Frequently Asked Questions About Proton Stereotactic Radiosurgery

This section answers the most common questions patients have about Proton Stereotactic Radiosurgery.

Q.  What is radiosurgery?
A.  Radiosurgery is a procedure that uses a neurosurgical technology known as stereotaxis to precisely aim an intense dose of radiation into a targeted abnormality, such as a brain tumor or vascular malformation. With this technique, the radiation dose to the normal tissues surrounding the target is minimized. Radiosurgical treatments are typically performed in one or two sessions.

Radiosurgery may be performed as an alternative to conventional neurosurgery for certain types of medical conditions. In other circumstances it may be used in conjunction with fractionated radiotherapy and/or surgery. Some of the determining factors are the nature of the disease, its location and extent. Brain tumors commonly treated by radiosurgery include benign lesions such as meningiomas, acoustic neuromas and pituitary adenomas, as well as a variety of malignant tumors including gliomas and metastases. Vascular abnormalities of the brain, especially arteriovenous malformations, are also frequently treated. Recent advancements allow for select lesions located throughout the body to be treated using radiosurgical techniques.

Radiosurgery can be performed with any type of beam of ionizing radiation. This includes photon beams such as gamma rays and X-rays, as well as particle beams, which include protons. When gamma or X-rays are directed at tissue, the radiation dose received by that tissue is most intense near the point of penetration. Progressively less radiation is delivered to the tissue as the beam passes more deeply into it. When protons are directed at tissue, the radiation dose gradually increases as the beam passes more deeply, then drops to near zero beyond the targeted depth.

In order to deliver a high dose of radiation to a target deep within the brain or other organ while sparing the surrounding tissues from the same radiation dose, it may be necessary to aim the beam at the target from multiple directions, thus focusing an intense spot of radiation on the target. This also allows the radiation dose to conform more closely to the margins of the target.

Q.  What is proton radiosurgery?
A.  Small targets that tend to be spherical can be effectively treated using gamma or X-ray radiosurgery. However, with larger and more irregularly shaped targets, it becomes increasingly difficult to deliver a uniform dose of radiation within the target and spare surrounding normal tissues. In this circumstance, the unique characteristics of proton radiation are a significant advantage for performing radiosurgery.

Protons have a physical advantage over gamma and X-rays when it comes to sparing normal tissue. 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 of the beam.

This energy can be very precisely controlled to cause the Bragg peak to fall within the tumor or other tissue that is 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.

In order to further reduce the amount of radiation received by normal tissues in the path of the proton beam, beams are aimed at the target from multiple directions. For each of these directions special devices called apertures are fabricated to shape the radiation to the target profile. Other devices called compensators control the proton’s penetration within tissue. These devices are custom designed for each patient to further help each beam conform to the unique shape of the targeted lesion.