Dr. Efstathiou is Professor of Radiation Oncology at Harvard Medical School (HMS) and Director of the Genitourinary (GU) Division in the Department of Radiation Oncology and Clinical Co-Director of The Claire and John Bertucci Center for GU Cancers at Massachusetts General Hospital. He holds a BS from Yale University, MD from HMS, DPhil from the University of Oxford, and completed his residency training in the Harvard Radiation Oncology Program.

His busy and robust clinical practice focuses on treatment of patients with prostate, bladder, testicular and other urologic cancers, as well as proton beam therapy and brachytherapy. Delivery of first-rate excellent patient care is a focus and his commitment to patient care, service and mentorship has been recognized as honoree for the MGH Cancer Center's "The One Hundred", as nominee for the Brian A. McGovern Award for Clinical Excellence, as Health Professional of the Game (for the Patriots and Red Sox), as a MGH Mentor with Distinction, and as recipient of the 2021 A. Clifford Barger Excellence in Mentoring Award from HMS.

He is an internationally recognized leader in the field of GU malignancies and Radiation Oncology. His research has informed clinical guidelines and made novel contributions to organ preservation for bladder cancer and technology assessment of advanced/emerging therapies for prostate cancer including proton therapy. He is the recipient of significant grant funding and serves as the Principal Investigator of multi-center, NCI-sponsored, randomized trials in prostate and bladder cancer. He also remains active in evaluating biomarkers for bladder and prostate cancer outcomes. Other projects of his include the development of web-based patient decision aids/support tools, integrating Patient Reported Outcome Measures (PROMs) into practice, evaluating the multidisciplinary approach to cancer care, and global oncology outreach efforts with Botswana.

He lectures nationally and internationally and holds leadership positions in Oncology including within the cooperative groups (NRG Oncology - Vice-Chair GU Steering Committee and Bladder Lead), professional societies (ASTRO, ASCO, ABR, ACRO), and the National Cancer Institute (NCI). He also serves on patient advocacy organizations including the Bladder Cancer Advocacy Network (BCAN) and on the Board of Directors for the Massachusetts Prostate Cancer Coalition (MPCC).