Giant mature mediastinal teratoma mimicking a pericardial cyst

Abstract

Background

Mature mediastinal teratomas are uncommon benign germ cell tumors. When the cystic component predominates and typical features such as fat or calcification are absent, diagnosis can be misleading.

Case Summary

A 17-year-old male had a brief syncopal episode two months prior. At presentation, his primary complaint was mild chest tightness. Chest computed tomography showed a 13.6 × 10.3 × 7.9 cm homogeneous cystic mass in the anterior mediastinum, closely attached to the pericardium. Attenuation measured 17 ± 15 Hounsfield units on non-contrast computed tomography, consistent with fluid density and showing no enhancement. Transesophageal echocardiography confirmed that the lesion compressed but did not invade the heart, indicating an extrapericardial mass. It was initially diagnosed as a pericardial cyst. Because of its large size and dense pericardial attachment, the tumor was resected via median sternotomy. Histology demonstrated mature cystic teratoma. Recovery was uneventful.

Conclusion

Cyst-dominant teratoma can closely resemble a pericardial cyst on computed tomography. Using complementary imaging such as transesophageal echocardiography or magnetic resonance imaging helps avoid misinterpretation. Early complete removal ensures accurate diagnosis and excellent prognosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13019-026-03916-1.

Introduction

Mediastinal teratoma represents about 10–20% of anterior mediastinal tumors and is most common in adolescents and young adults [1, 2]. Typical imaging features include fat, calcification, or mixed cystic–solid components [3]. When these signs are missing and the lesion appears completely cystic, it may be mistaken for a benign congenital cyst, such as a pericardial cyst [4–6].

Misdiagnosis can delay appropriate treatment and prolong patient anxiety. Although purely cystic teratomas have been reported in scattered case studies, systematic comparison of their imaging features, diagnostic pitfalls, and surgical outcomes remains limited. Most prior reports involved adult women presenting with incidental findings on routine imaging [4–7]. Cases in adolescent males, especially those with lesions exceeding 10 cm and tightly adherent to the pericardium, are rare.

We describe a large mature mediastinal teratoma in a 17‑year‑old male that was initially diagnosed as a pericardial cyst on computed tomography (CT). This case demonstrates how overlapping imaging findings can confuse the diagnosis, how transesophageal echocardiography (TEE) can clarify pericardial relationships, and how surgical strategy affects outcome. It also supports recent multimodality recommendations for evaluating pericardial and mediastinal lesions [8, 9] and emphasizes that large cystic masses in young males should prompt early surgical exploration. By comparing our case with similar reports in the literature, we aim to expand the clinical spectrum of cyst-dominant teratomas and provide practical guidance for clinicians managing such lesions.

Case report

Patient information and clinical findings

A 17-year-old male was referred for evaluation of an anterior mediastinal mass incidentally discovered during workup for a brief syncopal episode two months earlier. At presentation, he reported several weeks of mild chest tightness without recurrent syncope, chest pain, or dyspnea. Physical examination was unremarkable. Chest X-ray showed a smooth anterior mediastinal opacity occupying the left cardiophrenic angle (Fig. 1A).

Fig. 1.

Preoperative imaging findings. (A) Chest radiograph showing a homogeneous opacity occupying the left cardiophrenic angle with mild mediastinal shift to the right. (B) Sagittal contrast-enhanced CT demonstrating a large, well-defined cystic mass in the anterior mediastinum compressing the adjacent cardiac border. (C) Axial CT image illustrating the close relationship between the lesion and the pericardium without clear fat or calcification. (D) Transesophageal echocardiography revealing a cystic mass located anterior to the left atrium and outside the pericardial sac, showing heterogeneous internal echogenicity (arrow). CT, computed tomography

Diagnostic assessment

Contrast‑enhanced CT revealed a 13.6 × 10.3 × 7.9 cm well‑circumscribed cystic mass closely adherent to the pericardium. Non‑contrast CT revealed a homogeneous cystic lesion with an average attenuation value of 17 ± 15 Hounsfield units (HU), consistent with fluid density and showing no internal enhancement or solid components on contrast-enhanced sequences (Fig. 1B, C). Hounsfield unit measurements were obtained on non-contrast axial CT using three circular regions of interest (ROI, 1.0–1.5 cm²) placed centrally within the cystic lumen while carefully avoiding the cyst wall and any septations; the mean attenuation was 17 ± 15 HU.The thin, regular wall and absence of fat or calcification suggested a simple fluid-filled cyst. Based on these findings, the mass was initially considered most likely a pericardial cyst.

To better define its relationship with the heart and pericardium, TEE was performed. This showed a heterogeneous cystic structure compressing but not invading the right atrium and ventricle, confirming an extrapericardial location (Fig. 1D, Movie 1). The internal echogenicity was not entirely homogeneous, raising the possibility of more complex contents than pure serous fluid. Laboratory tests, including complete blood count and tumor markers (alpha-fetoprotein, human chorionic gonadotropin, carcinoembryonic antigen), were unremarkable.

Magnetic resonance imaging (MRI) was discussed but not performed for two reasons: first, TEE had already provided clear localization and demonstrated extrapericardial origin; second, the patient declined additional imaging due to financial concerns and preferred to proceed directly to surgical treatment given the symptomatic nature and large size of the mass.

Therapeutic intervention

Surgical resection was performed through a median sternotomy. This approach was chosen for several reasons. First, the tumor measured more than 10 cm in diameter, making complete thoracoscopic removal challenging. Second, preoperative imaging showed dense adhesion between the cyst and the left pericardium, raising concern that attempting video-assisted thoracoscopic surgery (VATS) might result in intraoperative rupture or incomplete excision. Median sternotomy provided optimal exposure for safe dissection and en bloc removal.

Intraoperatively, the cyst was found to adhere densely to the left pericardium. To avoid capsule violation and ensure complete excision, part of the pericardium was removed en bloc with the mass (Fig. 2A). The tumor had no invasion into surrounding structures, including the lung or heart. Grossly, the tumor was a unilocular cyst measuring approximately 13 × 10 cm and containing yellow sebaceous fluid (Fig. 2B).

Fig. 2.

Intraoperative and pathological findings. (A) Intraoperative view showing the large cystic tumor with dense adhesion to the pericardium but no invasion of surrounding structures. (B) Gross specimen after complete resection, demonstrating a unilocular cyst measuring approximately 13 × 10 cm. (C) Histopathological section (hematoxylin & eosin stain, ×100) showing mature epithelial and mesodermal components, including cartilage and sebaceous glands, consistent with mature teratoma. (D) Postoperative CT scan confirming complete excision and full re-expansion of the left lung. CT, computed tomography

The specimen was opened in the operating room to display its internal contents for educational purposes and to guide pathological sampling (Movie 2). Sectioning revealed sebaceous material, hair, and focal calcified areas, characteristic of a mature cystic teratoma.

Pathological findings

Microscopy revealed stratified squamous epithelium, sebaceous glands, respiratory‑type epithelium, smooth muscle, and mature cartilage, confirming a mature cystic teratoma derived from all three germ layers (Fig. 2C). No immature or malignant elements were identified. The final diagnosis was a benign mature cystic teratoma with no invasion.

Follow-up and outcomes

Recovery was uneventful. Postoperative CT confirmed complete resection and full lung re‑expansion (Fig. 2D). At 12-month follow‑up, the patient remained asymptomatic with no signs of recurrence on imaging.

Discussion

Literature context and comparison with similar cases

To contextualize our case, we conducted a targeted literature search in PubMed, Web of Science, and Google Scholar (up to November 2025) using the terms “mediastinal teratoma,” “cystic teratoma,” “pericardial cyst,” and “fat-poor teratoma.” We identified seven recent case reports (2022–2025) describing cyst-dominant mediastinal teratomas initially misdiagnosed as other benign cystic lesions [4–7, 10–12]. These cases are summarized in Table 1.Mature mediastinal teratomas usually contain solid and fat components [1, 2]. When cystic fluid dominates, distinguishing them from pericardial or thymic cysts can be difficult. Several reports have described such misdiagnoses that were resolved only after surgery and pathologic confirmation [4–7].

Table 1.

Comparative Summary of Reported Cyst-Dominant Mediastinal Teratomas

Study (First Author, Year)	Country	Age/Sex	Tumor Location	Presenting Symptom	Imaging Used	Initial Misdiagnosis	CT Imaging Features	Management	Outcome
Kym et al., 2024 [4]	South Korea	32/F	Left anterior mediastinum	Asymptomatic (routine check)	CT, VATS	Pericardial cyst	Large fluid-dominant cystic mass adjacent to pericardium with thin wall; no fat/calcium	VATS excision	Uneventful recovery; no recurrence
Su et al., 2024 [5]	China	28/M	Anterosuperior mediastinum	Chest discomfort	CT	Thymic cyst	Homogeneous low-attenuation mass; nearly fluid-density	Median sternotomy; complete resection	Good recovery; benign histology
Kakamad et al., 2025 [6]	Iraq	35/F	Right anterior mediastinum	Cough, chest pain	CT, CXR	Bronchogenic cyst	Thin-walled multiloculated cystic lesion; no fat or calcification	Right thoracotomy	Uneventful; no recurrence at 6 mo
Goyal et al., 2023 [7]	India	40/F	Anterosuperior mediastinum	Asymptomatic (health screening)	CT	Pericardial cyst	Well-defined cyst adjoining heart border; non-enhancing	Median sternotomy	Full recovery; benign teratoma
Natalie et al., 2023 [10]	USA	30/F	Anterior mediastinum	Asymptomatic	CT, MRI	Thymic cyst	Cystic lesion without fat signal on MRI	VATS resection	No recurrence at 7 mo
Ghareeb et al., 2023 [11]	Egypt	27/M	Right anterior	Chest pain	CT	Hydatid cyst	Intact capsule; heterogeneous cystic mass with soft-tissue nodules	Right thoracotomy	Benign teratoma; no recurrence
Manabe et al., 2022 [12]	Japan	25/M	Left anterior	Chest discomfort	CT	Pleural cyst	Calcified mass with fluid level; components of hair/sebum	Complete resection	Disease-free at 1 yr
Present case	China	17/M	Anterior mediastinum	Brief syncope 2 months prior; current mild chest tightness	CT, TEE	Pericardial cyst	Homogeneous fluid attenuation (17 ± 15 HU); no fat/calcium; no enhancement	Median sternotomy	No recurrence at 12 mo

CT, computed tomography; F, female; HU, Hounsfield units; M, male; MRI, magnetic resonance imaging; VATS, video-assisted thoracoscopic surgery

Most reported patients were female adults whose lesions were initially interpreted as simple cysts. Our adolescent male case with a giant unilocular lesion (> 13 cm) expands this pattern and emphasizes early multimodal assessment and surgery

Table 1 compares recent cases of cyst‑dominant teratomas. Kym et al. [4] reported a 32-year-old woman with a fluid-dominant anterior mediastinal mass initially thought to be a pericardial cyst. The lesion was successfully removed by VATS. Su et al. [5] described a 28-year-old man with a homogeneous low-attenuation mass nearly fluid-density on CT, preliminarily diagnosed as a thymic cyst but proven to be mature cystic teratoma after median sternotomy. Kakamad et al. [6] presented a 35-year-old woman with a multiloculated thin-walled cystic lesion without fat or calcification, initially mistaken for a bronchogenic cyst. Goyal et al. [7] reported a 40-year-old woman whose well-defined non-enhancing cyst adjoining the heart border was interpreted as a pericardial cyst before surgical excision confirmed teratoma. Additional cases from Natalie et al. [10], Ghareeb et al. [11], and Manabe et al. [12] further illustrate the diagnostic challenge posed by fat-poor or cyst-dominant teratomas.

Most reported patients were women in their third to fifth decade; none of the cases involved an adolescent male with a large unilocular lesion exceeding 13 cm. Our case broadens the clinical spectrum and highlights that age, sex distribution, and lesion size alone should not exclude teratoma from the differential diagnosis. The rarity of such presentation in young males underscores the importance of maintaining a high index of suspicion for germ cell tumors even when imaging suggests a simple benign cyst.

Diagnostic pitfalls and the value of multimodality imaging

A purely cystic mediastinal lesion may appear innocuous on CT. The absence of fat or calcification can suggest thymic or pericardial cysts and lead to delayed surgery [4, 5]. In our case, the homogeneous fluid attenuation of 17 ± 15 HU on non-contrast CT and lack of enhancement on contrast-enhanced CT strongly favored a simple fluid-filled cyst. However, TEE proved useful in showing that the mass was extrapericardial and had heterogeneous internal echogenicity, which raised suspicion for more complex contents. This finding was critical in reconsidering the diagnosis and proceeding with surgical resection rather than conservative observation.

CT remains the first‑line modality for characterizing mediastinal masses due to its wide availability, rapid acquisition, and excellent spatial resolution [3, 8]. However, multimodality correlation is vital when findings are equivocal. TEE is especially valuable when a lesion is adjacent to the heart or pericardium, providing real‑time assessment of motion, contact, and internal echotexture [8, 9]. MRI offers complementary information by better characterizing soft tissue components, identifying internal debris or septation, and distinguishing fluid from fat [13]. In our patient, MRI was not performed because TEE had already provided clear localization and the patient preferred to proceed directly to surgery. Nonetheless, in retrospect, MRI might have detected signal heterogeneity suggestive of proteinaceous or sebaceous material, potentially prompting earlier consideration of teratoma.

Recent expert consensus statements emphasize the role of multimodality imaging in pericardial and mediastinal disease [8, 9]. Klein et al. [9] outlined standardized criteria for integrating echocardiography, CT, and cardiac magnetic resonance to optimize diagnostic accuracy. Verhaert et al. [8] discussed how combining modalities can prevent misdiagnosis in cases where a single imaging technique yields ambiguous results. Our case supports these recommendations and illustrates how TEE can unmask diagnostic pitfalls in apparently simple cystic lesions.

Hounsfield unit data and imaging characteristics

Hounsfield unit measurements were obtained by placing three circular regions of interest (ROI, 1.0–1.5 cm²) in the central portion of the cystic lesion on axial non-contrast CT images, carefully avoiding the cyst wall and any internal septations.The measured attenuation of 17 ± 15 HU (mean ± standard deviation) on non-contrast CT fell within the range typically seen in simple fluid collections such as pericardial cysts (0–20 HU) [3]. Contrast-enhanced CT showed no internal enhancement, further supporting a benign cystic process. These features explain why the lesion was initially interpreted as a pericardial cyst. However, HU values alone cannot reliably distinguish pericardial cysts from cyst-dominant teratomas, especially when sebaceous or proteinaceous fluid yields similar attenuation to serous fluid [4, 10]. This underscores the limitation of relying solely on CT density measurements and the need for complementary imaging.

Surgical approach and rationale

Although thoracoscopic resection is increasingly used for benign mediastinal masses, median sternotomy offers better exposure for large, adherent teratomas [14, 15]. In our patient, the decision to perform median sternotomy was based on tumor size (> 10 cm) and anticipated dense pericardial adhesion seen on preoperative imaging. This approach prevented capsule rupture and enabled safe en bloc removal, including a portion of adherent pericardium.

VATS has been successfully applied in selected cases of mediastinal teratoma [4, 10], particularly when lesions are smaller, mobile, and well-encapsulated. However, for tumors exceeding 10 cm or demonstrating tight adhesion to vital structures, open surgery remains the safer option to ensure complete resection and avoid intraoperative complications [5, 14]. Su et al. [5] also chose median sternotomy for a similar case due to tumor size and proximity to the pericardium. In contrast, Kym et al. [4] successfully performed VATS excision in a 32-year-old woman whose mass was smaller and less adherent.

The specimen was opened intraoperatively to display internal contents for educational purposes and to guide pathological sampling. This practice allows direct visualization of sebaceous material, hair, and calcification, features highly suggestive of mature teratoma, and facilitates targeted tissue sampling for histopathological confirmation.

Pathological confirmation and prognostic implications

Histopathology confirmed a mature teratoma with elements from all three germ layers: ectoderm (stratified squamous epithelium, sebaceous glands), endoderm (respiratory-type epithelium), and mesoderm (smooth muscle, mature cartilage). The absence of immature or malignant components indicated a benign lesion with excellent prognosis [1, 2]. Complete excision is curative in such cases, with recurrence rates near zero [12, 14].

Our patient remains asymptomatic at 12-month follow-up with no radiographic evidence of recurrence. This outcome is consistent with prior reports showing that complete surgical resection of mature teratomas results in long-term disease-free survival [4–7, 10–12].

Clinical implications and recommendations

This case highlights several important clinical messages. First, purely cystic mediastinal masses in young patients should not automatically be presumed benign based on CT appearance alone. Teratoma should be considered in the differential diagnosis, particularly when lesions are large, occur in adolescents or young adults, or demonstrate atypical features such as heterogeneous internal echogenicity on ultrasound. Second, complementary imaging modalities such as TEE or MRI can provide critical diagnostic clues and prevent delays in appropriate surgical management. Third, early resection is advisable when a “simple” cyst exceeds 10 cm or is tightly attached to the pericardium, as observation may miss the opportunity for definitive diagnosis and cure.

For clinicians managing similar cases, we recommend the following approach: (1) Obtain multimodality imaging (CT plus TEE or MRI) when initial CT findings are equivocal or inconsistent with clinical presentation; (2) Consider surgical exploration for large (> 10 cm) cystic mediastinal masses, especially in adolescents and young adults, even if imaging suggests a benign lesion; (3) Choose median sternotomy over VATS when tumors are large or densely adherent to vital structures to ensure safe and complete resection; (4) Perform complete histopathological examination to confirm diagnosis and exclude immature or malignant components.

Limitations

This is a single case report with 12‑month follow‑up and without preoperative MRI; therefore, generalizability is limited, and MRI might have further characterized internal components.

Conclusion

Cystic mediastinal teratomas may appear identical to pericardial cysts on CT, especially when fluid is clear and density uniform. Combining CT with TEE or MRI enables accurate localization and appropriate surgical planning. Complete resection is associated with excellent long‑term outcomes in benign mature teratomas. Clinicians should consider teratoma in any large, simple-appearing mediastinal cyst, as “simple cysts can mislead—TEE and surgery can unmask them.” Our case expands the clinical spectrum of cyst-dominant teratomas to include adolescent males with giant unilocular lesions and provides practical guidance for multimodality imaging and surgical decision-making.

Author contributions

FL: Conceptualization, data collection, writing – original draft preparation, writing – review and editing. HL: Conceptualization, methodology, supervision, writing – review and editing, project administration, funding acquisition. Both authors have read and approved the final manuscript.

Funding

This study was supported by the Natural Science Foundation of Shandong Province of China (Grant No. ZR2020QH025). The funding body had no role in the design of the study, data collection, analysis, interpretation of data, or writing of the manuscript.

Data availability

All data generated or analyzed during this study are included in this published article and its supplementary information files. Additional clinical data are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Written informed consent was obtained from the patient and his legal guardian. This case report was granted an ethics review waiver by the Ethics Committee of Shandong Provincial Hospital, Shandong First Medical University, as it involves a single retrospective case without identifiable information, and was conducted in accordance with the Declaration of Helsinki.

Consent for publication

Written informed consent for publication of clinical details and accompanying images was obtained from the patient and his legal guardian. A copy of the consent form is available for review by the Editor-in-Chief of this journal.

Competing interests

The authors declare no competing interests.