Long-term follow-up results of the thymic cysts diagnosed by chest magnetic resonance imaging: single center retrospective study

Introduction

Recently, chest computed tomography (CT) is widely used in health screening programs and for surveillance of various malignancies. Consequently, incidental detection of anterior mediastinal nodular lesions has become more common. According to a Korean study by Yoon et al., among 56,358 people who underwent low-dose chest CT as part of a health checkup, the prevalence of incidental anterior mediastinal nodular lesions was 0.73%, and a considerable portion of the lesions were suspected to be benign (1). Previous reports have reported the prevalence of lesions to range from 0.45% to 0.89% (2,3).

Thymic cysts are the most common benign lesions in the anterior mediastinum, while thymomas are the most common solid tumors in this region and generally require surgical resection for accurate diagnosis and treatment. Thymic cysts typically present with characteristic features such as an oval shape, smooth contour, and midline location. However, thymic cysts are frequently misdiagnosed as thymomas when the CT attenuation value exceeds 20 Hounsfield units (HU) (4). Consequently, unnecessary nontherapeutic thymectomies have been performed due to the misinterpretation of benign lesions, such as lymphoma, thymic hyperplasia, and thymic cysts as thymomas based on chest CT (5). Chest magnetic resonance imaging (MRI) has emerged as a reliable diagnostic tool for differentiating these lesions (6).

The purpose of this study was to investigate the natural course of thymic cysts diagnosed using chest MRI and to evaluate the need for surgical intervention through long-term CT follow-up. We present this article in accordance with the STROBE reporting checklist (available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-915/rc).

Methods

Study design

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Institutional Review Board of Chonnam National University Hwasun Hospital (IRB approval No. CNUHH-2025-086). The requirement for individual informed consent was waived due to the retrospective nature of the study.

Since the mid-2010s, patients diagnosed with thymic cysts or small nodular lesions in the anterior mediastinum on chest CT at our institution have undergone chest MRI. Patients with probable or definitive cystic lesions confirmed on MRI did not undergo surgery, but were monitored with serial chest CT to evaluate changes in size, wall thickness, and nodular formation that could indicate malignant transformation.

This single-center retrospective study was conducted between January 2017 and April 2024. A total of 380 patients with suspected thymic cysts or small nodular lesions that could not be definitively excluded as solid masses were referred for chest MRI. After excluding 37 patients who either refused MRI or were lost to follow-up, the MRIs of 343 patients were reviewed. An additional 48 patients were excluded because of diagnoses other than thymic cysts: thymoma (n=20), indeterminate lesions (n=9), pericardial recess (n=12), pericardial cysts (n=2), teratoma (n=3), cystic thymoma (n=1), and vascular anomaly (n=1). The remaining 295 patients were diagnosed with probable or definite thymic cysts. Of these, 194 patients had adequate chest CT data for analysis. Patients were included in the study if they had a minimum follow-up period of 3 months and underwent at least one follow-up chest CT (Figure 1).

Figure 1 Flowchart of study inclusion. CT, computed tomography; MRI, magnetic resonance imaging.

Follow-up CT scans were scheduled based on lesion size and the clinical condition of the patient, with the first follow-up typically conducted at 9 months. If a previous contrast- enhanced chest CT was available, follow-up was performed without contrast; otherwise, contrast-enhanced CT was used to evaluate the cyst enhancement characteristics. Subsequent follow-ups were conducted every 1–2 years for a minimum of 5 years, with interval adjustments based on lesion size and observed changes.

Collected data included patient demographics, initial CT and MRI findings, lesion size at initial and follow-up imaging, follow-up duration, and surgical history. Lesion size was defined as the longest axial dimension measured on CT. Significant size changes (increased or decreased) were defined as a variation of ≥10% from the initial size, excluding differences of ≤1 mm. Although there is no clear definition of significant cyst growth, we referred to a criterion for pancreatic cysts, wherein a 20% increase in the long-axis diameter was considered meaningful for lesions ≥1.5 cm (7). Lesions were categorized into four groups based on size changes: unchanged, increased, decreased, or disappeared (Figure 2).

Figure 2 CT findings of representative cases from each group. (A) A 69-year-old female patient with a lesion that remained unchanged at 31 mm (blue arrows) over 36 months. (B) A 65-year-old female patient with a lesion that increased in size from 8 to 20 mm (red arrows) over 32 months. (C) A 43-year-old female patient with a lesion that decreased in size from 10 to 5 mm (yellow arrows) over 12 months. (D) An 80-year-old female patient with 12 mm lesion (white arrow) that disappeared over 12 months. CT, computed tomography.

Chest MRI protocol

The chest MRI protocol implemented at our hospital is shown in Table 1. The diagnostic MRI criteria for thymic cysts are as follows (Figure 3):

• Well-defined saccular or oval lesions without eccentric wall thickening;
• Signal hyperintensity on T2-weighted images;
• No contrast enhancement on pre- and post-contrast T1-weighted images;
• No diffusion restriction.

Figure 3 MRI findings of a representative case of a thymic cyst. Chest MRI of a 58-year-old female shows a 4.3 cm benign cyst in the thymic bed of the prevascular mediastinum. (A) High signal intensity corresponding to cerebrospinal fluid on fat-suppressed T2-weighted imaging. No diffusion restriction on (B) axial diffusion-weighted imaging and (C) apparent diffusion coefficient map. (D) The lesion shows no remarkable enhancement on post-contrast subtraction imaging. MRI, magnetic resonance imaging.

Statistical analyses

All statistical analyses were performed using Microsoft Excel (Microsoft, Redmond, WA, USA). In addition to demographic analysis, lesion size changes were analyzed using means, percentages, and standard deviations (SDs) along with maximum, minimum, and quartile values for each group.

Results

Patient characteristics

A total of 194 patients were diagnosed with probable or definitive thymic cysts via chest MRI. Except for one patient who underwent surgery due to anxiety and personal preference, no invasive treatments were performed. Patients underwent regular follow-up via chest CT. The mean patient age was 64.0 years (SD: 12.0 years), with 105 males (54.1%) and 89 females (45.9%). The mean follow-up duration was 1.84 years (range, 0.25–6.42 years; Table 2).

Classification and comparison of patient characteristics by cyst size change

Patients were classified into four groups based on cyst size changes: unchanged, increased, decreased, or disappeared (Table 3).

During the follow-up period, 162 of 194 patients (83.5%) showed no significant change in thymic cyst size (unchanged group). Of these, 94 (58.0%) were male, closely reflecting the overall sex ratio of the cohort. The mean follow-up period was 1.74 years [interquartile range (IQR), 0.64–2.73 years].

Thirteen patients (6.7%, eight females) showed a decrease in cyst size (decreased group) and five patients (2.6%, three females) showed complete cyst disappearance or reduction to a size too small to be evaluated (disappeared group). These patients represent 9.3% of the entire cohort, and had relatively high proportions of females (61.5% and 60.0%, respectively). The mean follow-up durations were 1.94 and 1.40 years, respectively.

Fourteen patients (7.2%) showed an increase in cyst size (increased group), with the highest proportion of females (71.4%) among the four categories. The mean follow-up period in this group (3.12 years) was relatively longer than the other groups, with a wide distribution (IQR, 1.04–4.67 years), indicating greater variability compared to other groups.

Comparison of cyst size changes across patient groups over time

Several observations were made when assessing thymic cyst size changes over time in the different patient groups (Table 4).

Overall cohort: the mean initial cyst size was 22.1 mm (IQR, 14.0–25.0 mm, SD: 14.4 mm). At final follow-up, the mean size slightly decreased to 21.2 mm (IQR, 13.0–25.0 mm, SD: 14.5 mm), with a mean size change of −0.7 mm (IQR, 0.0–0.0 mm, SD: 7.1 mm), indicating minimal overall size change.

Unchanged group: the mean initial cyst size was 21.0 mm (IQR, 14.0–25.0 mm, SD: 12.7 mm), and final follow-up size was 21.1 mm (IQR, 14.0–25.0 mm, SD: 12.8 mm), demonstrating minimal size variation, consistent with the overall cohort. One patient underwent video-assisted thoracoscopic surgery for cystectomy after 34 months of follow-up because of anxiety about the mass, and pathology confirmed it as a thymic cyst.

Decreased group: this group had the largest initial mean cyst size at 34.3 mm (IQR, 18.0–50.0 mm, SD: 25.7 mm). At final follow-up, the mean cyst size had significantly decreased to 18.9 mm (IQR, 2.0–35.0 mm, SD: 16.5 mm), with a mean size reduction of −15.4 mm (IQR, −5.0 to −13.0 mm, SD: 16.9 mm). This group exhibited the most substantial size change and size variation.

Disappeared group: the initial mean cyst size was 15.4 mm (IQR, 12.0–16.0 mm, SD: 6.5 mm), with most cysts clustered in the mid-teen range. This group included patients whose cysts either completely disappeared or became radiologically undetectable.

Increased group: the initial mean cyst size was 25.7 mm (IQR, 11.0–39.8 mm, SD: 16.6 mm). At final follow-up, the mean cyst size increased to 31.8 mm (IQR, 18.0–50.8 mm, SD: 22.8 mm). The mean size increase was 8.9 mm (IQR, 5.0–9.0 mm), with most cysts enlarging by <1 cm over the 3.12-year follow-up. Notably, no cases exhibited features indicative of malignancy, such as an increase in the solid component, wall thickening, enhancement, or septation.

Each group exhibited distinct patterns of cyst size change, with the majority showing minimal to no significant changes over time. Those with increased or decreased size tended to show broader variability in cyst dimensions.

Discussion

Thymic cysts are uncommon lesions, representing 1–3% of all mediastinal masses, that can be congenital or acquired and typically contain clear or mucinous fluid (8,9). In South Korea, where chest CT is frequently performed for health screening and cancer surveillance, incidental detection of anterior mediastinal masses has increased. Differentiation between benign cysts and potentially malignant thymomas is critical because thymomas require surgical intervention. The most common benign disease of the anterior mediastinum is thymic cysts. When a cyst is large or contains a relatively clear fluid, it is more easily identified on imaging; however, smaller or viscous cysts are often more challenging to diagnose using CT.

Wang et al. reported that CT diagnostic sensitivity may be affected by cyst characteristics, wherein cysts ≤3 cm in diameter and an unenhanced CT attenuation >20 HU were independent factors for incorrect diagnosis (8). Ackman et al. highlighted those features, such as location, shape, lobulation, mass effect, lymphadenopathy, and pericardial invasion, can help differentiate thymomas from cysts. However, when a mass is well circumscribed, round, and homogeneously hyperattenuating, distinguishing between a cyst and a solid tumor on CT becomes difficult, making MRI or biopsy more useful for differentiation (10).

The International Thymic Malignancy Interest Group introduced a multidisciplinary approach for mediastinal masses in 2017. They suggested that a well-circumscribed homogeneous lesion with attenuation between 0 and 20 HU near the thymic bed and exhibiting a round, oval, or saccular shape likely represents a thymic cyst. Cysts may demonstrate higher attenuation owing to hemorrhagic or proteinaceous components, necessitating MRI for more accurate characterization and to identify any solid internal components (11). Since then, several reports have demonstrated the superiority of MRI for the diagnosis and differentiation of thymic cysts (12-14).

Park et al. reported that MRI is useful for differentiating cystic from solid mediastinal lesions, evaluating adjacent invasion, and characterizing tissue components (13). Madan et al. showed that MRI outperformed CT in distinguishing non-neoplastic complex thymic cysts from malignant tumors (14). Kim et al. suggested a stepwise MRI-based approach for the diagnosis and further management of anterior mediastinal cystic lesions, incorporating contrast enhancement and diffusion-weighted imaging (15). Ahn et al. recently reported moderate inter-reader agreement for diagnosing thymic cysts on MRI in two tertiary referral centers and that MRI sequences, including the subtracted image and diffusion-weighted imaging/apparent diffusion coefficient map, may be optimal for improving agreement consistency (16).

Nonetheless, clinical data validating these imaging-based approaches remain limited. In 2021, Ackman et al. reported longitudinal changes in the appearance of unilocular thymic cysts on CT and MRI. During follow-up for more than 5 years, changes in size, CT attenuation, and MRI findings were observed; however, no irregular wall thickening, mural nodularity, or septations indicative of malignant transformation were observed (17). This is consistent with our results, especially in the increased group. Choe et al. reported that none of the simple cysts identified on MRI were tumors and that short-term follow-up can be recommended for complex cysts (18). Despite these findings, Ackman et al. acknowledged selection bias in their earlier study, while the latter study by Choe et al. lacked sufficient long-term data.

Because thymic cysts are benign, the necessity of surgery for thymic cysts remains debatable. Surgery is usually necessary only in symptomatic cases where they compress the surrounding structures or when differentiation from other solid tumors with possible malignancy is uncertain. Efforts to reduce unnecessary surgeries for thymic cysts are ongoing. Previous reports revealed that nontherapeutic thymectomy rates ranged from 27.8% to 43.8%, with thymic cysts accounting for 10.4% to 24.3% of cases (10,19). Ackman et al. later demonstrated that chest MRI not only improved diagnostic accuracy, but also influenced clinical decision-making by reducing the number of planned surgeries (20).

Choi et al. recently reported that low fluorodeoxyglucose uptake (maximum standardized uptake value ≤2.1) on positron emission tomography-CT is a stronger diagnostic marker for thymic cysts than CT features alone (21).

At our center, surgical resection was routinely performed for all thymic cysts and solid tumors prior to 2010. However, we began questioning the necessity of surgery for highly suspected benign cysts based on imaging studies. In cases where cystic lesions were not surgically treated and remained stable or decreased in size during follow-up, we opted to avoid surgery and focused instead on confirming the cystic nature of the lesions. Therefore, if a cystic lesion was suspected of malignancy on CT, MRI was performed. Upon confirmation, an asymptomatic cyst that did not compress surrounding structures was monitored through follow-up CT rather than immediate surgery, regardless of the size of the cyst. We believe that this approach effectively reduces the incidence of unnecessary surgeries.

This study had several limitations. First, it was a retrospective, single-center study, which may have introduced selection bias. However, as a tertiary referral center in the southern region of Korea, we were able to recruit a relatively large number of patients with thymic cysts. Second, image quality varied due to the inclusion of initial CT from external hospitals, and there were no clear guidelines for follow-up CT with a relatively short observation period. Third, the number of patients in each group other than the unchanged group was small, limiting statistical comparisons within or between groups. Fourth, CT follow-up limited detailed evaluation of internal cyst features. However, we believe that signs indicative of malignancy can be sufficiently observed on CT. Finally, there was no basis for pathological diagnosis for all patients. However, two patients who were diagnosed with cysts by MRI and underwent surgery immediately and were excluded from this study, and one patient who underwent surgery during follow-up were confirmed to have cysts in the pathological results. Further studies addressing these limitations should be performed to validate our findings.

Conclusions

Thymic cysts diagnosed using MRI can be safely monitored with routine chest CT without requiring immediate surgery. In our cohort, 7.2% of thymic cysts increased in size, but no cases showed malignant transformation. Furthermore, 6.7% of thymic cysts decreased in size, 2.6% disappeared entirely, and the remainder remained unchanged. These findings support a conservative approach for managing thymic cysts, reducing unnecessary surgeries. Further prospective studies are required to develop definitive guidelines for the long-term management of thymic cysts, including follow-up intervals and endpoints, to ensure optimal patient outcomes.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-915/rc

Data Sharing Statement: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-915/dss

Peer Review File: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-915/prf

Funding: This study was financially supported by Chonnam National University (grant No. 2025-0357-01).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-915/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Institutional Review Board of Chonnam National University Hwasun Hospital (IRB approval No. CNUHH-2025-086). The requirement for individual informed consent was waived due to the retrospective nature of the study.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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