General considerations

The preoperative diagnosis of MALT lymphoma is essential because the consequence differs from all other thyroid malignancies: instead of surgical ablation of the tumor only biopsy is required in order to histologically prove the diagnosis.
This is a classical clinico-cytological diagnosis: combining the clinical data with the cytological picture we can diagnose practically all MALT lymphomas while without the knowledge of clinical picture a cytological diagnosis is impossible.
MALT lymphoma presents as very hard, huge uni- or bilateral thyroid mass evolving within a few months. The clinical presentation is very close to anaplastic carcinoma, but the latter is more hard evolves more rapid and is unilateral. However, the differences are not striking.

Prevalence

Primary lymphoma of the thyroid is an uncommon malignancy; the incidence in the published series varies from 0.6% to 5% of thyroid malignancies, rarely between 5 and 10% (Fujimoto 1967, Staunton 1973, Shields 1977, Heiman 1978, Souhami 1980, Scholefield 1992, Singer 1998). The female to male ratio is in the range 3:1 to 10:1 (Compagno 1980, Rasbach 1985, Tupchong 1986, Skarsgard 1991, Junor 1992, Pyke 1992, Sholefield 1992, Tsang 1993).
The preoperative diagnosis of thyroid lymphomas involves a special consequence: in most cases, the diagnosis is followed by excision biopsy and not a curative surgical procedure.
A majority of the thyroid lymphomas, i.e. MALT lymphomas, belong in a common group from clinical and cytological aspect. Their clinical presentation is very similar to that of anaplastic carcinomas of the thyroid: the patients are over 60, and display a rapidly-enlarging, firm goiter. On the other hand, the cytological picture resembles that of Hashimoto's thyroiditis. This clinico-cytological presentation is very specific.
All other malignant lymphoproliferative thyroid diseases exhibit different clinical and cytological pictures, but in most cases the suspicion of lymphomas can be raised.

History of patients with malignant lymphomas

MALT lymphomas are primary diseases of the thyroid. They develop in most if not all cases in Hashimoto's goiter  (Rasbach 1985, Junor 1992, Scholefield 1992, Matsuzuka 1993). The reported incidence of Hashimoto's thyroiditis in patients with thyroid lymphoma is 25-100% (Singer 1998). There is a debate in the literature as to whether Hashimoto's thyroiditis is a predisposing factor (Williams 1981) or as to whether only a coincidence exists (LiVolsi 1990). The well-known relationship between Hashimoto's thyroiditis and MALT lymphomas is of only very limited help in the diagnosis of MALT lymphomas. In our practice, there have been  no patients occurred where a previous lymphocytic thyroiditis or hypothyroidism was known. On the other hand, at the time of the diagnosis of thyroid lymphoma, most patients presented evidences of thyroiditis and/or hypothyroidism.
The situation is similar in secondary lymphomas of the thyroid. In all but one patient, the underlying lymphoproliferative disease was not known.

Clinical presentation

As mentioned above, MALT lymphomas exhibit a very characteristic picture. A patient (in more than 95% of the cases a woman) is presents with a rapidly-developing, firm goiter. The disease involves both lobes of the thyroid, although not uniformly, and a unilateral presentation can be seen. The development is very rapid, in most cases involving 4-12 weeks. This is somewhat slower than in anaplastic cancer, but more rapid than in well-differentiated cancers of the thyroid.
The thyroid is very firm, but not stone-hard as in typical cases of anaplastic cancer.
Dyspnea, dysphagia or hoarseness are present at the first examination in around 50% of patients (Compagno 1980, Rasbach 1985, LiVolsi 1990, Skarsgard 1991, Singer 1998).
The above-mentioned features demonstrate that only small differences exist in the presentation of MALT lymphoma and anaplastic thyroid cancer.
Nevertheless, there are two features, both consequences of the underlying autoimmune thyroiditis, which may of help in the differentiation of thyroid MALT lymphoma from anaplastic cancer. In most patients, the TSH levels are elevated, reflecting hypothyroidism. The other feature is the US appearance. In MALT lymphomas, the whole thyroid is hypoechogenic, even the non-tumorous parts of the gland, because diffuse LT is also present, whereas in cases of anaplastic cancer the thyroid outside the tumor is intact.
Secondary thyroid lymphomas are present in a non-specific form. The tumor may develop over months or years. In most of the cases, the tumor appears as a nodule. Concomitant Hashimoto's thyroiditis may present only by chance.

Ultrasonography

MALT lymphomas present as an uni- or bilateral huge hypoechogenic mass. It affects in most cases a whole thyroid. The sonographic presentation does not differs from that observed in diffuse Hashimoto's thyroiditis except for the extreme enlargement. The degree of hypoechogenicity is severe in all cases. In contrast with anaplastic carcinoma necrosis is not present while fibrosis may present. The vascularization has no specificity in MALT-lymphomas.

Other lymphoproliferative disorders present in most cases as a solitary hypoechogenic nodule. In most if not all of the cases a concomitant enlargement of one or more ipsilateral neck lymph node is also present.

Cytology

MALT lymphomas A very cellular picture is seen, composed of a heterogeneous lymphoid population. Besides typical elements, atypical lymphoblasts are present. Oxyhilic cells may be seen if the smears are carefully examined. The heterogeneous lymphoid population and the presence of oxyphilic cells may cause significant difficulties in the cytodiagnosis. A knowledge of the clinical presentation is therefore mandatory.
The reported sensitivity of FNAC in thyroid lymphoma varies in the literature. Matsuzuka reported a correct diagnosis in 78% of the patients (Matsuzuka 1993 ), whereas Skarsgard et al. found that the FNAC was suspicious of thyroid lymphoma in 56% of the cases. (Skarsgard 1991). In our opinion, the less impressive results may be explained at least partially by a lack of the knowledge of the full clinical picture at the time of cytological analysis. Thyroid lymphoma is one of those cytological diagnoses where a correct cytological report is difficult to explain without a knowledge of the clinical data.

Other lymphoproliferative disorders The cytological picture of these lymphomas does not differ from those observed in the lymph nodes. The clinical and cytological picture depends on the type of the given tumor. In contrast with primary MALT lymphomas, epithelial cells are not present in the smear. The observation of lymphoglandular bodies may be of great help. These lymphomas are best regarded as secondary tumors of the thyroid.
Hodgkin's disease of the thyroid is considered to be a rare disease. Three cases occurred in our practice. At the time of the thyroid cytology the disease was known in only one patient. The clinical presentation was impressive in our cases: a rapidly-enlarging, very firm, solitary thyroid nodule.

Differential diagnostics of MALT lymphomas

Anaplastic carcinoma. If the cytopathologist is aware about the clinical presentation, and he or she must be aware about the extremely unusual, rapid evolution of a thyroid mass, than anaplastic carcinoma of the thyroid is the only disease involved in differential diagnostics. The cytological pattern of MALT lymphoma strikingly differs from anaplastic carcinoma as regards the high cellularity, the lack of pleomorphic epithelial cells, the presence of oxyphilic cells and first of all the presence of heterogeneous lymphoid population.

Hashimoto's thyroiditis. In contrast with the former possibility, the cytological presentation is the issue. I think that cytology is not able to discriminate between Hashimoto's thyroiditis and MALT lymphoma because it is practically the same. On the other hand, the knowledge of the clinical presentation differs strikingly. In our practice only a very few cases caused problems when a Hashimoto's thyroiditis presented in very rapid increase of the thyroid.