Because of its exceptional accuracy, proton therapy can in some cases enable doctors to safely deliver a higher daily dose of radiation to the tumor than is possible with X-rays. This technique, known as hypofractionation, can translate to faster cancer control and fewer treatment sessions.

'Painting' radiation onto tumors


Today, most proton therapy is delivered using the traditional "passive scatter" approach, in which the beam is shaped to match the tumor after exiting the treatment nozzle. With passive scatter, the beam passes through specially manufactured devices to form the beam's height, width and penetrating depth, before entering the patient.

The Scripps center will be the nation's first to exclusively treat patients with pencil-beam scanning, which is more precise and versatile than passive scatter. Instead of using physical devices outside of the treatment nozzle, pencil-beam technology electromagnetically shapes the beam inside the nozzle via computerized data files, developed from 3-D models of patients' tumors.

Pencil-beam technology sweeps the beam across the tumor in fine strokes, building up the radiation dose onto the tumor layer by layer. The penetrating depth of the beam is controlled digitally by an energy selection system, which varies the energy of the protons.

According to Dr. Rossi, using pencil-beam technology to treat tumors is like using a very fine paint brush to apply the radiation, whereas earlier proton technology is more like using a can of spray paint.

"We're essentially breaking down each tumor into thousands of tiny cubes, and then 'painting' each individual cube with radiation," said Dr. Rossi, who has treated more than 9,000 prostate cancer patients with proton therapy. "This approach spares even more normal tissue and enables us to treat larger and more irregularly shaped tumors than possible before. It also produces far fewer neutrons than passive scatter, which further reduces the probability of secondary malignancies. And it allows us to adapt treatment plans more quickly, as tumors change size during the course of treatments."

Powerhouse technology behind proton

The center's proton delivery system is both refined and powerful. Its technological centerpiece is a 95-ton superconducting cyclotron, where the proton beam is generated using hydrogen and oxygen to create a plasma stream. Protons are extracted and accelerated to roughly 100,000 miles per second. They are then delivered to a beam transport system, which uses a series of high-powered electromagnets to steer and transport the beam to one treatment room at a time using a vacuum line.

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