Bronchioloalveolar Carcinoma and Lung Adenocarcinoma: The Clinical Importance and Research Relevance of the 2004 World Health Organization Pathologic Criteria

Introduction:Advances in the pathology and computed tomography (CT) of lung adenocarcinoma and bronchioloalveolar carcinoma (BAC) have demonstrated important new prognostic features that have led to changes in classification and diagnostic criteria.Methods:The literature and a set of cases were reviewed by a pathology/CT review panel of pathologists and radiologists who met during a November 2004 International Association for the Study of Lung Cancer/American Society of Clinical Oncology consensus workshop in New York. The group addressed the question of whether sufficient data exist to modify the 2004 World Health Organization (WHO) classification of adenocarcinoma and BAC to define a “minimally invasive” adenocarcinoma with BAC. The problems of diffuse and/or multicentric BAC and adenocarcinoma were evaluated.Results:The clinical concept of BAC needs to be reevaluated with careful attention to the new 2004 WHO criteria because of the major clinical implications. Existing data indicate that patients with solitary, small, peripheral BAC have a 100% 5-year survival rate. The favorable prognostic impact of the restrictive criteria for BAC is already being detected in major epidemiologic data sets such as the Surveillance Epidemiology and End-Results registry. Most lung adenocarcinomas, including those with a BAC component, are invasive and consist of a mixture of histologic patterns. Therefore, they are best classified as adenocarcinoma, mixed subtype. This applies not only to adenocarcinomas with a solitary nodule presentation but also to tumors with a diffuse/multinodular pattern. The percentage of BAC versus invasive components in lung adenocarcinomas seems to be prognostically important. However, at the present time, a consensus definition of “minimally invasive” BAC with a favorable prognosis was not recommended by the panel, so the 1999/2004 WHO criteria for BAC remain unchanged. In small biopsy specimens or cytology specimens, recognition of a BAC component is possible. However, it is not possible to exclude an invasive component. The diagnosis of BAC requires thorough histologic sampling of the tumor.Conclusion:Advances in understanding of the pathology and CT features of BAC and adenocarcinoma have led to important changes in diagnostic criteria and classification of BAC and adenocarcinoma. These criteria need to be uniformly applied by pathologists, radiologists, clinicians, and researchers. The 2004 WHO classification of adenocarcinoma is readily applicable to research studies, but attention needs to be placed on the relative proportion of the adenocarcinoma subtypes. Other recently recognized prognostic features such as size of scar, size of invasive component, or pattern of invasion also seem to be important. More work is needed to determine the most important prognostic pathologic features in lung adenocarcinoma