Introduction Adenocarcinoma is the most common histologic type of lung cancer. To address advances in oncology, molecular biology, pathology, radiology, and surgery of lung adenocarcinoma, an international multidisciplinary classification was sponsored by the International Association for the Study of Lung Cancer, American Thoracic Society, and European Respiratory Society. This new adenocarcinoma classification is needed to provide uniform terminology and diagnostic criteria, especially for bronchioloalveolar carcinoma (BAC), the overall approach to small nonresection cancer specimens, and for multidisciplinary strategic management of tissue for molecular and immunohistochemical studies. Methods An international core panel of experts representing all three societies was formed with oncologists/pulmonologists, pathologists, radiologists, molecular biologists, and thoracic surgeons. A systematic review was performed under the guidance of the American Thoracic Society Documents Development and Implementation Committee. The search strategy identified 11,368 citations of which 312 articles met specified eligibility criteria and were retrieved for full text review. A series of meetings were held to discuss the development of the new classification, to develop the recommendations, and to write the current document. Recommendations for key questions were graded by strength and quality of the evidence according to the Grades of Recommendation, Assessment, Development, and Evaluation approach. Results The classification addresses both resection specimens, and small biopsies and cytology. The terms BAC and mixed subtype adenocarcinoma are no longer used. For resection specimens, new concepts are introduced such as adenocarcinoma in situ (AIS) and minimally invasive adenocarcinoma (MIA) for small solitary adenocarcinomas with either pure lepidic growth (AIS) or predominant lepidic growth with ≤5 mm invasion (MIA) to define patients who, if they undergo complete resection, will have 100% or near 100% disease-specific survival, respectively. AIS and MIA are usually nonmucinous but rarely may be mucinous. Invasive adenocarcinomas are classified by predominant pattern after using comprehensive histologic subtyping with lepidic (formerly most mixed subtype tumors with nonmucinous BAC), acinar, papillary, and solid patterns; micropapillary is added as a new histologic subtype. Variants include invasive mucinous adenocarcinoma (formerly mucinous BAC), colloid, fetal, and enteric adenocarcinoma. This classification provides guidance for small biopsies and cytology specimens, as approximately 70% of lung cancers are diagnosed in such samples. Non-small cell lung carcinomas (NSCLCs), in patients with advanced-stage disease, are to be classified into more specific types such as adenocarcinoma or squamous cell carcinoma, whenever possible for several reasons: (1) adenocarcinoma or NSCLC not otherwise specified should be tested for epidermal growth factor receptor (EGFR) mutations as the presence of these mutations is predictive o...

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- There was discussion and consensus opinion regarding guidelines for (1) distinguishing benign from malignant mesothelial proliferations (both epithelioid and spindle cell lesions), (2) cytologic diagnosis of MM, (3) recognition of the key histologic features of pleural and peritoneal MM, (4) use of histochemical and immunohistochemical stains in the diagnosis and differential diagnosis of MM, (5) differentiating epithelioid MM from various carcinomas (lung, breast, ovarian, and colonic adenocarcinomas, and squamous cell and renal cell carcinomas), (6) diagnosis of sarcomatoid MM, (7) use of molecular markers in the diagnosis of MM, (8) electron microscopy in the diagnosis of MM, and (9) some caveats and pitfalls in the diagnosis of MM. Immunohistochemical panels are integral to the diagnosis of MM, but the exact makeup of panels employed is dependent on the differential diagnosis and on the antibodies available in a given laboratory. Depending on the morphology, immunohistochemical panels should contain both positive and negative markers for mesothelial differentiation and for lesions considered in the differential diagnosis. Immunohistochemical markers should have either sensitivity or specificity greater than 80% for the lesions in question. Interpretation of positivity generally should take into account the localization of the stain (eg, nuclear versus cytoplasmic) and the percentage of cells staining (>10% is suggested for cytoplasmic and membranous markers). Selected molecular markers are now being used to distinguish benign from malignant mesothelial proliferations. These guidelines are meant to be a practical diagnostic reference for the pathologist; however, some new pathologic predictors of prognosis and response to therapy are also included.

The new International Association for the Study of Lung Cancer/American Thoracic Society/European Respiratory Society lung adenocarcinoma classification provides, for the first time, standardized terminology for lung cancer diagnosis in small biopsies and cytology; this was not primarily addressed by previous World Health Organization classifications. Until recently there have been no therapeutic implications to further classification of NSCLC, so little attention has been given to the distinction of adenocarcinoma and squamous cell carcinoma in small tissue samples. This situation has changed dramatically in recent years with the discovery of several therapeutic options that are available only to patients with adenocarcinoma or NSCLC, not otherwise specified, rather than squamous cell carcinoma. This includes recommendation for use of special stains as an aid to diagnosis, particularly in the setting of poorly differentiated tumors that do not show clear differentiation by routine light microscopy. A limited diagnostic workup is recommended to preserve as much tissue for molecular testing as possible. Most tumors can be classified using a single adenocarcinoma marker (eg, thyroid transcription factor 1 or mucin) and a single squamous marker (eg, p40 or p63). Carcinomas lacking clear differentiation by morphology and special stains are classified as NSCLC, not otherwise specified. Not otherwise specified carcinomas that stain with adenocarcinoma markers are classified as NSCLC, favor adenocarcinoma, and tumors that stain only with squamous markers are classified as NSCLC, favor squamous cell carcinoma. The need for every institution to develop a multidisciplinary tissue management strategy to obtain these small specimens and process them, not only for diagnosis but also for molecular testing and evaluation of markers of resistance to therapy, is emphasized.

Morphometric procedures were used to determine the number of cells, cell volume, cell diameter, and surface areas of the airways in human and rat lungs. Nuclear sizes of epithelial cells from human bronchi were significantly larger than other lung cell nuclei. The average volume of human ciliated cell nuclei was 310 +/- 30 microns 3 and 167 +/- 12 microns 3 in bronchi and bronchioles, respectively. The smaller nuclei of human bronchioles were comparable to those of alveolar cells. In the pseudostratified epithelium of human bronchi, basal cells had a large surface area in contact with the basement membrane (51.3 +/- 4.6 microns 2 per cell) when compared with ciliated (1.1 +/- 0.1 microns 2), goblet (7.6 +/- 1.2 microns 2), or other secretory cells (12.0 +/- 2.1 microns 2). In the first four airway generations distal to the trachea, basal cells account for 30% of the cells in human airway epithelium and 2% of the cells in rat airway epithelium. Total airway surface area from trachea to bronchioles was 2,471 +/- 320 and 27.2 +/- 1.7 cm2 in human and rat lungs, respectively. These direct measurements of airway surface area are less than half of the estimates based on current lung models. The total number of airway epithelial cells were 10.5 x 10(9) for human and 0.05 x 10(9) for rat lungs. For both species, there were 18 times more alveolar cells than bronchial epithelial cells.

The solitary pulmonary nodule is a common radiologic abnormality that is often detected incidentally. Although most solitary pulmonary nodules have benign causes, many represent stage I lung cancers and must be distinguished from benign nodules in an expeditious and cost-effective manner. Evaluation of specific morphologic features of a solitary pulmonary nodule with conventional imaging techniques can help differentiate benign from malignant nodules and obviate further costly assessment. Small size and smooth, well-defined margins are suggestive of but not diagnostic for benignity. Lobulated contour as well as an irregular or spiculated margin with distortion of adjacent vessels are typically associated with malignancy. There is considerable overlap in the internal characteristics (eg, attenuation, cavitation, wall thickness) of benign and malignant nodules. The presence of intranodular fat is a reliable indicator of a hamartoma. The presence and pattern of calcification can also help differentiate benign from malignant nodules. Computed tomography (CT) (particularly thin-section CT) is 10-20 times more sensitive than standard radiography and allows objective, quantitative assessment of calcification. Initial evaluation often results in nonspecific findings, in which case nodules are classified as indeterminate and require further evaluation to exclude malignancy. Growth rate assessment, Bayesian analysis, contrast material-enhanced CT, positron emission tomography, and transthoracic needle aspiration biopsy can be useful in this regard.

A new World Health Organization (WHO) Classification of Tumors of the Pleura has recently been published. While the histologic classification of pleural malignant mesothelioma remains the same in the 2015 WHO classification as it was in the 2004 classification, multiple new observations have been recorded. First, more detailed study has been performed of histologic subtyping of epithelioid mesothelioma. In particular, it has been recognized that the pleomorphic subtype is associated with a poor prognosis, similar to that of sarcomatoid malignant mesothelioma. Second, there is improved understanding of the role of immunohistochemistry in distinguishing mesothelioma from carcinomas of various sites. Third, the criteria for distinguishing malignant mesothelioma from reactive mesothelial proliferations has been further refined. Fourth, additional studies of sarcomatoid mesothelioma have defined the frequency and spectrum of various histologic and immunohistochemical features, including heterologous elements. Finally, pleural well-differentiated papillary mesotheliomas are better defined and cases with invasive foci are recognized. In addition, several promising observations in mesothelioma pathology and genetics have been made in the past decade. These are now the subject of further investigation to determine if they can be validated in ways that will significantly impact clinical practice. These include a preliminary study of grading, suggesting that nuclear atypia and mitotic count are independent prognostic markers. The discovery of inactivating mutations in the BRCA1-associated protein 1 gene in sporadic and hereditary mesothelioma has opened up a variety of novel molecular, clinical, and diagnostic investigations. One possible diagnostic application includes the setting of separating mesothelioma from reactive mesothelial proliferations, where it may play a role in conjunction with p16 FISH. Another useful discovery was that the NAB2-STAT6 fusion is characteristic of solitary fibrous tumors. This led to development of a STAT6 antibody that is a reliable immunohistochemical marker for solitary fibrous tumors. Genetic studies also led to the finding that WWTR1-CAMTA1 fusions are useful diagnostic markers for epithelioid hemangioendotheliomas, which can present as pleural-based masses. Finally, desmoid type fibromatosis, a locally aggressive tumor that can present in the pleura, has been shown to frequently have CTNNB1 gene mutations and express β-catenin by immunohistochemistry.

Histological subtyping of pulmonary adenocarcinoma has recently been updated based on predominant pattern, but data on reproducibility are required for validation. This study first assesses reproducibility in subtyping adenocarcinomas and then assesses further the distinction between invasive and non-invasive (wholly lepidic) pattern of adenocarcinoma, among an international group of pulmonary pathologists. Two ring studies were performed using a micro-photographic image-based method, evaluating selected images of lung adenocarcinoma histologic patterns. In the first study, 26 pathologists reviewed representative images of typical and ‘difficult’ histologic patterns. A total number of scores for the typical patterns combined (n = 94) and the difficult cases (n = 21) were 2444 and 546, respectively. The mean kappa score (± s.d.) for the five typical patterns combined and for difficult cases were 0.77 ± 0.07 and 0.38 ± 0.14, respectively. Although 70% of the observers identified 12–65% of typical images as single pattern, highest for solid and least for micropapillary, recognizing the predominant pattern was achieved in 92–100%, of the images except for micropapillary pattern (62%). For the second study on invasion, identified as a key problem area from the first study, 28 pathologists submitted and reviewed 64 images representing typical as well as ‘difficult’ examples. The kappa for typical and difficult cases was 0.55 ± 0.06 and 0.08 ± 0.02, respectively, with consistent subdivision by the same pathologists into invasive and non-invasive categories, due to differing interpretation of terminology defining invasion. In pulmonary adenocarcinomas with classic morphology, which comprise the majority of cases, there is good reproducibility in identifying a predominant pattern and fair reproducibility distinguishing invasive from in-situ (wholly lepidic) patterns. However, more precise definitions and better education on interpretation of existing terminology are required to improve recognition of purely in-situ disease, this being an area of increasing importance.