Proton Beam Therapy

Overview

The MGH Department of Radiation Oncology is an established leading institution treating with protons. In the 1960s and 1970s, pioneering MGH neurosurgeons and radiation oncologists, working with Harvard’s Cyclotron Laboratory, became the first in the world to discover how to harness the extraordinary behavior exhibited by high energy protons, a phenomenon known as the Bragg peak, for medical care. By using protons rather than conventional x-rays (photons), physicians could deliver radiation energy directly and precisely to tumors and cancerous cells, causing less damage to nearby healthy tissue.

One of the first hospitals in the world to establish its own proton therapy system, MGH transitioned from treating patients in an experimental setting to its state-of-the-art Francis H. Burr Proton Therapy Center. Since 2001, thousands of adults and children have received highly customized, team-based treatment here, with advances in imaging and technologies making proton beam therapy ever more precise.

Principles of Proton Beam Therapy

Irregularly shaped lesions located near critical structures, tumors in children, and large tumors near any critical organ are well suited for proton beam therapy. Protons have a physical advantage over gamma rays and x-rays when it comes to sparing normal tissues. Protons deposit most of their radiation energy in what is known as the Bragg Peak, which occurs at the point of greatest penetration of the protons in tissue. The exact depth to which protons penetrate, and at which the Bragg Peak occurs, is dependent on the energy or modulation of the proton beam. This energy can be very precisely controlled to place the Bragg Peak within a tumor or other tissues that are targeted to receive the radiation dose. Because the protons are absorbed at this point, normal tissues beyond the target receive very little or no radiation. Proton energy can be adjusted to match the depth of the target with a sharp drop in dose beyond the Bragg Peak.

Tumors can have very irregular shapes and can be located close to critical organs. Every patient’s tumor shape, size and location are unique. Patient specific hardware, which helps sculpt the proton beam, is customized to maximize the dose to the tumor while minimizing the dose to normal structures. The shaping of the proton beam can also be controlled by magnetically scanning across the tumor volume. Aiming proton beams, each with customized field shaping, from various directions further ensures that the dose to normal tissues is reduced as much as possible.

How Proton Therapy Works

Protons are hydrogen atoms whose electrons have been removed. Proton beam therapy uses a special machine called a cyclotron or synchrotron to energize protons. Protons are extracted from the cyclotron or synchrotron and directed with magnetic fields to the tumor. The depth of penetration of the protons is related to their energy and can be precisely controlled to match the location of the tumor.

Protons deliver the majority of their energy at a very narrow area within the body. This unique dose delivery property of protons is known as the Bragg Peak. We can manipulate the Bragg Peak area to deliver the desired radiation dose to the tumor itself without any exit dose beyond the tumor. Conventional external beam radiation therapy uses photons or x-rays that enter and exit through the body. The special properties of protons generally reduce the radiation dose to the uninvolved normal tissues surrounding the tumor.