The "WHO Blue Books" refer to the World Health Organization Classification of Tumours series. These are authoritative and concise reference books that classify tumours based on expert consensus and multidisciplinary input. They are published by the World Health Organization's International Agency for Research on Cancer (IARC) and are considered the gold standard for tumour diagnosis worldwide

. Key points about the WHO Blue Books:

• The series started in 1956, with the first editions published between 1967 and 1981 under the name International Histological Classification of Tumors.
• The current series is in its fifth edition (started in 2019), featuring updated classifications with a more systematic structure, including histopathology, molecular pathology, staging, and diagnostic criteria

• Each volume focuses on a major tumour group (e.g., digestive system, breast, soft tissue, bone, central nervous system) and covers up to 300 tumour types per volume.
• The editorial board, mostly practicing pathologists and specialists, regularly updates the classifications to reflect the latest scientific knowledge

• The books are available in print and online, with the fifth edition being the first to appear fully online with enhanced features like images and hyperlinks

• The WHO Blue Books serve a wide range of professionals involved in cancer care and research, including pathologists, oncologists, radiologists, geneticists, and epidemiologists

In summary, the WHO Blue Books are a comprehensive and internationally recognized series of tumour classification manuals published by WHO/IARC, essential for standardized cancer diagnosis and research globally