The WHO classification of thymoma is outlined herein.

The arrow on this CT scan indicates a homogeneously enhancing anterior mediastinal mass which turned out to be a thymoma.

This image clearly demonstrates the two components of thymomas, the neoplastic epithelial component (vesicular nuclei with abundant cytoplasm) and the lymphocytic component composed of non-neoplastic lymphocytes. There is a clear demarcation between the two populations. The epithelial cells are arranged in a fasciular pattern, and some are even trying to form pseudorosettes. This image demonstrates features of histologic subtype AB of bland epithelial cells forming pseudorosettes with a prominent lymphocytic component. Note, however, you must always look at the entire specimen to determine the correct histologic subtype.

This image demonstrates a storiform pattern of spindled, bland epithelial cells. This is a type A pattern, as the cells do not exhibit atypia. If the cells were atypical, this pattern would be more consistent with histological subtype B3.

Some pathologists say that a thymoma can be diagnosed grossly by the looking at the fibrous septa, which should form acute sharp angles. In this low power image, the lobules are separated by dense fibrous septae, which are arranged in acute angles.

A higher power from the above thymoma case demonstrates focal areas of spindled epithelial cells. without prominent atypia.

However, most of the thymoma was of this pattern, with a abundance of lymphocytes admixed with epithelial cells. Can you pick out the two populations? The epithelial cells have more abundance cytoplasms, and the lymphocytes are look like your usual lymphocytes (small, round, dark nuclei with very little cytoplasm). Because of the prominence of the lymphocytic component in much of this thymoma, it was signed out as a type B1.

A double immunolabel stain highlights the epithelial cells (brown reactivity) obscured by the lymphoid component (red reactivity)

Thymoma staging is depicted in this chart.

WHO Classification and morphological pattern. See the individual thymoma cases separated by subtype for additional images.

Here is the basic category of the WHO classification.

This is a nice example of a thymoma type A.

While this tumor is typical for a type AB

And for comparison, this is an example of a type B2 tumor.


The classification of thymomas can be discouragingly complex. I will not review the evolution of various schemes, but if you are interested, an excellent summary is provided on the first page of the Kondo paper cited in the reference section. The current WHO classification scheme divides thymomas into 6 histologic subtypes based on the morphology of the epithelial cells and the ratio of lymphocytes and epithelial cells. Remember that thymomas are tumors of the epithelial cells of the thymus. If the neoplastic component is of the thymocytes (thymus lymphocytes), the diagnosis is a lymphoma and NOT a thymoma.

The six histologic subtypes (A, AB, B1, B2, B3 and C) exist because recent studies have confirmed their prognostic value. Basically, as you move from A to C, the tumor increases in aggressiveness. For example, Kondo and colleagues examined 100 thymomas using the WHO classification system: the 10-year survival rates for subtypes A, AB, B1, B2, B3 and C were 100%, 100%, 94%, 94%, 92% and 58%, respectively.2 A study of 200 thymomas by Chen and colleagues in 2001 revealed results in a similar pattern with 5-year survivals of 100% for types A and AB, 96% for type B1, 75% for B2, 70% for B3 and 48% for B4.3

An independent predictor of prognosis is the clinical staging and the current scheme is by Masoaka. Obviously, the higher the stage, the higher the rate of recurrence and the poorer the prognosis. Interestingly, the histologic subtypes generally correspond with the staging, meaning the favorable histologic subtypes (A, AB, B1) mostly present in Stage I and II and the less favorable subtypes (B2, B3 and C) presenting in Stage III and IV. Note that this is just a rule of thumb; a number of thymomas with bland histology will present at higher stages, notably, a significant minority of B1 present at stage III.4


Occurs in middle-aged persons; affects men and women equally. Most common tumor of the mediastinum. Approximately 40% of patients present with symptoms related to mass effect (tumor impinging on mediastinal structures) and 20-45% present with myasthenia gravis.1,2 Note that myasthenia gravis is more closely associated with thymic hyperplasia that thymomas.


Surgery is the mainstay with chemo and radiation as adjuvant therapies.


Histologic subtype and stage are independant prognostic factors, with stage being more important.


1 Robbins

2 Kondo

3 Chen

4 Fletcher

** Case materials courtesy of Lida Crooks, MD at VAMC Albuquerque, New Mexico.

Last updated: 2012-02-14
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