Unilocular Thymic Cysts May Rarely Undergo Malignant Transformation

Over more than five years of follow-up, thin-walled unilocular thymic cysts, as initially defined by magnetic resonance imaging (MRI), never developed irregular wall thickening, mural nodularity or septations, according to a study recently published in Radiology. The cysts, however, did show mural calcification, along with change in size, computed tomography (CT) attenuation and MRI signal intensity over the same period.

“Our investigation highlights the intrinsic and longitudinal characteristics of unilocular thymic cysts that contribute to their misinterpretation as thymic neoplasms and lymphadenopathy at CT,” wrote Wariya Chintanapakdee, M.D., Chulalongkorn University in Bangkok, Thailand, and colleagues.

In this retrospective study, the researchers evaluated the imaging characteristics and longitudinal change of unilocular thymic cysts on CT and MRI over time. All chest MRI studies showing thymic cysts between July 2008 and December 2019 were included. If initial CT showed a thymic lesion, the patient was referred for MRI for characterization and the baseline MRI indicated a cystic lesion. 

The researchers evaluated 244 chest MRI studies in 140 patients with 142 unique cysts and 392 CT examinations. The median follow-up duration was 2.2 years, but a subcohort of 33 patients and 34 cysts underwent CT and/or MR imaging follow-up for more than five years. 

The results in the subcohort showed that unilocular thymic cysts can arise de novo, as did with one of 34 cysts. Additionally, volume changed in 91%, T1- weighted MRI signal intensity changed in 67% and CT attenuation changed in 43%. Further, 16% of cysts had or developed mural calcifications. In 98% of thymic cysts, wall enhancement was present at MRI, with a median wall thickness of two millimeters. The results of the full study cohort of 142 unilocular thymic cysts matched those of the subcohort.

The authors explained that fluctuation in thymic cyst characteristics over time may be due to chronic recurrent intralesional hemorrhage and subsequent resorption. “The variable CT attenuation, MRI signal characteristics, cyst wall thickness, occasional findings of T1 and/or T2 hypointensity of the thymic cyst wall, wall calcification and intracyst fluid levels that we found support this hypothesis,” the authors wrote.

Limitations to the study included the absence of pathologic proof of benignity in all but the two lesions that were resected and proven to represent thymic cysts.

“Awareness of the variable imaging characteristics and behavior of thymic cysts over time should help guide clinical management. In indeterminate cases, imaging surveillance or surgical resection may be appropriate to exclude malignancy,” the authors wrote.