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A woman with an abnormal mass in the thyroid gland

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Q1: What other investigations would you consider?

Peak expiratory flow measurement would give an objective assessment of tracheal stenosis by demonstrating extrathoracic airways obstruction.

Computed tomography of the neck was also carried out and revealed a heterogenous mass lesion arising from the left lobe of the thyroid gland surrounding the trachea, left common carotid artery, and extending retrosternally with patchy contrast enhancement. There was no evidence of lymphadenopathy. The tumour was far too advanced for a thyroidectomy and hence only an open biopsy and tracheostomy could be performed (normally, removal of a wedge of thyroid tissue down to the trachea is sufficient to relieve airways obstruction without resorting to tracheostomy).

Q2: What is the diagnosis and what is it commonly mistaken for?

Histology confirmed a highly malignant tumour extensively infiltrating the thyroid gland with extrathyroidal spread without any evidence of papillary or follicular differentiation. This appearance was initially thought to be due to anaplastic thyroid carcinoma, with which it is commonly mistaken. However, immunohistochemical stains revealed positive staining for lymphoid cells and negative for epithelial cells. Tumour cells had centroblastic morphology and were CD20, CD79a positive (CD3 negative) indicating a B-cell phenotype. Also, the proliferation fraction, as indicated by the high MIB-1 staining, confirmed a high grade non-Hodgkin's lymphoma. The other rare differential diagnosis is Riedel's thyroiditis, which would be excluded by open biopsy of the thyroid gland.

Q3: What is a common predisposing factor?

Autoimmune Hashimoto's thyroiditis has usually, although not invariably, been known to predispose to this form of lymphoma.

Q4: What is the treatment and prognosis of this condition?

Non-Hodgkin's lymphoma of the thyroid gland usually responds well to a combination of radiotherapy and chemotherapy or radiotherapy alone in localised stage 1 disease. The treatment of choice, however, is total thyroidectomy plus radiotherapy in patients where radical surgery is possible. Prognosis of this uncommon malignancy is usually good and much better than anaplastic thyroid carcinoma and hence every effort must be made to distinguish the two forms of malignancy.

Learning points

  • Primary non-Hodgkin's lymphoma is an uncommon malignancy of the thyroid gland and is usually, although not invariably, preceded by Hashimoto's thyroiditis.

  • It is very easily mistaken for anaplastic thyroid carcinoma on histology.

  • Immunohistochemical stains are very important in distinguishing between the two conditions as treatment and prognosis is entirely different.

  • Prognosis of this form of lymphoma is usually good as it responds well to radiotherapy and chemotherapy.

Discussion

Primary thyroid lymphoma is an uncommon disease representing 5% of all thyroid neoplasms and the majority are due to non-Hodgkin's lymphoma.1 They affect elderly patients (median age at presentation 61–73 years) and the reported female:male ratio varies from 2.6:1 to 8.4:1.2 3 Though it is widely accepted that the vast majority of thyroid lymphomas occur with the background of autoimmune Hashimoto's thyroiditis (either by the clinical history, histology, or serology), its presence has been reported as varying from 22%–100% and is by no means sufficient to establish a diagnosis.4 In recent years the recognition of mucosa associated lymphoid tissue (MALT) has drawn new attention to the pathogenesis of primary lymphomas of thyroid. MALT lymphomas are also known to transform into high grade lymphomas.4

Typically, the disease presents with a rapidly growing, painless thyroid mass with or without obstructive symptoms and variable thyroid dysfunction. Brownlie et al reported eight such cases over a 12 year period: one of them was thyrotoxic and the other hypothyroid.5 Matsuzuka et al found 27% of their patients to be hypothyroid and 14% had evidence of subclinical hypothyroidism on the basis of coexistence of Hashimoto's thyroiditis.6

According to Skarsgard et al, using log rank survival tests, statistically significant predictors of improved survival were tumour mass less than 10 cm, absence of dysphagia at presentation/mediastinal lymphadenopathy, and disease localised to the thyroid gland.7 Treatment of choice is total thyroidectomy plus radiotherapy in patients where radical surgery is possible or radiotherapy alone, which usually yields good results.1Most studies recommend combined radiotherapy and chemotherapy for localised thyroid lymphoma and radiotherapy alone for a small select group of patients with stage I disease and a small tumour bulk.

Though there was no histological evidence of Hashimoto's thyroiditis in our patient (lymphomatous growth had completely replaced the whole of the thyroid gland), she had biochemical evidence of subclinical hypothyroidism and strongly positive thyroperoxidase antibodies, which may be indicative of thyroiditis in the absence of a direct histological proof. Additionally, she had evidence of other autoimmune diseases namely, vitligo and insulin dependent diabetes, both of which highlight the autoimmune background that could have been instrumental in triggering her primary pathology. It has been quoted that whenever there is no previous evidence of Hashimoto's thyroiditis the typical presentation of primary thyroid lymphoma may be taken as suggestive of anaplastic thyroid carcinoma.4 Even histologically, anaplastic thyroid carcinoma is virtually indistinguishable from high grade non-Hodgkin's lymphoma on a haematoxylin and eosin stained section, hence a battery of immunohistochemical stains are needed to establish the exact histogenesis of the tumour cells so that proper treatment can be instituted immediately.

This form of lymphoma responds well to treatment and has a much better prognosis than anaplastic carcinoma, as is evident from the excellent results in our patient. She has received radiotherapy and is currently undergoing chemotherapy and has done very well so far with regression of her neck swelling and complete resolution of her symptoms, mainly dysphagia, hoarseness, and stridor.

Final diagnosis

High grade non-Hodgkin's lymphoma.

References

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