Physical Properties of Proton Radiation

Protons are positively charged particles that are isolated by stripping of the orbital electron from hydrogen. High-energy protons are directed to targets of interest where its energy is subsequently deposited. Proton radiation has a similar biological efficacy as photon radiation which is the most common form of clinically applied radiation therapy. Photons are high-energy X-rays and in contrast to protons, they have no charge or mass. Photons also deposit their energy in the tissue they irradiate. Photon-based therapies have previously been mentioned and include linear accelerator derived treatments and Gamma Knife® therapies.

The primary difference between protons and photons in clinical application is the greater ability of proton beams to conform to the target and thereby decrease unnecessary radiation exposure to surrounding nontarget normal tissues. Protons have finite path lengths. As they decelerate in speed within tissues, they transfer their energy. The greatest deposition of energy is within the last few millimeters of their path length. This final large release of energy is known as the Bragg peak (Fig. 54-1). The path length of a proton beam within a given tissue increases with the beam energy. Multiple proton beam energies can be used to create a spread out Bragg peak (SOBP) to irradiate the entire target to a relatively uniform dose. Because proton beams have a finite travel distance, there is essentially no radiation delivered downstream to where the proton stops. In contrast, photons are partially attenuated by the matter with which they pass through but they do not have a finite path length. To achieve an adequate dose delivery to clinical targets, photon beams deposit both a higher amount of energy upstream to the target and continue to deliver a significant amount of energy downstream to the target. Typically multiple radiation beams from different orientations are used to maintain an adequately low-dose deposition to the normal tissues between the skin surface and target. Overlapping radiation beams within the target achieves the higher and therapeutic dose desired.

FIGURE 54-1 Energy depth–dose profile for proton and photon beams. Proton beams (blue) have a characteristic large energy loss at the end of their beam path length known as the Bragg peak. Proton beams of different energies can be combined to deliver a spread out Bragg peak (SOBP, red) that covers the width of a target at the desired therapeutic dose. Photons (green) continue to lose energy with depth from tissue absorption but do not have the defined path length as compared to protons.

The increased conformality of protons in dose distribution has two primary benefits. It may reduce the risk of adverse effects derived from normal tissue exposure to ionizing radiation. This is the major advantage of protons versus photons for patients with benign meningiomas. Although there is still energy deposited between tissue entry and target, the elimination of all downstream radiation exposure significantly reduces the volume of normal tissues irradiated. This also allows for the selection on many more beam arrangements in proton therapy because downstream irradiation of radiation-sensitive structures is not a concern. When treating irregularly shaped targets such as skull-base meningiomas, proton radiation has both the benefit of diverse beam arrangements and minimization of irradiation of neighboring tissues such as the brain and eyes. Dosimetric studies comparing modern photon-based radiation therapies with proton radiation find that no more than three proton beams are required for meningiomas plans whereas five or six photon beams are required for optimization of each technique.¹ Proton planning is associated with lower volume of normal tissues exposed to low-dose excess radiation. A second study comparing proton and photon planning of intracranial tumors including five cases of meningiomas also found superior conformality with protons.² Although these studies are purely planning studies, it is expected that they will translate into reduction of potential late effects of normal tissue irradiation.

A second important feature of proton therapy is its ability to deliver higher doses than photons while maintaining an equivalent risk of normal tissue injury probability. This is an important concept for managing patients with atypical and malignant meningiomas. It is a direct consequence of decrease in normal tissue volume irradiated and dose delivered to these tissues. With proton radiation, the target dose can be increased while the surrounding normal tissue risk of complication can be maintained equivalent to or less than alternatively achieved with photon radiation. In diseases in which treatment efficacy was limited by insufficient dose, proton radiation may increase the effectiveness of radiotherapy as a treatment modality. As mentioned earlier, in the setting of management of atypical or malignant histologies of meningiomas where doses in excess of 60 Gy may be beneficial to improving local control, the both improved local control and reduction of normal tissue injury can be expected with the use of proton radiation.³

Similar to photon-based radiation therapies, proton radiation can be delivered in multiple treatments as fractionated therapy or in a single fraction as stereotactic radiosurgery. The clinical indication defines the dose and fractionation pattern. Short-course treatments involving high doses per fraction for a few fractions is termed hypofractionated radiation therapy and is an option between traditional fractionated therapy of typically 5 to 6 weeks and single-fraction radiosurgery. Multiple factors including that of tumor location, size and shape, proximity of normal tissues, and patient health are considered to determine treatment dose and fractionation.

Despite the physical properties of protons that would seem to make it a clearly superior form of external beam radiation delivery, the complexity and cost of creating and maintaining such treatment facilities has limited their initial application in the clinics. As of 2008, there are five open clinical proton treatment facilities in the United States. This is an increase from only two centers available prior to 2005. The growing appreciation of proton radiation’s superior dosimetry and clinical implication has fueled the development of several more new proton therapy centers expected to open in the coming years.

Immobilization Techniques for Proton Radiation Therapy

Patients must be comfortably, safely, and securely positioned in a reproducible position for daily radiation treatments. Commercial products exist to facilitate efficient and patient friendly immobilization techniques but the limited number of proton treatment facilities has led to the development of various custom immobilizations tailored to specific treatment facilities. Proton beams are more sensitive than photons to the shape and density variations of immobilization materials and must be accounted for during both design of immobilization and treatment planning.