A pathologist experienced in thoracic oncology is an essential member of the thoracic team. Surgical resection related to lung cancer and other pulmonary pathology accounts for the largest proportion of current thoracic practice. The goals of pathologic analysis of surgical lung specimens are to classify the lung cancer, determine the extent of its invasion (i.e., pleural, lymphovascular, soft tissue, or chest wall), and establish the status of the surgical margins for cancer involvement.¹ Accurate disease identification and staging are of pinnacle importance to the decision-making process and influence the diagnosis (benign or malignant), course of treatment, selection of optimal surgical approach, and pursuit of appropriate adjuvant and neoadjuvant therapies such as chemotherapy, radiation, and other innovative approaches to treatment. Further, determination of the specific molecular abnormalities of the tumor is critical for predicting sensitivity or resistance to a growing number of drugable targets primarily tyrosine kinase inhibitors (TKIs).^2,3 After a malignancy has been identified, the pathologist must determine whether the tumor is primary or metastatic. Most tumors found in the lung represent metastatic foci from distant primaries, such as breast and colon cancer, as opposed to a primary lung malignancy. While the pathologic features of metastatic versus primary adenocarcinoma may be similar, for example, the treatment course is not. Immunohistochemistry (IHC) is required to make the distinction and has proved to be an invaluable diagnostic adjunct. Primary malignant tumors of the lung are most often of epithelial or mesenchymal origin. The epithelial tumors are broadly divided into small-cell lung cancer (SCLC) and non–small-cell lung cancer (NSCLC). NSCLC is further classified as squamous cell carcinoma (SCC), adenocarcinoma, and large-cell carcinoma (LCC).⁴ The World Health Organization (WHO) tumor classification system has historically provided the foundation for the classification of lung tumors, including histologic types, clinical features, staging considerations as well as the molecular, genetic, and epidemiologic aspects of lung cancer.^4–6

The pathologist plays a fundamental role in the preoperative, intraoperative, and postoperative evaluation. The preoperative evaluation includes examination of one of the following specimens: bronchial brushings, bronchial washings, fine-needle aspiration biopsy, core needle biopsy, endobronchial biopsy, and transbronchial biopsy. Because lung tumors demonstrate a great deal of heterogeneity, accurate classification depends on sampling technique: If the pathology sample is limited, sometimes the only categorization that can be made is the distinction between NSCLC and SCLC. The generic term “non–small-cell lung cancer (NSCLC)” should be avoided as a single diagnostic term. In small biopsy samples of poorly differentiated carcinomas where IHC is used, the following terms are acceptable: “NSCLC favor adenocarcinoma” or “NSCLC favor SCC.”^6,7 Mutational testing (e.g., epidermal growth factor receptor [EGFR], anaplastic lymphoma kinases [ALKs]) should be performed in this setting. Lymph node status is one of the most important prognostic features in patients with NSCLC.^8,9 Since mediastinoscopy with pathologic examination of lymph nodes remains the “gold standard” for the evaluation of lymph node status in patients with NSCLC, mediastinal lymph nodes are sampled during the preoperative evaluation and provide information important to staging and therapeutic options.^10–13

The intraoperative evaluation of the surgical pathology specimen is performed by frozen-section examination, which can be analyzed immediately, and findings are communicated to the operating room. Lobectomy or pneumonectomy specimens are routinely evaluated intraoperatively to determine the status of the surgical resection margin, to diagnose incidental nodules discovered at the time of surgery, and to evaluate regional lymph nodes.

The postoperative evaluation reveals pathologic characteristics necessary for classification of tumor type, staging, and prognostic factors. The pathology diagnostic report should include the histologic classification as described by the WHO for carcinomas of the lung with squamous morphology, neuroendocrine differentiation, and other variant carcinomas. The recently published classification of adenocarcinoma should be used for this tumor subtype in resection specimens and small biopsy specimens.⁶ Use of bronchioloalveolar carcinoma (BAC) terminology is strongly discouraged. The parameters considered in the surgical pathology report are histologic type, histopathologic grade, visceral pleural invasion (Fig. 69-1), venous/lymphatic vessel invasion (Fig. 69-2), and involvement of mediastinal lymph nodes and TNM stage groupings.¹⁴

Although the WHO pathologic classification of lung tumors published in 2004 was the basis for categorizing lung tumors,⁵ there were a number of pitfalls that made it difficult to utilize the classification system: (1) the term bronchioloalveolar carcinoma (BAC) was used for widely divergent clinical, radiologic, pathologic, and molecular subsets of patients, (2) the “mixed subtype” category accounted for more than 90% of all resected lung adenocarcinomas, despite great heterogeneity in clinical, radiologic, pathologic, and molecular features, and (3) the WHO classification was based primarily on resection specimens, despite the fact that 70% of patients were presenting in advanced stages where the only type of tissue samples were obtained by biopsy or cytology.

The recent tumor classification system issued in 2011,⁶ addresses the inadequacies of the 2004 classification and provides the foundation for tumor diagnosis and patient therapy and a critical basis for epidemiologic, molecular, and clinical studies. The most important changes in the 2004 revised classification system were (1) eliminate the term, bronchioloalveolar carcinoma, (2) define the term adenocarcinoma in situ (AIS), (3) define the term minimally invasive adenocarcinoma (MIA), (4) revive the term lepidic, (5) promote comprehensive histologic subtyping, (6) emphasize and introduce the term micropapillary carcinoma, (7) detach the term mucinous adenocarcinoma, and (8) discourage use of the term NSCLC and subclassify the tumors in as much detail as possible.

For the first time in the history of lung cancer evaluation, the classification of lung cancer will be applicable to either (a) resected specimens or (b) small biopsy and cytology specimens. In the revised WHO classification system, (1) lesions attributed to preinvasive lung cancer now include atypical adenomatous hyperplasia (AAH) and AIS (formerly BAC); (2) the term bronchioloalveolar carcinoma (BAC) has been completely eliminated; (3) two new entities, both with favorable prognoses, were added to the category of adenocarcinoma, namely, AIS and MIA; and (4) the formerly known mucinous BAC category was renamed invasive mucinous adenocarcinoma and “shifted” to the category of “Variants of Invasive Adenocarcinoma” (Table 69-1). Since adenocarcinomas are extremely heterogeneous, extensive sampling is necessary to identify each of these histologies in the surgical pathology specimen. The surgical pathology report always should include the histologic classification published by the WHO for carcinomas of the lung (Table 69-2).

In contrast to surgical resections, the biopsy or cytology specimens have a limited amount of tissue and therefore the strategy and prioritization should focus on molecular tests that could identify potentially drugable molecular targets. For tumors that show clear morphologic features of adenocarcinoma or SCC, the standard terms are used. However, if the tumor only shows a carcinoma with no clear squamous or glandular features (NSCLC-NOS), a minimal immunohistochemical workup is recommended using a single adenocarcinoma marker and squamous marker. At the moment, the best markers for adenocarcinoma and SCC are TTF-1 and p63, respectively.¹⁵ One of the key aspects that potentially impacts radiologists is the need to obtain sufficient tissue not only for diagnosis, but also for molecular studies. This requires a strategic and multidisciplinary approach so the method of biopsy results in either a core biopsy or a cell block from tissue samples obtained for cytology.

Adenocarcinoma in situ (AIS), formerly BAC, is an important subtype of pulmonary adenocarcinoma. This cancer has received increasing attention in recent years owing to its increasing incidence and rate of sensitivity to epidermal growth factor—TKIs.¹⁶ AIS is a primary lung tumor with a peripheral location, well-differentiated cytology, lepidic growth pattern and a tendency for both aerogenous and lymphatic spread. The key feature is preservation of the underlying architecture of the lung with no invasion.

Minimally invasive adenocarcinoma (MIA) was introduced to define patients with a near 100% 5-year disease-free survival. It is defined as a lepidic predominant tumor measuring 3 cm or less that has an invasive component of 5 mm or less.¹⁷ MIA is characterized by a combination of ground glass opacity (GGO) and a central solid opacity, with the solid component measuring 5 mm or less. Nonmucinous MIA (Fig. 69-3), is more common than mucinous MIA and most often appears as a GGO. Mucinous MIA (Fig. 69-4) appears radiologically as a solid or part-solid nodule.

Invasive adenocarcinoma changes were inserted in the classification of invasive adenocarcinomas. Overtly invasive adenocarcinomas are classified according to the predominant subtype after the use of comprehensive histologic subtyping to estimate the percentages of the various components in a semiquantitative fashion in 5% to 10% increments. The term lepidic predominant adenocarcinoma consists of mixed subtype tumors containing a predominant lepidic growth pattern of type II pneumocytes and/or Clara cells that have an invasive component >5 mm. A micropapillary predominant subtype is added because it has been recognized as a poor prognostic category. Signet ring and clear cell carcinoma subtypes are now recorded as cytologic features whenever present with a comment about the percentage identified.

Tumors with Neuroendocrine Morphology

Neuroendocrine tumors of the lung are a distinctive subset of lung cancers characterized by varying degrees of neuroendocrine morphologic, immunohistochemical, and ultrastructural features.¹⁸ This category includes a wide spectrum of tumor types: low-grade typical carcinoid (TC) (Fig. 69-5), intermediate-grade atypical carcinoid (AC) (Fig. 69-6), and two high-grade tumors, large-cell neuroendocrine lung carcinoma (LCNEC) (Fig. 69-7) and SCLC (Fig. 69-8).¹⁹ Accurate classification of neuroendocrine tumors has prognostic importance. The grade of malignancy of neuroendocrine tumors progresses in the following order: TC, AC, LCNEC, and SCLC.¹⁹ No prognostic difference was noted between LCNEC and SCLC. The carcinoid nomenclature is preferred by the WHO over terms such as well-differentiated neuroendocrine carcinoma because it provides continuity with established terminology familiar to clinicians.⁵ In the 2004 WHO classification, TC and AC are categorized together under the heading of carcinoid tumors; LCNEC is listed as a subtype of LCC, and SCLC is retained as an independent category. Histologically, the neuroendocrine features consist of an organoid or trabecular growth pattern, peripheral palisading of tumor cells around the periphery of tumor nests, and the formation of rosette structures.

TC is defined as a neuroendocrine tumor with fewer than 2 mitoses per 2 mm² and no necrosis (Fig. 69-5). AC is defined as a neuroendocrine tumor that meets one of the two criteria: 2 to 10 mitoses per 2 mm² or necrosis (Fig. 69-6). In contrast to the high-grade neuroendocrine tumors, TC and AC do not occur in combination with other types of carcinoma. The number of mitoses and necrosis may be present only focally within a given tumor. Therefore, accurate classification of carcinoid tumors into TC or AC may not be possible in limited biopsy specimens with scant diagnostic material, and a definite diagnosis may require larger fragments of tumor. In these situations, it is recommended that small biopsies be signed as “carcinoid tumor” and the appropriate classification be performed on thorough examination of the resected specimens.^5,18

LCNEC is defined as a neuroendocrine tumor with more than 10 mitoses per 2 mm² and cytologic features of LCC (Fig. 69-7). These features include cells with polygonal shape, abundant cytoplasm, and prominent nucleoli. Evidence of neuroendocrine differentiation must be demonstrated by performing IHC for the specific neuroendocrine markers chromogranin and synaptophysin.⁴ Only tumors that show both neuroendocrine morphology and positive staining should be classified as LCNEC. It is important to note that up to 20% of conventional adenocarcinoma, small-cell carcinoma (SCC), or LCC will stain with neuroendocrine markers. Such tumors have been designated as NSCLC with neuroendocrine differentiation.

SCLC is defined as a neuroendocrine tumor with more than 10 mitoses per 2 mm² and small-cell cytologic features (Fig. 69-8). Cells have an oval or vaguely spindled shape and have scant cytoplasm. Nuclei are hyperchromatic and have absent or very small nucleoli (Fig. 69-8). Crush artifact may be prominent on small biopsies, but this is not pathognomonic for the diagnosis of SCC. In larger core biopsies or resected specimens, the cells may appear slightly larger than in a transbronchial biopsy and may have distinct cytoplasm. Numerous prominent nucleoli and large cells should not be seen.

LCNEC and SCLC may occur in combination with other NSCLCs as well as with each other. Such tumors are termed combined LCNEC, combined SCLC, and combined SCLC/LCNEC, respectively.^4,20 While the two high-grade neuroendocrine tumors show numerous similarities, they are retained in separate classifications because LCNEC currently has not been shown to respond to chemotherapy in the same fashion as SCLC. Surgery is the currently preferred treatment for LCNEC, although further studies are ongoing.^21–24

Immunohistochemical Staining

Although the concordance is generally good between the histologic subtype and the immunophenotype seen in small biopsies compared with surgical resection specimens, caution is advised in attempting to subtype small biopsies with limited material or cases with an ambiguous immunophenotype. IHC should be used to differentiate primary pulmonary adenocarcinoma from SCC or LCC, from metastatic carcinoma, and from malignant mesothelioma; and to determine whether neuroendocrine differentiation is present.^15,25,26 Limited use of IHC studies in small tissue samples is strongly recommended, thereby preserving critical tumor tissue for molecular studies particularly in patients with advanced stage disease. A limited panel of p63 and TTF-1 should suffice for most diagnostic problems.¹⁵

Differentiation between Primary Pulmonary Adenocarcinoma and Metastatic Adenocarcinoma

The morphologic features of primary adenocarcinoma of the lung may be similar to the features of an adenocarcinoma that is metastatic from a distant primary site. Although the presence of multiple nodules often leads to the presumptive diagnosis of metastases, multifocal adenocarcinoma is not rare and needs to be distinguished from metastases. Furthermore, patients with a solitary pulmonary nodule may have metastatic adenocarcinoma to the lung as the first presentation of disease.