The New IASLC-ATS-ERS Lung Adenocarcinoma Classification: What the Surgeon Should Know

In 2011, a new histologic classification of lung adenocarcinomas was proposed from a joint working group of the International Association for the Study of Lung Cancer, American Thoracic Society, and European Respiratory Society, based on the recommendation of an international and multidisciplinary panel. This classification proposed a method of comprehensive histologic subtyping (lepidic, acinar, papillary, micropapillary, and solid pattern) based on semiquantitative assessment of histologic patterns (in 5% increments), with the ultimate goal of choosing a single, predominant pattern. Prognostic subsets could then be described for the classification. Patients with completely resected adenocarcinoma in situ and minimally invasive adenocarcinomas experienced low risk of recurrence. Patients with micropapillary or solid predominant tumors have a high risk of recurrence or cancer-related death. Patients with acinar and papillary predominant tumors comprise an intermediate-risk group. Herein, we review the outline of the proposed International Association for the Study of Lung Cancer, American Thoracic Society, and European Respiratory Society classification, a summary of published validation studies of this new classification, and then discuss the key surgical issues; we mainly focused on limited resection as an adequate treatment for early-stage lung adenocarcinomas, as well as preoperative and intraoperative diagnoses. We also review the published studies that identified the importance of histologic subtypes in predicting recurrence, both rates and patterns, in early-stage lung adenocarcinomas. This new classification for the most common type of lung cancer is useful for surgeons, as its implementation would require only hematoxylin-and-eosin histology slides, which is the common type of stain used in hospitals. It can be implemented with routine pathology evaluation and with no additional costs.

Introduction

Lung adenocarcinoma, which is the most frequent histologic subtype of non–small cell lung cancer (NSCLC) encountered by surgeons, is heterogeneous in clinical and radiologic presentation, histologic appearance, surgical outcome, and molecular biological profile.¹ Over the past decade, the single most important factor that has helped determine clinical management and prognosis for patients with lung cancer has been TNM staging.² Although the histologic classification of lung cancer has been periodically revised by the World Health Organization, the last classification, which was published in 2004, had approximately 90% of lung adenocarcinomas classified as a single category—mixed subtype.¹

Although there have been several advances over the past decade in defining the molecular alterations within lung adenocarcinomas, some of which have aided in the development of targeted therapies such as EGFR mutation in patients with advanced disease, none of the molecular patterns have proven useful for a uniform classification system of lung adenocarcinoma in surgically resected tumors. There is mounting evidence that suggests that histologic patterns in lung adenocarcinomas can be used to define prognostically variable subsets. With the pressing need for an update, a new classification for lung adenocarcinomas was developed by a joint working group of the International Association for the Study of Lung Cancer, the American Thoracic Society, and the European Respiratory Society (IASLC-ATS-ERS) in 2011.¹