A 36-year-old male presented with a 1-week history of fevers, chills, productive cough, chest pain, dyspnoea on exertion, loss of appetite, and generalized weakness. The chest pain was described as being located in the middle of the back, pressure like in quality, constant in duration, 5/10 in severity, non-radiating, and non-pleuritic. The patient’s history is notable for mediastinal PEComa status post resection, essential hypertension, and paroxysmal atrial fibrillation status post appendage ligation currently not on anticoagulation. Of note, the initial occurrence of PEComa was originally misdiagnosed as a rhabdomyoma, and it was correctly identified as a PEComa upon re-evaluation of the corresponding histopathological slides during the current presentation. Since the evaluation and management of the first presentation occurred at an outside facility, details regarding the final diagnosis are limited. Additionally, indication for appendage ligation, performed at the same time the PEComa was resected, is also unclear, but since the patient is young and has a low CHA2DS2-VASc score of 1, the most likely reason is being off lifelong anticoagulation. Given the patient had a history of a mediastinal PEComa, tumour recurrence was highest in the differential. Other aetiologies included pneumonia. Vital signs were unremarkable. Physical exam was notable for an irregularly irregular rhythm and sternotomy scar. Laboratory data, including inflammatory markers, were unremarkable. Electrocardiogram was significant for normal sinus rhythm, first-degree atrioventricular block, and left atrial enlargement. Chest radiograph was unremarkable. Based on concerns for tumour recurrence, a transthoracic echocardiogram was obtained, and it showed a globular mass involving the posterior left atrium and pulmonary veins and measuring 20 cm circumferentially. Since the mass appeared to involve posterior cardiac structures, and transoesophageal echocardiography, compared with transthoracic echocardiography, offers better visualization of posterior cardiac structures due to close proximity of the oesophagus to the posteromedial heart with lack of intervening lung and bone, a transoesophageal echocardiogram was performed. The echocardiogram showed a large globular bilobed mass arising from the posterior aspect of the left atrium near the roof, pushing onto the interatrial septum into the right atrium, and either extending into or arising from the right upper pulmonary vein, with one lobe measuring 36 mm × 36 mm (). Given ambiguity about structures affected by the mass as well as features of the mass, a cardiac magnetic resonance imaging (cMRI), which offers high spatial and temporal resolution, especially compared with ultrasound, and the ability to characterize tissue, was ordered. The cMRI showed a 40 mm × 53 mm bilobed posterior mediastinal mass abutting the roof of the left atrium and right atrium, compressing the right inferior pulmonary vein, and extending superiorly to the main pulmonary artery and inferiorly to the inferior vena cava (). T1-weighted images demonstrated that the mass had a similar appearance to cardiac myocytes. T2-weighted images demonstrated that the mass was hyperintense to skeletal muscle. First-pass perfusion signal was elevated, highlighting that the mass was hypervascularized. Delayed enhancement was present, reflecting the rich stroma and neovascularization of the mass. Inversion recovery sequencing revealed no scars. These findings, together, suggested recurrent PEComa, thymoma, teratoma, or leiomyoma. Furthermore, the imaging features, specifically a well-defined mass without invasion of adjacent cardiac structures and pericardial effusion/thickening, favoured a localized lesion over a metastatic one. To stage the cardiac mass, a computed tomography angiography of the chest, abdomen, and pelvis was performed, and it re-demonstrated the mass without any evidence of metastasis. Cardiothoracic surgery (CTS) was consulted, who, upon thorough review of all available data and after discussing the case during weekly tumour board rounds, recommended surgical resection of the mass given its large size, invasive features (i.e. compressing the right inferior pulmonary vein, right and left atria, and inferior vena cava), and proximity to the oesophagus. Of note, analysis of the operative report related to resection of the initial mediastinal PEComa revealed that the initial mass and recurrent mass originated from the same location. Since the patient had the prior mass resected via a median sternotomy, the repeat resection was performed via a right lateral posterior thoracotomy while on cardiopulmonary bypass using peripheral cannulation. Furthermore, hypothermia was induced so the heart could be safely fibrillated, allowing for reconstruction of the pulmonary vein and left atrium without arresting the heart. Intraoperative findings were consistent with imaging findings. Patient had an uncomplicated recovery course, and he was safely discharged home on postoperative Day 6. Pathologic examination of this tumour revealed a portion of intact tumour that was partially encapsulated with areas of capsular disruption and a separate aggregate of tumour fragments (). The cut surface of the tumour was tan, firm, and slightly friable in areas with focal areas of haemorrhage. Microscopic examination showed tumour cells to have an epithelioid morphology with abundant clear to eosinophilic cytoplasm and scattered prominent nuclear atypia (). Immunohistochemical stains highlighting myogenic differentiation, such as smooth muscle actin and desmin, were positive, and the proliferation index by ki-67 was 20–30%; no definite melanocytic differentiation was identified. No definite invasion of the myocardium was identified. In summary, despite the lack of definitive melanocytic differentiation, these findings were most consistent with a malignant PEComa. Due to the piecemeal nature of the resection, definite evaluation of margins was not possible. Following discussion of the case and final pathology report during tumour board rounds, the healthcare team believed it was in the best interest of the patient to follow-up with medical oncology and radiology oncology for further recommendations, including initiation of everolimus and radiation, respectively. Patient was seen in CTS clinic ∼1 month after he was discharged from the hospital, and he reported doing well, including complete alleviation of all the original presenting symptoms. Additionally, the pathology report and recommendations were shared, and patient acknowledged both.