Construct the adequate antibody panel adressing the problem. This requires proper interpretation of morphology in a context of clinical data and a knowledge of immunophenotypes of the tumors including their possible immunophenotypic variations. In cases with less characteristic morphology it may be wise to order your immunostains in two or even three steps starting with the couple of broad spectrum antibodies and later completting with antibodies essential for more precise diagnosis or relevant for therapy.
A) The case represents bronchial carcinoid but the diagnosis of small cell neuroendocrine carcinoma is a potential pitfall if the staining for proliferation marker KI-67 is not ordered.
HE. Bronchial biopsy with traumatized tumor.
PANCK (AE1/AE3/5D3). Staining in majority of tumor cells in some places with “dot-like” pattern.
Chromogranin A. Strong granular positive staining in cytoplasm of all tumor cells. 10x.
Synaptophysin. Strong positive granular cytoplasmic staining of all tumor cells. 10x.
Ki67. Staining shows low labelling index much lower than expected for small cell neuroendocrine carcinoma and supports the diagnosis of carcinoid tumor. 10x.
B) Merkel cell carcinoma especially in metastatic setting may be mistakenly diagnosed as B-cell acute lymphatic leukemia/ lymphoblastic lymphoma based on common positive staining in Merkel cell carcinoma for TdT (>80%) and PAX5 (>70%) if the antibody panel does not include epithelial nor endocrine markers. Same wrong interpretation may happen in cases small cell carcinoma (TdT+ <10% and PAX5+ >80%) or granulocytic sarcoma/acute myeloid leukemia (TdT+ in 10% and PAX5+ in 30%).
Hemacolor. Smear from pleural fluid.
HE. Section from the cellblock from pleural fluid.
PAX5. Positive nuclear staining in all cells. Low magnification.
PAX5. Staining in the cell block. High magnification.
TdT. Positive nuclear staining in almost all cells.
Cytokeratin 20 (CK20). Positive “dot-like” staining i almost all cells.
Synaptophysin. Staining in the cell block.
Fig. 7. (D5F3). Staining in the cell block showing strong cytoplasmic positivity. High magnification.
C) Merkel cell carcinoma and infiltrate of B-cell chronic lymphatic leukemia in the skin biopsy. There is a risk to misdiagnose leukemic infiltrate as reactive if the panel does not include hematolymphoid markers.
HE. Merkel cell carcinoma and infiltrate of B-cell chronic lymphatic leukemia.
HE. Merkel cell carcinoma and infiltrate of B-cell chronic lymphatic leukemia in the skin.
CD23. Positivity for CD23 together with positivity for CD5 and CD20 as well as morphology fits well with the diagnosis of B-cell chronic lymphatic leukemia in addition to Merkel cell carcinoma.
CD5. Staining is positive in both reactive T-cells and B-cells, which strongly indicates the neoplastic infiltrate of CD5+ B-cell lymphoma. Skin biopsy with infiltrate of B-CLL and Merkel cell carcinoma
Cytokeratin 20 (CK20). Positive “dot-like” staining in Merkel cell carcinoma, and as expected the infiltrate of B-cell chronic lymphatic leukemia shows no staining.
Synaptophysin. Positive staining of Merkel cell carcinoma and scarttered neuroendocrine tumor cells but the infiltrate of B-cell chronic lymphatic leukemia. is non reactive as expected.
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