Dr. Yoon was an attending surgeon in the Division of Surgical Oncology at Massachusetts General Hospital (MGH) until September of 2009. At MGH, he was an active member of both the Center for Sarcoma and Center for Gastrointestinal Oncology. His clinical practice focused on patients with soft tissue sarcomas and gastric cancer, and he also has significant experience in treating patients with other upper gastrointestinal malignancies and melanoma.

In addition to the clinical care of patients, Dr. Yoon is also highly involved in both clinical research and translational research. He is the principal investigator of a National Cancer Institute sponsored clinical trial examining the use of targeted biological agents to improve the effects of radiation therapy for sarcomas, and also participated in several clinical trials at MGH examining the use of proton beam radiation therapy for sarcomas in difficult anatomic locations. Dr. Yoon has served on sarcoma committees for the American College of Surgeons Oncology Group and the Radiation Therapy Oncology Group. Dr. Yoon received additional training in gastric cancer surgery at the Seoul National University Hospital (South Korea), which performs nearly 1000 surgeries for gastric cancer each year. He is the principal investigator of a planned multi-institutional clinical trial examining the role of more extensive lymph node dissections in the surgical treatment of gastric cancer and a member of the International Gastric Cancer Association.