Correct answer is: Primary thyroid lymphoma.

Review of the imaging studies

A transverse US scan of the thyroid (Fig 1) shows a large, lobulated hypoechoic mass in the right thyroid lobe (large arrows). The mass invades the soft tissues surrounding the common carotid artery (CCA). The isthmus and left thyroid lobe are essentially of normal appearance (small arrows). A longitudinal US scan of the right thyroid lobe (Fig 2) shows a diffusely enlarged right lobe which is totally infiltrated by a large heterogeneous hypoechoic mass (arrows). Color Doppler US of the right lobe (Fig 3) demonstrates abundant color flow signals in the mass (arrows). Note that the mass invades the thyroid capsule. ¹⁸F-FDG PET/CT of the same patient (Fig 4) shows a significantly increased metabolic activity in the right lobe of the thyroid (arrow) (SUVmax = 46.1), which is highly suggestive of malignancy. No other sites showed distinct areas of increased FDG-metabolism.

Sonographic Appearance of Thyroid Lymphoma

Thyroid lymphoma is most commonly described as bilateral or unilateral asymmetric goiter. The lesions typically show heterogeneous echotexture and marked hypoechogenicity with posterior acoustic enhancement and may invade the thyroid capsule. There is increased chaotic vascularity which is shown on the color Doppler flow image.

Clinical Course of the Patient

The patient received a US-guided core needle biopsy 1 day after the ultrasound examination, and the pathological study revealed that the tumor was consistent with large B cell lymphoma. Since there is no evidence of lymphomatous lesions in other parts of the body and bone marrow biopsy showed no abnormalities, the patient was diagnosed as a case of primary thyroid lymphoma. She subsequently received chemotherapy and total remission was achieved. She was doing well one year after chemotherapy.

Differential Diagnoses

A goiter with markedly hypoechoic, and heterogeneous internal echoes could also be observed in Hashimoto’s thyroiditis. However, rapid growth and pressure symptoms are rare in diffuse Hashimoto’s thyroiditis.

Nodular goiter may present with multiple solid nodules with large size (5 cm in greatest diameter). However, there will not be thyroid capsule invasion or increased chaotic vascularity.

Goiter and rapid progression are also observed in undifferentiated thyroid carcinoma. Undifferentiated thyroid carcinoma often presents with microcalcifications and cystic changes on sonograms.

Conclusion

The clinical presentation with rapid thyroid enlargement and breathing difficulties, may indicate PTL. The US features of PTL include asymmetric goiter, heterogeneous echotexture, and marked hypoechogenicity with posterior acoustic enhancement.  The thyroid capsule invasion and the focal chaotic vascularity further raise suspicion for lymphoma.

Conflicts of Interest

The authors declare no conflict of interest

References

1. Xia Y, Wang L, et al. Sonographic appearance of primary thyroid lymphoma-preliminary experience. PLoS One.2014,9(12):e114080.
2. Peng C, Chen Yang C, et al. Multimodal Sonographic Appearance and Survival Outcomes of 69 Cases of Primary Thyroid Lymphoma Over 10 Years. J Ultrasound Med. 2022, 41(12):3031-3040.