A proton is a positively charged particle found in the nucleus of an atom. The protons used in proton therapy are derived from stripping a hydrogen atom of its electron. When protons interact with electrons in the atoms of cancer cells, they impart energy to the electrons, causing them to leave the atom and undergo a series of interactions (ionizing events) that result in damage to the DNA of the cancer cell.
Damaging the DNA destroys specific cell functions, which include the ability to divide or proliferate. A cancer cell's ability to repair molecular injury is frequently inferior to that of cells in normal tissues. As a result, cancer cells accumulate permanent damage and subsequent cell death occurs. As the cell dies, so does the tumor.
Traditional radiation therapy affects everything in its path, so doctors have to limit the dose delivered to the tumor in order to minimize damage to surrounding healthy tissue. With proton therapy, the beam is accelerated to specific energies that determine how deeply in the body protons will deposit their maximum energy.
Protons enter the body with a low dose of radiation, which increases when the beam slows down within the designated target tumor and then protons stop. Compared to an X-ray beam, a proton beam has a low “entrance dose” (the dose delivered from the surface of the skin to the front of the tumor), a high dose designed to cover the entire tumor and no “exit dose” beyond the tumor.The combined effect is greater precision in targeting the tumor with a more potent dose of radiation. The accuracy of proton therapy for treatment delivery is within approximately one millimeter.
Pencil Beam Scanning
Pencil beam scanning, also known as spot scanning, has the ability to treat the most complex of tumors, while leaving healthy tissue and critical structures untouched. Powerful scanning magnets direct thousands of ultra-fine proton beams from multiple directions toward the tumor, creating a “U” shape around healthy tissue and avoiding sensitive areas entirely during treatment. Intricate treatment planning allows the protons to deposit their potent dose of radiation to the exact dimensions of the tumor. Side effects common after radiation therapy are reduced and healthy organs are preserved because the radiation is confined to the tumor.
MD Anderson is currently using pencil beam scanning to treat cancers of the prostate, brain, base of the skull and eye. As the technology evolves, pencil beam will ultimately be applied to any tumor site where proton therapy is used, particularly for treating solid tumors in children, who are generally more sensitive to the adverse effects of radiation.
Patient Education Resources
ProtonPals: an online support group for proton therapy patients
Pencil Beam Scanning (3:55)
Proton Therapy (9:30)