AB005. Thymic carcinoma arising in a multilocular thymic cyst that previously went through complete remission after antibiotics treatment

Background Multilocular thymic cysts are sometimes associated with thymic epithelial tumors (TETs) and may be misdiagnosed as benign lesions. Cystic TETs generally progress during the course of follow-up. Case Description A 55-year-old woman was referred to our hospital in 2017. Her symptoms and physical exams were unremarkable. No abnormal lab results were detected other than elevated ESR (76 mm/lh). Contrast chest computed tomography (CT) showed an anterior mediastinal mass of 3×2×5 cm3 with heterogenous attenuation. The cystic feature was confirmed on contrast magnetic resonance imaging (MRI). Positron emission tomography (PET)-CT was also performed and there was no uptake in the lesion. Benign thymic cyst with infection was among the differential diagnoses. Therefore, intravenous antibiotics were administered for a week. A follow-up CT performed a month later showed a radical change: no visible lesion was present in the anterior mediastinum. Patient was cautioned that malignancy was still possible and that regular follow-up was necessary. The patient did not have another chest CT until the end of 2020. Follow-up chest CT showed lobulated anterior mediastinal mass with multiple pleural implants, highly suggestive of malignancy. Thymectomy plus pleurectomy was performed. Patient was discharged on post-operative day 10. Diagnosis: the pathology was thymic squamous cell carcinoma. The tumor invaded right pleura and pericardium, and pleural implants were confirmed metastasis (T2N0M1a, Stage IVa). Resection status was R0. Adjuvant radiation and chemotherapy were administered. Patient experienced recurrence after 10 months. Conclusions Multilocular thymic cysts can lead to misdiagnosis. Regular follow-up is needed if upfront surgery lacks evidence of malignancy and is deemed inappropriate at first. In cystic lesions or small lesions, it is sometimes difficult to make a clinical decision between surgery and follow-up based on imaging alone. Rash decision might result in unnecessary surgery or disease progression. Novel diagnostic tools might provide insights into the decision- making of these patients.