WHO soft tissue and bone, sixth edition: 19 new entities and 'pattern' replaces 'variant'

The edition in brief

The sixth edition of the WHO Classification of Soft Tissue and Bone Tumours, summarised in the American Journal of Surgical Pathology on 7 October 2026, adds 19 new tumour types, covers 62 sarcomas, and replaces the term 'variant' with 'morphological pattern' for appearances that do not change behaviour; 'subtype' is reserved for differences that should change treatment or outcome. Essential and desirable criteria have been refined with different resource settings in mind. In 98 resected lung neuroendocrine tumours from a Dutch registry, a panel of OTP, CD44 and Ki-67 on the preoperative biopsy flagged 17 of 19 later relapses as high risk, against 2 of 19 by WHO typical/atypical classification, with better interobserver agreement and biopsy-resection concordance. A meta-analysis of 15 studies found AI models classified membranous nephropathy, IgA nephropathy and minimal change disease with high accuracy on biopsy images, but head-to-head evidence against pathologists was sparse and mostly retrospective. A 20-case study showed MHC-II, MxA and p62 could be stained reliably on formalin-fixed paraffin muscle, though MHC-I and membrane attack complex failed, which matters where frozen sections are unavailable. The pearl covers fixation time for breast biomarker testing.