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.

After a malignancy has been identified, the pathologist must determine whether the tumor is primary versus 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 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, adenocarcinoma, and large cell carcinoma (LCC).²

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. Lymph node status is one of the most important prognostic features in patients with NSCLC.^3,4 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.^5–8

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 parameters considered in the surgical pathology report are histologic type, histopathologic grade, visceral pleural invasion (Fig. 9-1), venous/lymphatic vessel invasion (Fig. 9-2), and extracapsular extension of positive mediastinal lymph nodes, TNM stage, and stage grouping.⁹

The World Health Organization's (WHO) pathologic classification of lung tumors was revised in 1999, 18 years after the previous edition. The 1999 revision contained numerous changes and revisions to account for the vast progress in lung cancer research in the preceding 18 years.¹⁰ This tumor classification system provides the foundation for tumor diagnosis and patient therapy and a critical basis for epidemiologic and clinical studies.¹¹ The most important changes in the 1999 revised classification system were (1) clarification of the preinvasive lesions in lung neoplasia, (2) reclassification of adenocarcinoma, (3) addition of two new entities to the category of large cell neuroendocrine carcinoma (LCNEC), and (4) addition of a new category called pleomorphic carcinoma.

In the revised WHO classification system, (1) lesions attributed to preinvasive lung cancer included squamous dysplasia, atypical adenomatous hyperplasia, and diffuse idiopathic pulmonary neuroendocrine cell hyperplasia; (2) the definition of bronchioloalveolar carcinoma (BAC) was restricted to noninvasive tumors; (3) two new entities, both with unfavorable prognoses, were added to the category of large cell carcinoma, that is, LCNEC and basaloid carcinoma; and (4) pleomorphic carcinoma was defined as a group of poorly differentiated non–small cell carcinomas with heterogeneous elements, such as spindle cells and/or giant cells. Carcinosarcoma and pulmonary blastoma also were included in this category.

Extensive sampling is necessary to correctly identify each of these elements of the surgical pathology specimen. In 2004, a revised WHO classification was created after a consensus meeting in Lyon, France. The 2004 modification retains much of the same nomenclature as the 1999 edition and maintains the same major diagnostic groups.^2,12 The surgical pathology report always should include the histologic classification published by the WHO for carcinomas of the lung¹³ (Table 9-1).

*Most of the lung neoplasms are malignant epithelial tumors.

BRONCHIOLOALVEOLAR CARCINOMA

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–tyrosine kinase inhibitors.¹⁴

Since 1999, the WHO classification has been more restrictive in the definition of BAC and includes only noninvasive tumors, where the neoplastic cells spread out along preexisting alveolar structures (lepidic spread).¹³ Pure BAC requires absence of invasion of stroma, pleura, or lymphatic spaces.¹¹ Tumors that include some, but not all, of these features are classified as adenocarcinoma of mixed type with a predominant BAC pattern.¹⁵

BAC is divided into three subtypes: mucinous (Fig. 9-3), nonmucinous (Fig. 9-4), and a mixed mucinous and nonmucinous, or indeterminate, form.¹³ As with conventional lung adenocarcinoma, nonmucinous BAC expresses thyroid transcription factor 1 (TTF-1). Mucinous BACs, however, may have an aberrant immunophenotype: They express CK20 but reportedly lack TTF-1 and CK7 expression.¹⁶

Both subtypes of BAC should be distinguished morphologically from primary adenocarcinoma of the conventional type, metastatic adenocarcinoma, and other benign and/or reactive conditions. As with pulmonary adenocarcinoma of the conventional type, BACs are usually CK7+ and CK20– and therefore distinguishable from CK7– and CK20+ metastatic adenocarcinoma of the colorectum. However, mucinous BAC is often CK7+/CK20+.¹⁷ CDX-2, which is a highly sensitive and specific marker of adenocarcinomas of intestinal origin, helps to distinguish mucinous BAC from metastatic primary gastrointestinal cancers.

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.¹² This category includes a wide spectrum of tumor types: low-grade typical carcinoid (TC) (Fig. 9-5), intermediate-grade atypical carcinoid (AC) (Fig. 9-6), and two high-grade tumors, LCNEC (Fig. 9-7) and SCLC (Fig. 9-8).¹⁸ 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.¹⁸ 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.¹³ 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 (see Fig. 9-5). AC is defined as a neuroendocrine tumor that meets one of the two criteria: 2-10 mitoses per 2 mm² or necrosis (see Fig. 9-6). 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.^12,13

LCNEC is defined as a neuroendocrine tumor with more than 10 mitoses per 2 mm² and cytologic features of LCC (see Fig. 9-7). These features include cells with polygonal shape, abundant cytoplasm, and prominent nucleoli. Evidence of neuroendocrine differentiation must be demonstrated by performing immunohistochemistry for the specific neuroendocrine markers chromogranin and synaptophysin.² 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 non–small cell lung carcinoma with neuroendocrine differentiation.

SCLC is defined as a neuroendocrine tumor with more than 10 mitoses per 2 mm² and small cell cytologic features (see Fig. 9-8). Cells have an oval or vaguely spindled shape and have scant cytoplasm. Nuclei are hyperchromatic and have absent or very small nucleoli (see Fig. 9-8). 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.¹⁹ 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 study of this relatively newly defined tumor is ongoing.^20–23

Immunohistochemical Staining

Immunohistochemistry is used routinely in the pathology laboratory for the diagnosis of lung tumors. Immunostains are used (1) to differentiate primary pulmonary adenocarcinoma from metastatic adenocarcinoma to the lung, (2) to distinguish adenocarcinoma from malignant mesothelioma, and (3) to determine the neuroendocrine status of tumors.

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.

TTF-1 is a homeodomain-containing transcription factor that regulates tissue-specific expression of surfactant apoprotein A, surfactant apoprotein B, surfactant apoprotein C, Clara cell antigen, and T1. TTF-1 is very important in distinguishing primary from metastatic adenocarcinoma because most of the cases of primary carcinomas are positive, whereas metastatic adenocarcinoma to lung is virtually always TTF-1-negative (Fig. 9-9A ). Lung cancer subtypes have different TTF-1 expression: 75% positive in adenocarcinoma, 85% positive in SCC, and rarely in the squamous cell and LCC.

Pulmonary adenocarcinoma of the lung is usually CK7+ and CK20– and therefore distinguishable from CK7– and CK20+ metastatic adenocarcinoma of the colorectum. CDX-2 is a highly specific and sensitive marker for metastatic gastrointestinal malignancies that could be used to differentiate these gastric entities from primary lung tumors (Fig. 9-9B ).

Although squamous cell carcinoma (Fig. 9-10) is usually p63-positive, there is no marker to date that is able to distinguish between primary and metastatic squamous cell carcinoma. However, p63 is an important immunostain in cases of poorly differentiated NSCLC, where the distinction between adenocarcinoma and squamous cell carcinoma is virtually impossible to make on the basis of hematoxylin and eosin-stained slides alone (Fig. 9-11).

DETERMINING THE NEUROENDOCRINE STATUS OF TUMORS

Both chromogranin (reacts with cytoplasmic neuroendocrine granules) and synaptophysin (reacts with a cell membrane glycoprotein) are used to diagnose the neuroendocrine tumors of the lung. All typical and atypical carcinoid tumors stain with chromogranin and synaptophysin, whereas SCC is negative in 25% of cases (Figs. 9-12 and 9-13). Neuroendocrine markers are valuable in diagnosing NSCLC with neuroendocrine differentiation,a specific group of poorly differentiated tumors without a neuroendocrine morphology.

DISTINGUISHING BETWEEN MALIGNANT MESOTHELIOMA AND LUNG ADENOCARCINOMA

Immunohistochemistry is most valuable in distinguishing between malignant mesothelioma and lung adenocarcinoma. A panel of four markers, two positive in mesothelioma and two negative in mesothelioma (but positive in adenocarcinoma), is used routinely in the pathology laboratory. The stains that are negative in mesothelioma but positive in adenocarcinoma include carcinoembryonic antigen, B72.3, Ber-EP4, and MOC31. The stains that are sensitive and specific for mesothelioma are WT-1, calretinin, D2-40, and cytokeratin 5/6 (Figs. 9-14, 9-15, and 9-16).

SUMMARY

Surgical excision remains the only therapeutic modality that can cure selected lung cancer patients. Pathologists play an important role in the surgical management of patients with lung cancer from preoperative diagnosis and staging, to intraoperative evaluation of the extent of distant disease and margin status, to postoperative assessment of tumor genetic alterations. They offer guidance in selecting appropriate surgical therapies and chemotherapeutic regimens, as well as in identifying morphologic and molecular prognostic markers and predictors of response to therapeutic agents. They are expected to provide accurate diagnosis and classification of the patient's lung cancer. Most important, pathologists are required to provide pathologic staging information in lung resection specimens, surgical resection margin status, and information about lung cancer subtype.

Tumor stage, as determined by current American Joint Committee on Cancer guidelines, is considered to be one of the most important prognostic factors for patients with lung neoplasms. Frozen-section examination of mediastinal lymph nodes obtained by mediastinoscopy is used routinely to determine whether NSCLC patients will undergo tumor excision. Determination of the pT and pN status of the resected surgical pathology specimen determines whether the patient will be treated with postoperative chemotherapy and/or radiation therapy. Furthermore, classification algorithms based on the presence of genetic alterations found in lung cancer can be used to identify drugs or therapeutic agents targeting these alterations.

REFERENCES