Dr. Theodossiou earned his medical degree from the University of Athens while he completed his internship and residency at Grant Hospital in Chicago. He also completed a Hematology fellowship at Mount Sinai Medical Center in New York and an Oncology fellowship at the National Cancer Institute, NIH, in Bethesda, Maryland.

Dr. Theodossiou is certified in Internal Medicine, Hematology and Medical Oncology. He has been on staff at Ochsner Medical since June 2004 and practices at The Gayle and Tom Benson Cancer Center, Ochsner Medical Center.