Andrew L. Pecora, MD
It worked in the “Wizard of Oz”; that is, it took a house to kill the Wicked Witch of the East; whereas, I doubt a marble would have been effective. Similarly, when using high energy protons (1.6 x 10-27 kg) in place of photons (electrons, 9.1 x 10-31 kg), the sheer larger relative mass and nuanced distribution of energy (Bragg peak) attributed to protons would seemingly confer a clear advantage to protons over photons when trying to kill cancer. As with many new advances in medicine, the relevant debate is not efficacy or safety but cost and value.
Proton therapy is exciting because it’s a powerful new tool.
Proton development work began in the early 1950s when physicists were able to create enough energy using powerful magnets to accelerate protons to high speeds. Remember from high school physics that anything with mass requires increasingly more energy to reach the speed of light, so by definition electrons (lighter) can go faster than protons (heavier) at any given energy level. Also remember Einstein’s famous equation E=mc2, which is the key to why protons weighing 10,000 times more than electrons do not need to travel as fast as electrons to deliver an equal wallop. One last piece of physics trivia is that it took a very large (building size) array of magnets and a lot of money (over $100 million) to bring to market the first proton beam unit. The story did not stop there, because advances in technology have enabled new proton beam units to come to market with even more powerful magnets and other features, allowing units to be much smaller and less expensive (around $25 million). Nonetheless, cost versus value remains the issue of the day.
There are clear success stories in proton beam therapy. The Bragg peak distribution of energy allows access to deep-seated tumors (ie, base of brain) and tumors in difficult anatomic areas (ie, head and neck cancer) without excessive damage to normal tissue. Unlike photons that deliver their energy along the entire tract of the beam, protons deposit their energy in a very narrow tract with little before or after the Bragg peak of distribution. So, with a seeming advantage, why all of the controversy? Studies of older and more expensive forms of proton
technology have not shown clear efficacy or toxicity advantages that might justify the cost. Given the capital investment required to assemble and operate even a modern proton beam unit, a large number of patients would need to be treated daily in order to generate a return on investment, or else the patient charge for proton therapy would have to be significantly greater than current reimbursement for photonbased intensity modulated radiation therapy. So far, the market is not supporting a significantly greater charge, so it has become a volume play.
The problem is that pediatric brain tumors—for which proton therapy is clearly superior—are rare, and common malignancies may not require proton therapy—at least, not at the current value equation.
As the cost comes down due to new technologies and as studies mature, maybe the value equation for common malignancies will favor proton beam therapy. However, it is hard to imagine how throwing a house will ever be as cost-efficient as throwing a marble.