A graduate of USC and New York Medical College, Dr. Hesselgesser completed his residency in radiation oncology at LA County USC Medical Center. Board-certified in radiation oncology in 1975, he is a member of the American Medical Association, the American Society of Clinical Oncology (ASCO), the American College of Radiology, the American Society of Therapeutic Radiology and Oncology (ASTRO) and the American Endocurietherapy Society. Dr. Hesselgesser is a specialist in radioactive implants with high and low dose rate Iridium-192, and has lectured internationally on the topic, including its use in lung, brain and nasopharyngeal tumors. He has performed over 250 I-125 prostate implants, one of the largest series done by a single physician.

Dr. Hesselgesser practices with North Oaks Radiation Oncology Center, in Thousand Oaks and Westlake Radiation Oncology Center in Westlake Village.