Recently, the US Food and Drug Administration (FDA) approved the Ventana MMR RxDx Panel as the first immunohistochemical companion diagnostic test for identification of tumors with mismatch repair (MMR) status. The aim of this study was to investigate the accuracy of this test in comparison with polymerase chain reaction (PCR)-based microsatellite instability (MSI) analysis. We assessed the MMR/MSI concordance rate in 140 cases of endometrioid carcinoma. MMR status was evaluated by immunohistochemistry (MMR-IHC), and MSI status was evaluated by PCR-based analysis (MSI-PCR). Potential molecular mechanisms responsible for MSH6 staining variations were also analyzed. Immunohistochemistry showed that 34 tumors (24.3%) were MMRd; these included 26 with combined MLH1/PMS2 loss, 2 with combined MSH2/MSH6 loss, and 6 with isolated MSH6 loss. Heterogeneous MSH6 loss was found in 10 tumors and was recognized only in tumors with combined MLH1/PMS2 loss. Eight of 10 tumors with heterogeneous MSH6 loss harbored MSH6 C8 tract instability, suggesting a secondary somatic event after MLH1/PMS2 loss. MSI-PCR revealed that 102 tumors were MSS, 4 were MSI-low, and 34 were MSI-high. Consequently, MMR-IHC and MSI-PCR showed perfect concordance (kappa=0.080, P <0.0001). However, 10 of the 34 MSI-high tumors, including the 6 tumors with isolated MSH6 loss, showed only minimal microsatellite shift by MSI-PCR, which may have been erroneously interpreted as MSS or MSI-low. On the basis of these findings, we consider that the FDA-approved immunohistochemical panel can detect MMR variations consistently and is more accurate than MSI-PCR for determining the applicability of immune checkpoint inhibitors for treatment of endometrioid carcinomas.
Carcinoma, Endometrioid , Colorectal Neoplasms , United States , Female , Humans , Carcinoma, Endometrioid/diagnosis , Carcinoma, Endometrioid/genetics , Mismatch Repair Endonuclease PMS2 , United States Food and Drug Administration , Microsatellite Instability , DNA Mismatch Repair , Phenotype , MutL Protein Homolog 1/genetics , MutS Homolog 2 Protein/genetics , Colorectal Neoplasms/pathology
We report a rare case of a 69-year-old woman in whom diffuse large B-cell lymphoma (DLBCL) originated from the uterus and involved the urinary bladder. The cervical smears of the case mostly consisted of discohesive atypical round cells, which were highly suggestive of lymphoma; however, in voided urine smears, a majority of the cells formed large aggregates of degenerated cells, mimicking those of urothelial carcinoma (UC). The smears also represented some small loose clusters, in which tumor cells formed short chains with nuclear molding, mimicking those of small cell carcinoma. The cytodiagnosis got definitive when we identified the atypical cells that showed CD20+/CD3-/cytokeratin-/NSE- immunophenotype. These are of particular concern as they may have misleading similarities to other epithelial neoplasms when examining lymphoma involving the urinary bladder. Accordingly, this case highlights the importance of immunocytochemistry to rule out malignant lymphoma when encountering large and/or small loose clusters of atypical round cells on urinary cytology.
