Dr. Cullen specializes in head and neck cancer. A graduate of Dartmouth College and Harvard Medical School, Dr. Cullen completed his internship and residency at Beth Israel Hospital in Boston and received additional training at the National Cancer Institute.

He served as interim director of the Lombardi Cancer Center at Georgetown University from October 2000 to September 2002 and was professor of medicine, oncology, and otolaryngology at Georgetown University School of Medicine until joining the University of Maryland in 2004.

Under his leadership, the University of Maryland Greenebaum Cancer Center received NCI Cancer Center designation in 2008. The center was ranked #11 out of 900 cancer programs nationally in the 2012 U.S. News & World Report's "Best Hospitals" list. Dr. Cullen is a member of the American Cancer Society National Board of Directors, is on the board of directors of the Association of American Cancer Institutes, and was appointed by President Obama as a member of the National Cancer Advisory Board.