SESSION TITLE: Tuesday Fellows Case Report Posters SESSION TYPE: Fellow Case Report Posters PRESENTED ON: 10/22/2019 01:00 PM - 02:00 PM INTRODUCTION: Rhabdomyosarcoma (RMS) is a rare malignancy in adults. Data regarding treatment regimens is limited, making management of this malignancy difficult. CASE PRESENTATION: A 61 y/o female presented for evaluation of an anterior mediastinal mass. She was experiencing a dry cough, chest discomfort and a 25-pound weight loss for 2 years. Exam was unremarkable with an O2 saturation of 96% on room air. Chest CT revealed a 10.8cm anterior mediastinal mass with central necrosis (Figure 1). She underwent CT guided biopsy of the mediastinal mass and pathology reported a pleomorphic spindle cell lesion with focal myogenic differentiation. While awaiting outside pathology report, the patient developed worsening SOB and was hospitalized. She was intubated after admission for respiratory distress. CXR showed enlargement in her mediastinal mass (Figure 2). After intubation she required vasopressor support and temporary prone ventilation. CT surgery took the patient for emergent mediastinal exploration with excision of her mass. The mass was abutting the posterior sternum and growing into the pericardium. Most of the mass was removed with decompression of the airway and surrounding mediastinal structures. Final surgical pathology reported pleomorphic rhabdomyosarcoma (RMS). Further imaging did not reveal any metastatic disease. Given the rarity of this tumor and its unique location, the patient was referred to higher level of care for treatment planning. She eventually received one round of chemotherapy but continued to decline and died a few days after treatment. DISCUSSION: RMS are primarily pediatric neoplasms, although they can rarely occur in adults.1 Histologically, the WHO divides RMS into 4 sub-types: embryonal, alveolar, pleomorphic and spindle cell/sclerosing.1 Pleomorphic RMS (PRMS) are quite rare, accounting for 3% of soft tissue sarcomas and less than 1% of adult malignancies.2 Mediastinal PRMS are extraordinarily rare with well under 100 reported cases.3 PRMS has a tendency to metastasize to the lungs, lymph nodes and bone marrow with metastatic disease often present at initial diagnosis.2 Standard treatment is surgical resection with wide margins. When surgery is incomplete or unable to be performed, chemotherapy and/or radiation can be pursued.2 Given the rarity of this tumor, there is limited data to guide chemotherapy, with many studies reporting it is refractory to standard chemotherapy medications. PRMS is one of the most aggressive types of RMS with a reported 5-year mortality rate of 27-44%.2 CONCLUSIONS: PRMS in adults is a rare, aggressive malignancy with few guidelines for treatment. As our case illustrates, it can present in atypical locations, making treatment even more complex. As with our patient, appropriate preparation regarding proper positioning, medications, ventilation strategy and volume status must also be considered when managing patients with mediastinal masses. Reference #1: Rudzinski, Erin R., et al. “The World Health Organization Classification of Skeletal Muscle Tumors in Pediatric Rhabdomyosarcoma: A Report From the Children's Oncology Group.” Archives of Pathology & Laboratory Medicine, vol. 139, no. 10, 2015, pp. 1281–1287., https://doi.org/10.5858/arpa.2014-0475-oa. Reference #2: Xi, Shiwen, and Weihua Tong. “Pleomorphic Rhabdomyosarcoma Metastasis to Small Intestine Causing Intussusception.” Medicine, vol. 97, no. 51, 2018, https://doi.org/10.1097/md.0000000000013648. Reference #3: Nishijima, Yoshimi, et al. “Pleomorphic Rhabdomyosarcoma Arising in the Anterior Mediastinum: A Case Report with Cytological Features of Imprint and Liquid-Based Cytology Specimens.” Diagnostic Cytopathology, vol. 45, no. 4, 2016, pp. 333–338., https://doi.org/10.1002/dc.23654. DISCLOSURES: My spouse/partner as a Speaker/Speaker's relationship with Salix Please note: $1001 - $5000 Added 03/13/2019 by Ebtesam Islam, source=Web Response, value=Speaker fee No relevant relationships by Audra Schwalk, source=Web Response