KRT20 (Keratin 20)

Detection of CK20 mRNA-positive CTCs/DTCs in PB and/or BM one month after surgical resection constitutes an independent negative prognostic factor for both long-term DFS and CSS in patients with non-metastatic CRC.

Integration of histomorphology with a comprehensive immunohistochemical panel supported a final diagnosis of large-cell variant MCC. This case underscores the importance of recognizing atypical morphologic and immunophenotypic presentations of MCC and reinforces the need for broad immunohistochemical evaluation to ensure appropriate management of this aggressive malignancy.

We encountered a rare case of non-mucinous enteric-type thymic adenocarcinoma harboring a pathogenic TP53 mutation. Further studies are required to enunciate the features of this subtype of thymic carcinoma.

Methylation analysis confirmed the proximity of the two cases with other skin cancers. To conclude, we report herein the first description of primary cutaneous small cell carcinoma POU2F3 subtype.

Immunohistochemistry showed positivity for CD20, Bcl2, Bcl6, and MUM1, with cytokeratin 20 (CK20) negativity and a high percentage (80%-90%) of C-MYC positivity, which was consistent with a primary cutaneous diffuse large B-cell lymphoma (PCDLBC-LT). Given the findings, the patient was referred to oncology for further management.

Thyroid transcription factor 1 (TTF-1) was negative in all instances, supporting the diagnosis of MCC. This case series illustrates the diagnostic value and limitations of PanCK and TTF-1 staining in MCC in cases where CK20 is negative and discusses strategies for diagnosing this rare variant of MCC.

We propose that the initial IHC panel should include TRPS1, CK7 and CK20. In TRPS1-negative cases, additional immunostains should be performed: CDX2 and SATB2 for colonic; p63, GATA3 and uroplakin II/III for urothelial; and PSA and NKX3.1 for prostatic secondary EMPD.

Immunoreactivity with either marker in small biopsy specimens can support a diagnosis of malignancy, especially in morphologically equivocal or limited samples. Absence of staining does not exclude malignancy, but immunoreactivity with CDX2 and/or CK20 provides strong supportive evidence for a malignant process.

We also noted that the histological type of this case was medullary carcinoma. Furthermore, as far as we could find out, this is the first reported case of colon cancer metastasis to the breast successfully treated with immune checkpoint inhibitors.

Pulmonary enteric adenocarcinoma (PEAC), also known as adenocarcinoma-enteric type of the lung, is a rare variant of non-small cell lung cancer (NSCLC) that shows similar morphological and immunohistochemical (IHC) features to colorectal adenocarcinoma and necessitates gastroenteroscopy to exclude enteric-origin lesions...However, their molecular features and therapeutic biomarkers are still unexplored, and their diagnosis and treatment remain difficult. Thus, systematic review and combined analysis data were obtained from previously published literature.

The case emphasizes the importance of correlating clinical, histopathological and radiological findings. Rescreening for malignancy may be prudent in recurrent primary Paget disease.

The optimal cost-effective panel is POU4F3 with keratin AE1/AE3 or pan-keratin, which achieves 100% sensitivity while reducing reliance on less effective stains. Adoption of this focused IHC panel may serve to standardize SLN evaluation for MCC and improve diagnostic accuracy and efficiency.