case selection by high immunohistochemistry score. In one series of lung adenocarcinoma with MET amplification, response to crizotinib was demonstrated.32
Mutations in MET of exon 14 or its flanking introns that result in skipping of exon 14 is an oncogenic event in adenocarcinoma and PSC. Skipping of exon 14 results in loss of a tyrosine ubiquitination site at position 1003 and thus loss of Casitas B-lineage lymphoma–mediated ubiquitination.33 This interrupts normal cycling and lysosomal degradation of MET, whose levels are thus preserved and can drive tumorigenesis; this alteration is mutually exclusive with other driver mutations.34 While present in about 4% of lung adenocarcinomas, this alteration is seen in PSC at a rate that varies from 5% to up to 32% in different series.2,21,35–37 The reason for this range of mutation frequency may include case inclusion (eg, more adenocarcinoma containing), method of mutation detection, or as-yet-undiscovered regional variation. Coexistence with MET amplification is sometimes but not always seen. Importantly, tumor response to crizotinib has been reported in these MET exon 14–skipping cases.2,21
Overall, it does appear that PSCs with targetable mutations generally have an adenocarcinomatous component.38 In addition to these targetable alterations, TP53 mutation is frequently encountered in PSC and is likely the most frequently encountered mutation in these tumors.2,21,29 TP53 mutations are also seen in carcinosarcomas and pulmonary blastoma.39
Another promising area in the treatment of PSC is immunotherapy. Programmed death ligand-1 (PD-L1) immunoreactivity is seen in these tumors at a high rate; high tumor mutation burden is also reported.2 PD-L1 immunoreactivity is frequent in PSCs—up to 90% of cases.40,41 One caveat to these data is that the studies used different antibody clones and cutoff values (5% and 10%) than currently in place for clinical PD-L1 testing. However, there are reports of responses to immune checkpoint inhibitors such as pembrolizumab in PSC.42,43
Another set of tumors with mixed histology are adenosquamous carcinomas. These tumors are defined as containing both adenocarcinomatous and squamous carcinoma components (at least 10%) of each. These are uncommon tumors, representing less than 3% of all lung carcinomas.44
Most patients with adenosquamous carcinoma are older than 50 years, with few (<3%) occurring in patients younger than 40 years. There is a male predominance and most patients are cigarette smokers.45 Tumors are most often peripheral by radiology46 and gross pathology47 (Figure 3, A and B).
These tumors are thought to be more aggressive than single-histology adenocarcinoma or squamous carcinoma,48 with poorer outcome.49,50 The adenocarcinoma and squamous carcinoma are often of similar grade and can be intermixed or distinct areas (Figure 4, A and B).
As in sarcomatoid carcinoma, small-sample diagnosis can once again be a challenge. Use of the term non–small cell carcinoma, not otherwise specified needs to be appended with a recognition that both adenocarcinoma and squamous cell carcinoma are present. In some instances, this may be suggested by immunohistochemistry or mucicarmine staining only. Also interesting are circumstances in which biopsy of the lung/bronchus and sampling of the lymph node yield different histologies (squamous versus adenocarcinoma) in what appears to be only 1 primary tumor. Here, the individual diagnoses of squamous cell carcinoma and adenocarcinoma need to be incorporated into a combined statement that adenosquamous carcinoma is possible.
There are several other histologic challenges in the diagnosis of adenosquamous carcinoma. While undersampling can lead to diagnosis of squamous cell carcinoma, the false assumption that a solid growth pattern equates squamous carcinoma can also be diagnostically erroneous. Immunohistochemistry can reclassify solid pattern tumors as adenocarcinoma, large cell carcinoma, and in some instances, demonstrate adenosquamous histology.51 However, undersampling, as occurs with small samples, remains a problem in up to 20% of cases, even with immunohistochemistry.52 In some instances, tumors that metastasize or grow in the interstitium, including squamous cell carcinoma, can induce a pneumocyte reaction that can be sufficiently atypical as to raise a concern for lepidic pattern adenocarcinoma (Figure 4, C). In most adenosquamous carcinomas the adenocarcinoma and squamous component are of similar grade/differentiation, so this disparity can be a feature against the diagnosis. Another pitfall is the presence of squamous morules (Figure 4, D), which again are of lower grade than the associated adenocarcinoma, for example, in β-catenin–positive, well-differentiated fetal adenocarcinoma.
As so defined, these tumors harbor mutations in EGFR and KRAS at a rate similar to that of adenocarcinoma, with similar regional variation.53–56 For example, in Asian series, rates are lower than that of adenocarcinoma, but still high57 relative to squamous carcinoma. Driver mutations are found in both components.53,55,58 Several series have shown that response rates to EGFR TKI in EGFR-mutated adenosquamous is high, as in EGFR-positive adenocarcinoma.59–61
Molecular studies of squamous cell carcinoma suggest a very low rate of EGFR mutation (2%–5%); it has been suggested that all such cases are either undersampled adenosquamous carcinoma or inappropriately classified.62 However, an increasing number of series (including TCGA data) indicate a variable but roughly 2% to 10% rate of EGFR mutation (including exon 19 deletions and L861Q mutations) in squamous carcinoma.63–65 In some cases, a minor adenocarcinomatous component may be present but may not meet the 10% requirement for the adenosquamous classification. Classification may be important—while adenosquamous carcinomas have a rate of EGFR TKI response similar to adenocarcinoma, these squamous carcinomas with EGFR mutation do not have the same TKI response rate as adenocarcinoma.63,66–68 This raises a question as to whether this truly represents misclassification/undersampling or instead a distinct subset of exclusively or extensively squamous carcinomas with EGFR mutation. It may be that this subgroup has poorer EGFR TKI response rate, and therefore they are not just misclassifications.
Adenosquamous histology can be acquired after EGFR TKI therapy and acquisition of EGFR T790M mutation.69
ADENOSQUAMOUS CARCINOMA VERSUS HIGH-GRADE MUCOEPIDERMOID CARCINOMA
Mucoepidermoid carcinomas also show a combination of gland-forming and squamous histology. However, in contrast to adenosquamous carcinoma, these tumors occur at a relatively younger age, with a median of about 40 years, with about equal sex ratio, and are generally central large airway tumors (Figure 5, A) rather than peripheral nodules.70,71 In this regard, in many cases the location of tumor and the epidemiology is sufficiently distinct to favor mucoepidermoid carcinoma over adenosquamous carcinoma.
Grossly, these are exophytic endobronchial circumscribed masses; with higher grades they are more solid than cystic.72 Histologically, solid sheetlike areas of bland epithelioid cells can be mixed with mucus-filled cysts. Columnar and goblet cells can also be seen amidst intermediate or squamous cells (Figure 5, B). With higher grade, tumor histology becomes more solid (Figure 5, C), requiring mucicarmine stain to highlight intracytoplasmic mucin (Figure 5, D). These areas are p40 positive by immunohistochemistry. However, carcinoma in situ and overtly keratinizing areas are not seen.71
Low-grade tumors are associated with improved outcome relative to high-grade tumors.73 In low-grade tumors, neither the gland-forming nor the solid/squamous component is difficult to identify. However, in higher-grade tumors, the squamous component resembles nonkeratinizing squamous cell carcinoma, and gland-forming elements can be relative sparse.
Immunohistochemistry can be useful. While thyroid transcription factor 1 (TTF1)74,75 and Napsin A are seen in adenosquamous carcinoma, these show negativity in mucoepidermoid carcinoma. If carefully applied, the morphologic and immunohistochemical characterization separates mucoepidermoid carcinoma from adenosquamous carcinoma into distinct clinical and molecular categories. The MECT1-MAML2 fusion (t11;19)(q21;p13) is characteristic of mucoepidermoid carcinoma and is seen in low-grade tumors.76,77 However, high-grade tumors are not consistently found to harbor the translocation, raising the possibility that high-grade tumors are of different origin or are misclassified adenosquamous carcinoma.74,76 This possibility is supported by series that exclude TTF1- or Napsin A–positive tumors. These series report a higher rate of MECT1-MAML2 fusion, independent of grade.75 Other alterations, such as EML-ALK translocation, have been identified in mucoepidermoid carcinoma, but only rarely.78
The classification of mixed histology tumors requires adherence to guidelines and recognition of unusual histologic patterns to avoid misclassification and overinclusion of tumors with more routine histology. This careful approach leads to more consistent clinicopathologic correlations and molecular associations, which have prognostic and predictive significance.
The author has no relevant financial interest in the products or companies described in this article.
Presented in part at the 2017 Pulmonary Pathology Society Biennial Meeting; June 13–16, 2017; Chicago, Illinois.