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The book that decides what a breast tumour is called. Its fifth edition (2019) is the one UK reports are written against in 2026; its sixth edition appeared in April 2026 and changed the vocabulary, the HER2 reporting categories and the rules for several rare types.
The WHO Classification of Tumours of the Breast is the reference every pathology report, guideline and trial protocol implicitly cites when it names a tumour. The fifth edition (2019, summarised for pathologists by Tan 2020) is the edition the UK reporting dataset in force is written against: it keeps invasive breast carcinoma of no special type as the large default group, into which it folded the former medullary, oncocytic, lipid-rich, glycogen-rich, sebaceous and pleomorphic carcinomas as patterns rather than types, and it keeps the special types, lobular, tubular, mucinous, cribriform, papillary, secretory, adenoid cystic, apocrine, micropapillary, metaplastic and neuroendocrine, as separate entities. Nearly every breast page here is written to that edition.
The sixth edition was published in April 2026 (Quinn 2026). Five of its changes matter to a reader holding a report. It fixes the vocabulary: a subtype differs clinically, histologically or genetically in a way that changes treatment or outcome, a pattern differs only in appearance, and the word variant is discouraged for anything but a molecular or genetic alteration, so a report or a website using variant to mean an unusual appearance is using the older, looser sense. It updates the HER2 reporting categories after the DESTINY-Breast04 and DESTINY-Breast06 trials, so that cases with no membrane staining at all are distinguished from cases with any membrane staining, because that boundary now decides who is offered trastuzumab deruxtecan. It recognises invasive lobular carcinoma with extracellular mucin as an emerging subtype that behaves more aggressively than classical lobular cancer. It moves mucinous carcinomas that are high grade, oestrogen receptor negative or HER2 positive out of mucinous carcinoma and into carcinoma of no special type with mucin production. It abandons the unified neuroendocrine neoplasm model the fifth edition applied across organs, on the ground that it is difficult to apply to the breast, and stops recommending routine neuroendocrine staining of ordinary carcinoma. And it lowers the bar for malignant phyllodes tumour from all five adverse histological features to four of the five.
None of that has reached a British report yet. The Royal College of Pathologists dataset in force (G148, version 3, November 2024) states that its histological subtypes were updated in line with the fifth edition, and it is due for full review in November 2026. So in 2026 a UK report is a fifth-edition report, and the sixth edition is what the next one will be written against.
Most cancers start in the ducts and drain first to the axillary nodes, which is why the armpit is checked and a sentinel node is sampled.
Same organ: Triple-negative breast cancer (TNBC), HR-positive / HER2-negative breast cancer, HER2-positive breast cancer, Male breast cancer, Ductal carcinoma in situ (DCIS), High-risk early HR-positive breast cancer, HR-positive metastatic breast cancer after CDK4/6 inhibitors, HER2-low and HER2-ultralow metastatic breast cancer, Early HER2-positive breast cancer, HER2-positive breast cancer with brain metastases, Early triple-negative breast cancer, Metastatic triple-negative breast cancer, Basal-like 1 triple-negative breast cancer (BL1), Basal-like 2 triple-negative breast cancer (BL2), Mesenchymal triple-negative breast cancer (M), Mesenchymal stem-like triple-negative breast cancer (MSL), Luminal androgen receptor triple-negative breast cancer (LAR), Immunomodulatory triple-negative breast cancer (IM), Metaplastic breast carcinoma, Carcinoma with medullary pattern (medullary breast cancer), Adenoid cystic carcinoma of the breast, Apocrine carcinoma of the breast, Secretory carcinoma of the breast, BRCA-associated triple-negative breast cancer, Inflammatory breast cancer, Paget disease of the nipple, Phyllodes tumour of the breast, Invasive lobular carcinoma of the breast, Invasive breast carcinoma of no special type (invasive ductal carcinoma), Tubular carcinoma of the breast, Mucinous carcinoma of the breast, Papillary carcinomas of the breast (encapsulated, solid and invasive papillary), Invasive cribriform carcinoma of the breast, Invasive micropapillary carcinoma of the breast, Neuroendocrine neoplasms of the breast, Lobular carcinoma in situ (LCIS)
Showing the organ this term concerns: Breast cancer (all types).
Shares Mucinous carcinoma of the breast, Invasive breast carcinoma of no special type (invasive ductal carcinoma), HER2-positive breast cancer, Breast cancer (all types).
Shares Invasive breast carcinoma of no special type (invasive ductal carcinoma), Nottingham grade (breast cancer grade 1, 2 and 3), Invasive lobular carcinoma of the breast, Breast cancer (all types).
Shares HER2 testing in the UK: reflex ISH, the ratio against the copy number, and where the UK differs from ASCO/CAP, Invasive breast carcinoma of no special type (invasive ductal carcinoma), Nottingham grade (breast cancer grade 1, 2 and 3), HER2-positive breast cancer.
Shares Mucinous carcinoma of the breast, Neuroendocrine neoplasms of the breast, Breast cancer (all types), HR-positive / HER2-negative breast cancer.
Shares Mucinous carcinoma of the breast, Invasive lobular carcinoma of the breast, Breast cancer (all types), HR-positive / HER2-negative breast cancer.
Shares Invasive breast carcinoma of no special type (invasive ductal carcinoma), Nottingham grade (breast cancer grade 1, 2 and 3), Invasive lobular carcinoma of the breast, Breast cancer (all types).
Shares HER2 testing in the UK: reflex ISH, the ratio against the copy number, and where the UK differs from ASCO/CAP, HER2-low and HER2-ultralow, HER2-positive breast cancer, Breast cancer (all types).
Shares Lobular carcinoma in situ (LCIS), Invasive lobular carcinoma of the breast, Breast cancer (all types).