Granular dot-like staining with MLH1 immunohistochemistry is a clone-dependent artefact.

Immunohistochemistry (IHC) for DNA mismatch repair proteins MLH1, PMS2, MSH2, and MSH6 is used for microsatellite instability (MSI) screening in colorectal carcinoma (CRC) and endometrial carcinoma (EC). Loss of PMS2, with retained MLH1 staining occurs in germline mutations of PMS2 gene, and is an indication for genetic testing. We report a pitfall of immunohistochemical interpretation in an EC, initially regarded as MLH1-positive and PMS2-negative. Review of the MLH1-IHC (M1-clone) revealed a granular, dot-like, nuclear staining. On repeating the MLH1-IHC with a different clone (ES05-clone), complete negativity was noted, and on molecular testing, MLH1 promotor methylation was detected. The dot-like pattern was therefore adjudged a clone-dependent artefact. On reviewing the archived MLH1-IHC slides, we observed the same dot-like pattern in two CRCs; in both cases the M1-clone had been used. Awareness of this artefact may prevent reporting errors, and unnecessary referrals for germline mutation testing.

Routine histopathological examination after female-to-male gender-confirming mastectomy.

BACKGROUND: The number of transmen seeking gender-confirming surgery has risen steadily throughout the last decade. Pathologists are increasingly confronted with transmale mastectomy specimens. It is not clear whether routine histopathological examination is useful. This study explored the possible benefit of routine investigation through detailed description of lesions encountered in mastectomy specimens after female-to-male gender-confirming surgery. METHODS: Breast tissue from a cohort of transmen was reviewed. The presence of benign and malignant breast lesions was recorded. The number of terminal duct-lobule units (TDLUs) per ten low-power fields (LPFs) was quantified. Information on hormone therapy and morphometry was retrieved for selected patients. RESULTS: The cohort included 344 subjects with a mean age of 25·8 (range 16-61) years at the time of surgery; the age at surgery decreased significantly over time. Older individuals presented with a significantly higher number of breast lesions. The number of TDLUs per LPF was lower in heavier breasts, but did not correlate with age. Breast lesions, either benign or malignant, were present in 166 individuals (48·3 per cent). Invasive breast cancer was found in two (0·6 per cent); one tumour was an unexpected finding. The number of breast lesions encountered on histopathological examination increased significantly when more tissue blocks were taken. CONCLUSION: The discovery of an unexpected breast cancer in a 31-year-old transman emphasizes the importance of thorough routine histopathological examination of mastectomy specimens. The number of tissue blocks taken should be based on age and breast weight.