Clinical History
Female patient aged 72 y. A left lobectomy was performed because of a nodule measuring about 4cm in maximum diameter. At operation, the nodule was beige and soft in consistency. The pathological diagnosis was confirmed a posteriori by the clinicians.

Diagnosis
Hashimoto’s thyroiditis

Discussion
This is a typical case of Hashimoto’s thyroiditis. At low power the diagnosis can be made easily (fig A). A lymphoid infiltration with active germinal centres (fig C), areas of oncocytic metaplasia forming occasional nodules (fig B) and fibrous bands are identified.(fig D)

fig. A	fig. B
fig. C	fig. D

Clinical History
Female patient aged 76 y and presenting a progressive increase in the thyroid’s volume. At gross examination, the gland was nodular but had an homogeneous light beige fish flesh aspect.

Diagnosis
Low grade Marginal zone B-cell lymphoma of MALT ( MZBL ).

Discussion
The picture is quite different in this case. The lymphoid infiltration is massive (fig A).

It shows focally a nodular pattern (fig C) but these nodules are not normal follicles with germinal centres, neither follicles like those observed in follicular centre-cell lymphomas. They are follicles colonised by malignant lymphomatous cells (vide infra).

In this slide, the malignant cells are small as seen in fig D, therefore the diagnosis of low grade lymphoma.

Another essential criteria is the presence of numerous lympho-epithelial lesions (LEL) (fig B). As seen in the picture, this lesion is characterised by penetration of the malignant cells through the wall into the lumen of a thyroid follicle. A cytokeratine iommunostaining has been used in order to reveal the follicles immersed in the lymphomatous infiltrate.
It must be said that the finding of an occasional lympho-epithelial lesion is acceptable in cases of chronic lymphocytic thryoiditis but a great number of LEL like in the present case is fairly diagnostic of lymphoma.

fig. A	fig. B
fig. C	fig. D

Primary lymphomas of the thyroid (PTL) occur predominantly in middle aged and older women in a setting of Hashimoto’s disease. They represent 5% of thyroid cancers and 2,5 to 7% of extranodal lymphomas PTL are presently believed to belong to the group of lymphomas deriving from the mucosa-associated lymphoid tissue (MALT),and called accordingly MALT-lymphomas.

PTL are essentially B-cell lymphomas containing moncytoid B-cells and centrocyte-like cells.

They may show an important plasmacytoid differentiation with numerous Dutcher bodies. They may present normal reactive follicles or follicles colonized by lymphomatous cells. The identification of these colonised follicles can be made by the Bcl2 immunohistochemical staining which is positive in normal germinal centers and in follicles of follicular center-cell lymphomas but negative in colonized follicles.

PTL are devided in 3 groups : 

1) MZBL : The low grade lymphoma of MALT called marginal zone B-cell lymphoma, made essentially of small lymphoid cells of the centrocytic-like or marginal type.

2) DLBCLA : The high grade lymphoma of MALT made mostly of large cells of the immunoblastic or/and centroblastic type ; they are labelled as diffuse large B-cell lymphomas.

3) Mixed MZBL and DLBCL : A high grade lymphoma derived from a low grade lymphoma with zones of passage from the one form to the other.

High grade lymphomas form the majority ( 60% ) but despite this fact, the prognosis of high grade lymphomas depends more on the stage than on the type. Indeed stage IE high grade lymphomas are faring quite well when compared to nodal lymphomas of a similar type.

Other factors seem to play a role in the prognosis such as necrosis, apoptosis, number of mitoses, and presence of vascular invasion.

As mentionned previously, stigmata of Hashimoto’s thyroiditis can be found in the majority of cases even in the high grade PTL but they are better evidenced in low grade lymphomas.

Light- chain restriction is present in PTL but cannot always be demonstrated , as for instance in fixed material.

Bibliography

Fonseca A, Sambade C
Primary lymphoma of the thyroid gland : a review with emphasis on diagnostic features.
Arch Anat Cytol Path 46 :94-99, 1998

Deringer GA, Thompson LDR, Frommelt RA, Bijwaard KE et al.
Malignant lymphoma of the thyroid gland. A clinicopathologic study of 108 cases.
Am J Surg Pathol 24 : 623-639, 2000