Micronodular thymoma with lymphoid stroma / B-cell hyperplasia

Definition

Small tumour nodules are embedded within sheets of mature B cells with germinal centres. This was initially considered to be a variant of spindle cell thymoma1 but the tumour nests may show a spectrum of appearances4, possibly corresponding to the spectrum of usual thymic neoplasms.

Epidemiology

This type accounts for only about 1% of all thymomas.

Clinical features

In a series of 18 cases, all were incidental findings in asymptomatic patients1. There is no association with myasthenia gravis or other autoimmune disorders1,3.

Macroscopic appearances

Most cases are encapsulated but some show infiltration of perithymic fat or pleural implants. These tumours commonly contain cystic areas.

Histopathology

The spindle cell variant is composed of small nodules of bland cells, lacking mitotic activity, as seen in conventional spindle cell thymoma. They do not form large lobules or fibrous septa but may locally coalesce. Only a few scattered lymphocytes are present within these nodules.

There may be numerous polygonal cells admixed with the spindle cells. They may form rosettes. The polygonal cells cells have large nuclei with nucleoli4. Alternatively, the polygonal cells may show atypia and focal keratinisation4. In some cases, the nests are irregular and the cells are cytologically atypical with features of lymphoepithelioma-like carcinoma4.

The lymphoid stroma contains prominent germinal centres. Plasma cells are prominent4. These follicles may show onion skin hyalinisation, resembling Castleman's disease3. These lymphoid areas are devoid of epithelial cells3.

In some studies, the cysts lack an epithelial lining1. In others, they are described as being associated with glandular differentiation, the lining cells being picked out by EMA2.

One case was in combination a type AB thymoma4, another with a type B2 thymoma2. In one case, there was transition to lymphoepithelioma-like carcinoma.

Some cases may progress to low grade lymphoma (MALT lymphoma or follicular lymphoma)0.

Immunohistochemistry

Epithelial cells :

 

Cam5.2

positive in 60% of cases: positive in cysts0, strongly positive1, 11/114

 

AE1/AE3

11/114

broad spectrum cytokeratin

weak focal positivity1

CK5/6

positive0

CK7

positive in cysts0

CK8

positive in cysts0

CK19

positive0

EMA

positive in cysts0, negative1

CD5

7/114

CD20

negative in epithelial cells0, 0/53

CD57

positive in 60% of cases0, 5/53

bcl-2

10/11

chromogranin

0/9

Synaptophysin

2/9

   
   

 

Lymphoid stroma. There is disagreement as to whether the lymphoid stroma is composed purely of B cells1 or is a mix of B and T cells3.

 

LCA

positive1

 

CD20

positive1,3

UCHL-1

a few positive cells1

CD99

a few positive cells within the spindle cell nodules only1, 8/114

CD1a

positive immature lymphocytes form a narrow band around the epithelial nodules0, a few positive cells within the spindle cell nodules only1, negative3

CD3

negative1, a population of positive cells3

CD5

negative1

CD10

positive immature lymphocytes form a narrow band around the epithelial nodules0, germinal centre B-cells are positive0

CD99

positive immature lymphocytes form a narrow band around the epithelial nodules0

TdT

positive immature lymphocytes form a narrow band around the epithelial nodules0

bcl-2

strong positivity of the mantles of the lymphoid follicles1

kappa and lambda

polytypic1

   
   

Another study describes the lymphoid stroma as being composed of both B-cell areas with germinal centres and T-cell areas with large immature (CD1a+/CD99+) lymphocytes2.

Differential diagnosis

Management

Local resection is curative.

Prognosis

Limited follow up indicates a benign behavior.

References

0 Tumours of the Lung, Pleura, Thymus and Heart. WHO Classification of Tumours. IARC Press 2004.

1Suster, S. and C. A. Moran (1999). "Micronodular thymoma with lymphoid B-cell hyperplasia: clinicopathologic and immunohistochemical study of eighteen cases of a distinctive morphologic variant of thymic epithelial neoplasm." Am J Surg Pathol 23(8): 955-62.

2Thomas De Montpreville, V., L. Zemoura, et al. (2002). "[Thymoma with epithelial micronodules and lymphoid hyperplasia: six cases of a rare and equivocal subtype]." Ann Pathol 22(3): 177-82.

3Pan, C. C., W. Y. Chen, et al. (2001). "Spindle cell and mixed spindle/lymphocytic thymomas: an integrated clinicopathologic and immunohistochemical study of 81 cases." Am J Surg Pathol 25(1): 111-20.

4Tateyama, H., Y. Saito, et al. (2001). "The spectrum of micronodular thymic epithelial tumours with lymphoid B-cell hyperplasia." Histopathology 38(6): 519-27.

Mende, S., M. Moschopulos, et al. (2004). "Ectopic micronodular thymoma with lymphoid stroma." Virchows Arch.

 

This page last revised 4.1.2006.

©SMUHT/PW Bishop