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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:

  1. Increased mitotic activity (4 or more /10 HPF)
  2. "True" (coagulative) tumor necrosis in contrast to ischemic or biopsy-induced necrosis
  3. 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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