Introduction to 2021 WHO Classification of Thoracic Tumors

The WHO classification system of tumors is adopted worldwide and is fundamental to how tumor or tumor-like conditions are clinically managed or treated. It is considered the international accepted standard for the diagnosis of tumors for clinical care of patients and education and research into the etiology, prevention, and treatment of cancers. The International Agency for Research on Cancer (IARC) has been responsible for the publication of the WHO classification of tumors books, often known as the Blue Book, since the third edition of the WHO Classification of Tumors of the Lung, Pleura, Thymus, and Heart was published in 2004,1 and the fourth edition in 2015.