Mediastinal cystic teratoma masquerading as a hydatid cyst

Abstract

A case is presented in which a preoperatively diagnosed hydatid cyst was found to be a mature cystic teratoma on pathological examination. Diagnostic dilemmas surrounding each disease are discussed.

Background

Differentiating the type of mediastinal mass is challenging given the variety of possible aetiologies. We present a unique case whereby a patient with serological and radiographic findings consistent with echinococcal disease was found to have a mediastinal mature cystic teratoma on pathological examination.

Case presentation

An otherwise healthy, active, 35-year-old man presented for management of a large cystic mass in the anterior right mediastinum that was found during workup for dyspnoea and cough (figure 1A). The patient had an extensive travel history throughout North America, the Caribbean, Europe, Australia, Africa and the Middle East. Blood test did not demonstrate eosinophilia, but antihydatid IgG serology was weakly positive. MRI demonstrated a 10.8×9.8 cm internally non-enhancing thick-walled lesion with internal debris suspicious for an infectious process (figure 1B). Thus, the patient underwent an empiric course of albendazole for presumed hydatid cyst disease.

Figure 1.

CT scan (A) and MRI (B) demonstrating a 10.8×9.8 cm internally non-enhancing thick-walled lesion with internal debris.

The patient subsequently underwent a right thoracotomy with resection of the lesion. The mass was adherent to the right pericardium which was resected en bloc. The pericardial defect was closed with a GORE-TEX patch. Gross pathological examination revealed a 12.5×10.8×9.5 cm firm-walled cyst weighing 704.7 g and filled with 50 mL of turbid, debris-filled, brown fluid (figure 2A). The histopathological analysis demonstrated a mature cystic teratoma containing sebaceous glands, fat, respiratory-type epithelium, squamous epithelium and mucinous glands (figure 2B) with fibrosis, inflammation and calcifications in the cyst wall.

Figure 2.

Gross (A) and histological (B) figures demonstrating an ovoid cyst with sebaceous glands (ˆ), fibroadipose tissue (’), respiratory-type epithelium (*), squamous epithelium (†) and mucinous glands (‡) consistent with mature cystic teratoma.

Outcome and follow-up

The patient was discharged home uneventfully 3 days postoperatively. He was seen on postoperative day 20 for outpatient follow-up and reported resolution of his dyspnoea. Because of the diagnosis of mature cystic teratoma, serum human chorionic gonadotropin (hCG) and alpha fetoprotein (AFP) levels were measured at this appointment and were found to be normal. The patient had no family history of malignancy and no clinical suspicion for malignancy (ie, weight loss, night sweats or testicular masses) so further tumour markers including carcinoembryonic antigen (CEA), cancer antigen 19-9 (CA 19-9) and cancer antigen 125 (CA 125) were not measured. The patient continued to do well at most recent contact approximately 18 months postoperatively.

Discussion

Mature cystic teratomas are the most common mediastinal germ cell tumour and, unlike other mediastinal tumours, have no gender predilection. The mediastinum is the most common extragonadal site of teratomas and most patients are asymptomatic.¹ Mature cystic teratomas contain mature tissue from at least two of the three embryonic germ cell layers (endoderm, mesoderm and ectoderm) and no immature tissue or any other type of germ cell tumour found on thorough histopathological examination. The diagnosis may be suspected if imaging demonstrates a well-circumscribed anterior mediastinal mass with internal fat and/or sebaceous elements. The presence of a fat/fluid level within the cyst on CT or x-ray is specific for teratoma but is not sensitive.² This patient’s tumour demonstrated the presence of mature sebaceous glands, mucinous glands, squamous epithelium (ectodermal origin), fat (mesodermal origin) and respiratory-type epithelium (endodermal origin) consistent with a mature cystic teratoma. Our patient’s tumour contained very little fat which explains why it was not appreciated on MRI. After the diagnosis of cystic teratoma, additional workup should be considered to rule out metastases of gonadal origin or the presence of an unsampled mixed germ cell tumour. Our patient had no further evidence of malignancy on imaging or physical examination, so we were satisfied that the lesion was unlikely metastatic. Furthermore, his negative AFP and hCG levels reassured us that the chances of him having unsampled mixed germ cell tumour were low. Further workup was therefore deferred.

Hydatid cyst disease (also referred to as hydatidosis) is a zoonotic infection caused by the larval stage of the tapeworm Echinococcus granulosus. The tapeworm is found worldwide and is considered highly endemic throughout the Mediterranean, Iran, India, Chine, Chile and Argentina.³ Humans are intermediate hosts and are infected after ingestion of the parasite’s eggs, which usually occurs through contact with dogs or sheep. Notably, the patient in the current report is a dog owner but had not recently travelled to a highly endemic region. He had, however, recently travelled to the United Arab Emirates. The larva embeds itself primarily in the liver or lungs and creates a protective cyst. Though commonly found in the lungs, mediastinal localisation of hydatid cysts is relatively rare with approximately 100 cases reported in the English literature.⁴

There is no standardised method to diagnose hydatid cyst disease. The traditional method was the Casoni intradermal test which has been mostly abandoned given its low sensitivity and potential for severe allergic reactions. A number of other immunological tests have been developed, yet each of these tests come with their own advantages and disadvantages. For example, the immunoelectrophoresis test is approximately 90% sensitive in hepatic hydatidosis but only 70% sensitive in pulmonary hydatidosis and results take 3–4 days. Indirect agglutination tests have sensitivities nearing 100% for both hepatic and pulmonary foci of disease but low specificity. Western blotting is a laborious process but has high specificity. ELISA testing for the presence of antihydatid IgG is highly specific and sensitive and results are obtained only in 2 hours.³ However, antihydatid IgG seropositivity only indicates present or past exposure to E. granulosus and does not necessarily indicate active infection. Furthermore, false positives can occur in patients with collagen vascular disease, cirrhosis and schistosomiasis and there is a strong cross-reaction between Echinococcus and cysticercosis.⁵ Though there are no pathognomic radiographic signs, mediastinal hydatid disease should be suspected in large septated cystic lesions in patients who have been to highly endemic regions and who have serological evidence of hydatidosis.^{6 7}

This case demonstrates the challenges associated with the accurate diagnosis of mediastinal masses. Cystic teratomas and hydatid cysts may mimic each other and differentiating the two requires careful interpretation of the patient’s clinical context, radiographic imaging and serological data. The differential diagnosis of mediastinal cysts includes benign congenital cysts (ie, oesophageal duplication, bronchogenic and thymic) and malignancies with cystic degeneration (most commonly after treatment with chemoradiation).⁸ Practitioners should not rely on a single serological or radiographic study to diagnose hydatid cyst disease but should thoughtfully consider the limitations of the available tests. Additionally, surgical exploration and resection should be considered to provide definitive diagnosis and treatment for both pathologies.

Learning points.

• Mature cystic teratomas are the most common mediastinal germ cell tumour.

• Antihydatid IgG positivity does not mean active hydatid disease—only previous exposure to Echinococcus granulosus.

• Surgical exploration and resection should be considered in cases of suspected mediastinal hydatid cysts or teratomas to provide a definitive diagnosis and treatment for both pathologies.