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June 1, 2015

Proton Beam Therapy: What does a Physicist do?

By Melissa Abrams Caulfield

Josh Stoker, Ph.D., Radiation Oncology

Josh Stoker, Ph.D., Radiation Oncology

Proton Beam Therapy:  What does a Physicist do?

Physics is the study of the fundamental particles and forces, such as electrons, protons and quantum mechanics, that comprise the world. These are not phenomena typically associated with the medical field. However, a team of physicists is involved in patient care every day at Mayo Clinic.

Using physics tools in cancer care

A medical physicist is a scientist who applies physics to medicine, usually in radiation oncology or medical imaging. A subset of medical physicists in the Department of Radiation Oncology works on the technical aspects of Mayo Clinic’s Proton Beam Therapy program.

The accelerators are the backbone of proton beam therapy and, in Mayo’s program, are used to create a narrow (pencil-sized) beam of protons that is precisely delivered in state-of-the-art radiation therapy to cancer patients.

Proton beam therapy requires many imaging modalities and computer systems to analyze images and calculate radiation doses. The physicists in the Proton Beam Therapy program verify that each piece of equipment works properly.

Making sure proton beams are delivered safely and accurately

“Mayo Clinic has constructed top-of-the line facilities, providing an optimal combination of features found in existing proton beam therapy systems from around the world and introducing new features to improve treatment efficacy,” says Josh Stoker, Ph.D., Radiation Oncology, who is a physicist on the Arizona campus.

Dr. Stoker has worked with particle beams for more than seven years, including one year at Mayo Clinic and two years at MD Anderson in Houston. He describes some of a physicist’s more hands-on roles — from participation in patient treatment design to running computer simulations of radiation interactions in various tissues.

Jon Kruse, Ph.D. Radiation Oncology
  Jon Kruse, Ph.D., Radiation Oncology

“We make sure that when the radiation therapist turns on the beam, the correct dose goes to the right location,” says Jon Kruse, Ph.D., Radiation Oncology, who is a physicist on the Rochester campus. He began working in Mayo Clinic’s Proton Beam Therapy program in 2006.

“Proton beam therapy is a little more complex than standard X-ray radiation therapy, because, unlike X-rays, protons travel a certain distance in the body and stop,” says Dr. Kruse. “The depth a proton beam reaches depends on the beam energy and types of tissue in the beam path. From a CT scan of the patient, we can determine the physical properties of each bit of tissue in the beam path and calculate how much energy the proton beam needs to reach the tumor. ”

The proton therapy team is vigilant in watching for changes that may alter the range of a proton beam in the patient over weeks of daily treatments. These include the patient gaining or losing weight, shrinkage of the tumor in response to treatment, or filling of a patient’s sinuses from a cold. The Mayo Clinic Proton Beam Therapy program in Rochester begins treating patients in late June, and the Arizona campus begins seeing patients in February 2016.

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