A SHEEP IN WOLF'S CLOTHING: BENIGN TERATOMA MASQUERADING AS MALIGNANT THYMOMA

Abstract

TOPIC: Disorders of the Mediastinum TYPE: Medical Student/Resident Case Reports INTRODUCTION: Metastatic parietal pleural implants can be seen in patients with thymoma. Benign teratomas are not associated with parietal pleural implants. We describe resection of a benign teratoma, appearing as a locally advanced thymoma with parietal pleural implants. CASE PRESENTATION: A 37-year-old healthy female was found to have an incidental large pericardial mass on screening breast MRI. A CT Chest showed a 9.4 cm well-defined hypoattenuating calcified cystic lesion with mass effect on the right atrium (Figure 1). Her preliminary diagnosis was thymoma. In the operating room, thoracoscopy demonstrated a large mass with involvement of the pulmonary parenchyma, pericardium, parietal pleural implants, and visceral pleural implants. (Figure 2A, B, C). Robotic en bloc resection was performed. The mass was mobilized with the resection of portions of the upper and middle lobes, pericardium and away from the remaining thoracic structures including the phrenic nerves (Figure 2C, D). Intra-operative frozen section of the mass revealed benign teratoma. Frozen section of the parietal pleural implants revealed fibrosis with teratomatous elements on final pathology (Figure 3). A single chest drain was placed. The patient did well postoperatively and was discharged on postoperative day four. DISCUSSION: To our knowledge, this is the first report of a benign mature teratoma masquerading as a metastatic thymoma. Approximately half of all mediastinal masses in adults are in the anterior mediastinum. The most common mediastinal tumor in adults is thymoma. Thymoma, although considered a low-grade neoplasm, has a propensity for locally invasiveness and recurrence. For these reasons, all thymomas should be approached as aggressive(1). In contrast, benign mature teratomas are well-circumscribed and indolent. Although there is potential for malignant transformation, complete resection results in cure(2). The current recommendation for diagnosis of anterior mediastinal mass is up front resection unless neoadjuvant or medical therapy is being pursued as treatment(1). Preoperative imaging, while helpful, cannot reliably delineate between thymoma and teratoma(3). The higher incidence of thymoma in adults makes the preoperative diagnosis of thymoma more likely. CONCLUSIONS: This case highlights the importance of performing an R0 resection given the known aggressiveness of thymomas as well as highlighting that things are not always as they seem. REFERENCE #1: Detterbeck FC ed. Pathology of the Mediastinum. New York: Cambridge University Press, 2014 REFERENCE #2: Lewis BD, Hurt RD, Payne WS et al Benign Teratomas of the Mediastinum J Thorac Cardiovasc Surg 1983; 86 (5): 727-731. REFERENCE #3: Tomiyana N, Honda O, Tsubamoto M et al Anterior Mediastinal Tumors: Diagnostic Accuracy of CT and MRI. Eur J Radiol. 2009; 69 (2): 280-288. DISCLOSURES: No relevant relationships by Mia DeBarros, source=Web Response No relevant relationships by Grant Fischer, source=Web Response No relevant relationships by M. Blair Marshall, source=Web Response No relevant relationships by Sue Wang, source=Web Response