The WHO 2015 classification of lung tumours: Evolution of tumour classification in lung cancer

Abstract

There are many important changes in the 2015 WHO classification of lung tumours, reflecting the numerous advances in tumour genetics and therapy over the past decade.1 A classification system for small biopsies and cytology is provided for the first time, with emphasis on integration of molecular testing and usage of a limited panel of immunohistochemistry when needed. Adenocarcinoma classification has changed significantly with definitions for adenocarcinoma in situ (AIS) and minimally invasive adenocarcinoma (MIA), and discontinuation of the term bronchioloalveolar carcinoma. Invasive adenocarcinomas are classified according to the predominant pattern, shown to have prognostic significance and also to predict response to adjuvant chemotherapy. These changes will also likely impact on the next TNM staging system in terms of multiple primary tumours and measurement of tumour invasive size. Large cell carcinoma is now restricted to resected tumours that lack clear morphological and immunohistochemical differentiation, with reclassification of those that do to solid adenocarcinoma and non-keratinising squamous cell carcinoma. Squamous cell carcinoma classification is simplified to keratinising, non-keratinising and basaloid subtypes, with the non-keratinising tumours ideally requiring immunohistochemical confirmation. Criteria for diagnosing neuroendocrine tumours remain essentially unchanged but these tumours are now grouped in one category. There are many important changes in the 2015 WHO classification of lung tumours, reflecting the numerous advances in tumour genetics and therapy over the past decade.1 A classification system for small biopsies and cytology is provided for the first time, with emphasis on integration of molecular testing and usage of a limited panel of immunohistochemistry when needed. Adenocarcinoma classification has changed significantly with definitions for adenocarcinoma in situ (AIS) and minimally invasive adenocarcinoma (MIA), and discontinuation of the term bronchioloalveolar carcinoma. Invasive adenocarcinomas are classified according to the predominant pattern, shown to have prognostic significance and also to predict response to adjuvant chemotherapy. These changes will also likely impact on the next TNM staging system in terms of multiple primary tumours and measurement of tumour invasive size. Large cell carcinoma is now restricted to resected tumours that lack clear morphological and immunohistochemical differentiation, with reclassification of those that do to solid adenocarcinoma and non-keratinising squamous cell carcinoma. Squamous cell carcinoma classification is simplified to keratinising, non-keratinising and basaloid subtypes, with the non-keratinising tumours ideally requiring immunohistochemical confirmation. Criteria for diagnosing neuroendocrine tumours remain essentially unchanged but these tumours are now grouped in one category.