SOME NOTES DEPICTED FROM A RECENT ARTICLE PUBLISHED IN JOURNAL OF THORACIC ONCOLOGY (see title below):
ITIMG consensus statement on the use of the WHO histological classification of thymoma and thymic carcinoma: refined definitions, histological criteria, and reporting.
A.MARX and al
J thorac Oncolo 2014,; 9: 596-611
TYPE A THYMOMA:
CD20 is expressed in epithelial cells
Absence of cortex-specific markers
A NEW CONCEPT OF "ATYPICAL" TYPE A THYMOMA:
Agreed criteria of "atypia" were:
- Increased mitotic activity (4 or more /10 HPF)
- "True" (coagulative) tumor necrosis in contrast to ischemic or biopsy-induced necrosis
- Necrosis may predict agresseviness
SPECTRUM OF COMMON HISTOLOGICAL PATTERN OF CONVENTIONAL WHO TYPE A THYMOMA:
Often, several patterns occur in the same tumor:
Spindle cell pattern/Microcystic/Rosetting/Hemangiopericytoma like/Gandular/Adenod/Mucoi/Whorls forming/and finally synovial sarcoma mike pattern
Type A THYMOMA VERSUS SPINDLE CELL B3 THYMOMA:
- Predominant and abundant peri vascular spaces (PVSs), would strongly favor a diagnosis of TYPE B3 THYMOMA, whereas uniform nuclei, abondance of capillary vessels, rosette formation, cystic spaces, and epithelial expression of CD20, would favor TYPE A THYMOMA
- Distinction between Atypical A thymoma type and spindle cell B3 thymoma is more difficult: IHC required.
Notes:
1/
By definition, Thymomas with moderate number of TdT+ T cells in up of 10% are counted among AB Thymomas
ANY NUMBER AREA WITH SUCH NUMBER OF TdT+ CELLS IS INCOMPATIBLE WITH ADIAGNOSIS OF TYPE A THYMOMA
Term: "Bordeland" A and AB Thymoma: focal, moderarte number of TdT+ cells T cells
N.B:
The term of Borderland is used in the article, to "refers to cases in which a decision between two diagnosis is diffiuclt, usually because dagnosis criteria are quantitaive rather than qualitative."
2/
- HASSAL'S Corpuscules are almost always absent in type A Thymoma, while they occur in 50% of B1 Thymoma
- B3 Thymomas typically show lobular growth, conspicious PVS, minor/moderate nuclear atypia, lack of intra cellular bridges, presence of TdT+ Tcells and lack of expression of CD5, CD117, GLUT1, and MUC1, in neoplastic epithelial cells.
- Tumors that lack TdT+ Tcells in the available histological material but otherwise show features of typical B3 Thymoma and CD5/CD117 negativity, should be called B3 Thymomas
- B3 Thymoma-Like Tumors, with expression of CD5 and/or CD117, and lack of TdT + T cells: In the absence of two features of thymus squamous cell carcinoma (TSCC) (i.e, clear-cut nuclear atypia and intercellular bridges), and lack of important feature of B3 Thymoma ( i.e, TdT+ T cells), these tumors were labeled as " B3/TSCC Borderline TETs
- If a comparable case would show relevant atypia, a diagnosis of "TSCC with organoid features", was considered the more apropriate designation
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