Differences between Protons and X-rays

James Metz, MD
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: July 31, 2006

The main difference between protons and X-rays is based on the physical properties of the beam itself. Protons are large particles with a positive charge that penetrate matter to a finite depth based on the energy of the beam. X-rays are electromagnetic waves that have no mass or charge and are able to penetrate completely through tissue while losing some energy. These physical properties have a significant bearing on the treatment of patients.

The depth of treatment in tissue for protons is related to a quantity known as the Bragg Peak. This is due to a buildup of dose in the final few millimeters of the proton range. The depth of the Bragg Peak is dependent on the energy of the beam; with increasing energy, the Bragg Peak is located deeper in tissue. Figure 1 shows the Bragg Peak. As you can see, the entrance dose is relatively low, but as the beam penetrates deeper in tissue, there is a sharp rise in dose deposited. This is followed by a rapid stop in dose deposition. The beam stops at this point. Thus no tissue is treated beyond the Bragg Peak. This peak needs to be "spread out" to fit the width of the target to be clinically useful. Thus a special wheel, called a modulator is placed in the beam to spread out the Bragg Peak to the desired size. Figure 2 shows a spread out Bragg Peak. Figure 3 shows the relationship between an unmodulated Bragg Peak, modulated spread out Bragg Peak, and standard X-rays.

Extensive studies have been performed to determine the biologic differences between protons and X-rays. A standard measure called the relative biologic effect (RBE) is used to compare the biologic effects of various radiation sources. A RBE of 1 is seen for standard X-rays. Neutrons have a much higher RBE of 3. It turns out protons can be thought of exactly the same as X-rays in terms of its biologic effects because the calculated RBE is 1.1. Another measure of effect in biologic systems is the oxygen enhancement ratio (OER). Again, there is no difference in OER between protons and standard X-rays. The bottom line is that the only difference between protons and standard X-rays lies in the physical properties of the beam and not the biologic effects in tissue.