Metastatic mesothelioma to the thyroid

Abstract

A 69 year-old male patient with a history of malignant mesothelioma treated with chemotherapy and surgical resection with removal of the right lung and right pleural pneumonectomy was clinically in remission for 1 ½ years. A positron emission tomography (PET) scan revealed limited uptake in the right pleural space, thought to represent post-surgical changes, and uptake in the left thyroid lobe. Thyroid ultrasound revealed a solid left lobe nodule with peripheral vascularity and absent microcalcifications. Fine needle aspiration cytology showed a microfollicular arrangement of cytologically bland cells with variable Hürthle cell changes initially interpreted as suspicious for Hürthle cell neoplasm. Review at multidisciplinary conference raised the possibility of metastatic mesothelioma, supported by immunohistochemical studies in the cell block. The patient opted for left hemithyroidectomy with isthmusectomy which confirmed malignant mesothelioma. Repeat PET scan 6 months later revealed no further uptake in the thyroid bed, with limited uptake in the right pleural space. Metastatic tumors to the thyroid are uncommon with only one previous description of metastasis to the thyroid by mesothelioma. Metastasis of cytologically low grade tumors such as mesothelioma present problems for cytology due to the potential for overlap with the variable appearances of thyroid neoplasms. The value (if any) of ancillary tests, including mutation testing, expression profiling and immunohistochemistry is discussed.

Keywords: Nuclear medicine-imaging; pathology-thyroid cytology; thyroid cancer-clinical; thyroid cancer-genetics.

Figure 1

Thyroid ultrasound: Left lobe nodule well circumscribed measuring 2.9 × 1.55 × 1.98 cm without calcifications but with peripheral vascularity

Figure 2

Thyroid FNA ThinPrep: On the left are the mesothelioma cells, which are in three-dimensional groupings compared to the Hürthle cell population on the right, that forms two dimensional sheets. The mesothelioma cells have waxy cytoplasm with multiple nucleoli and more nuclear variation. The Hürthle cell population has abundant granular cytoplasm and generally one nucleolus. Papanicolaou stain ×600

Figure 3

Thyroid FNA cell block: Panel A shows two distinct populations, with Hürthle cells (thin arrow) and mesothelioma cells (open arrow). Panels B and C show calretinin and keratin 5/6 staining, respectively, in the mesothelioma cell population, but not in the Hürthle cell population. (H and E ×400)

Figure 4

Surgical Resection: Panel A (×40) shows nodular thyroid tissue. Panel B (×40): Two populations of cells seen within the nodule. Hürthle cells (black arrow) shows abundant pink cytoplasm. However, the tissue contains mostly mesothelial cells (white arrow) which are smaller polygonal cells with less cytoplasm. Panel C (×100): Mesothelial cells are positive for keratin 5/6. Panel D (×100): Mesothelial cells are positive for calretinin. Other immunostains (not shown) that were positive in the mesothelial cells include CK7 and HBME1, while TTF-1 and Thyroglobulin were both negative