Question: What is proton beam therapy?
Answer: One possible therapy for certain conditions, including certain cancers, is to ablate/destroy abnormal/problematic cells/tissue with delivered energy.
Many people are familiar with X-ray radiation treatment, XRT, which does this by delivering the energy with photons, massless particles. This energy is delivered along the entire pathway of the beam, so en route and after leaving the target tissue some of the energy is delivered to normal body tissue. In order to maximize the energy delivered to the target tissue while minimizing damage to normal tissue, multiple different beams from different angles may be utilized.
Protons are positively charged particles with a mass of 1.7 x 10-27 kilograms, which sounds very small but is actually fairly big for a subatomic particle. When a charged particle, in this case a proton, moves through matter it releases a large percentage of its energy after penetrating through a certain depth, called the Bragg peak; this depth depends on the density of the tissue that the particle passes through and the energy of the particle. So, to utilize this physics in a beneficial way, imaging, a CT or an MRI, is done to map the density of tissue along the selected pathway to the targeted lesion, and the energy of the proton, the speed to which is it accelerated in the cyclotron, specifically needed to accurately “hit” the desired target is calculated.
Therefore, proton beam therapy has the potential to maximize the therapeutic effect at the target while minimizing any toxic/side effects from energy deposition along the beam pathway. However, medical studies that verify improved outcomes while minimizing risks for PBT vs. XRT are limited.
Considerations for PBT include:
• Increased cost: it is more difficult, and hence much more expensive, to accelerate protons to the required energies vs. creating photons of certain wavelengths (energy).
• Precise measurements of tissue density — which can change from one therapeutic session to the next as some tissue is destroyed from the prior treatments — require accurate imaging.
• The patient must be precisely positioned and remain very still to be sure that the exact beam pathway is utilized — sometimes the patient will be fitted with a special device in order to aid in this precise positioning.
• Use of this therapy is challenging in organs that may move — for example the lungs, which move with each breathe.
• There is the possibility of delivering more therapeutic energy with PBT than with XRT since the toxic effects along the x-ray beam pathway may limit how much total energy can be delivered.
PBT is usually done as an outpatient treatment, sometimes as the sole treatment and sometimes as part of a broader therapeutic plan — for example in addition to surgery and/or chemotherapy. After the planning imaging and identification of the best pathway for the beam, the patient is brought into the treatment room and very carefully positioned. Depending on the specific dose and other therapeutic decisions, PBT is usually delivered in one to five sessions, each lasting 15 to 30 minutes or less. Patients treated with PBT are not radioactive, and after each of their treatment sessions, they usually go home to continue with the rest of their treatment care plan.
PBT is painless. However, since some percentage of the energy is delivered to the skin at beam entry and to the tissue on the pathway to the target — very little is delivered after the Bragg peak at the target area — there can be some redness, swelling, irritation or other side effects.
PBT is usually only recommended for target caners/lesions in the brain, spinal cord, eye, head, neck, liver, prostate, pelvis, lung and possible others. As more experience and clinical data is collected — PBT has been used for over 10 years, but due to the cost and expertise needed for it, it is available only at certain treatment centers — other conditions may be treated with this therapeutic modality.
Jeff Hersh, Ph.D., M.D., can be reached at DrHersh@juno.com.