In multifocal prostate cancer, mismatch repair loss sat in the highest-grade focus, so that is the block to stain

The edition in brief

A single-institution series of 32 treatment-naive mismatch repair-deficient (MMRd) prostate adenocarcinomas found 38% were grade group 5, but 28% were grade group 2, and 78% had cribriform or intraductal carcinoma. Loss mostly involved MSH2/MSH6. Five of 15 patients tested had Lynch syndrome. Where several blocks were stained, MMR status differed between foci in 8 of 14, with deficiency always in the highest-grade focus. A Dutch national expert panel reviewing 1,146 Barrett's oesophagus consultations found p53 immunohistochemistry reproducible (kappa 0.65 among 13 pathologists) and decision-changing: it moved 37% of indefinite-for-dysplasia biopsies to low-grade dysplasia and 17% to non-dysplastic. FOG1 stained 91% of gastro-oesophageal adenocarcinomas but under 8% of colonic ones, while about half of gastro-oesophageal tumours were CDX2 negative or low. In 29 cervical neuroendocrine carcinomas, 27 of 28 tested were HPV positive, and all of those showed partial Rb loss, unlike the complete loss typical of lung small cell carcinoma. Nanopore sequencing gave same-day methylation classes for paediatric CNS tumours that matched histology at family level in every case. The pearl covers documenting which block went for biomarker testing.