Where Does Proton Therapy Go from Here?

Michael Spiotto, MD, PhD

By Michael Spiotto, MD, PhD

A recent Wall Street Journal article reported that three major insurers stopped covering proton beam therapy for early stage prostate cancer.

This may be the first of many blows to proton beam radiotherapy.

Radiotherapy is usually given with PHOTON-based radiation beams. Photon beams, like light, do not have any mass and, therefore, megavoltage photon beams can travel a long way in tissue without impacting the beams attenuation.

By contrast, protons are particles that have mass and abruptly stop upon reaching a certain depth in the tissue. When protons reach a tumor, these beams stop without damaging the other deeper organs.

When photons reach a tumor, these beams continue through it and deliver radiation to deeper organs, potentially resulting in damage to normal tissues.

While protons appear similar to sleek smart bombs, and photons seem akin to unsophisticated carpet bombing, the technological advances of intensity-modulated radiotherapy have minimized the differences between the two.

In pediatrics, proton radiotherapy can minimize radiation to normal tissues, which is important to minimize long-term toxicities for kids who subsequently may live for several decades once they are cured.

Furthermore, proton radiotherapy is likely advantageous for certain skull base tumors, such as chordomas, where high radiation doses are delivered to tumors that are adjacent to critical structures, such as the brainstem and spinal cord.

Since it’s unclear what the medical difference is between intensity modulated radiotherapy or proton therapy, why is proton therapy being prescribed for prostate cancer.

Presently, active surveillance is often used for very low-risk prostate cancers or low-risk cancers in patients with short life expectancies. Treatment for low-risk prostate cancers is usually limited to surgery for younger patients and radiotherapy for older patients.

So, proton therapy would remain applicable for a subset of patients who may not live long enough to develop long-term toxicities due to conventional photon radiotherapy. The dosimetric benefits of proton therapy may not even be clinically relevant versus photon therapy, given improvements in intensity-modulated radiotherapy.

So back to the Wall Street Journal story: why would proton therapy, then, be recommended for early stage prostate cancer?

A major reason is economics.

Each proton facility can cost $100 million to $225 million, and a steady stream of patients is needed to foot the bill.

Since the number of pediatric cases and rare tumors, which benefit from proton therapy, are not sufficient to finance each proton facility, these enterprises have to rely on treating more common tumors, such as prostate cancers.

As there are currently 11 facilities open and another nine under development, many more patients are needed to cover this multi-billion dollar enterprise.

There are medical uses for proton radiotherapy, but is early stage prostate cancer one of them?

With active PSA screening, prostate cancers are most commonly diagnosed in the early stage. But many are slow growing and, with surveillance, present little risk to the health of the patient.
Yet when many patients hear a diagnosis of cancer, often their first response is to seek treatment. And proton centers are not about to discourage radiotherapy.

Going forward, it remains unclear how proton therapy will adjust to the changing health care environment. It is likely beneficial for specific cancers and, therefore, the need still exists.
However, if the more common cancers, such as prostate, are not covered by insurers, only a few regional centers will likely remain. Will these be sufficient to satisfy the cases where proton therapy is the best course of treatment?

The next few years will be interesting for how the dynamic between insurers and proton centers plays out.

Michael Spiotto, MD, PhD, is an Assistant Professor of Radiation & Cellular Oncology at the University of Chicago Medicine. He specializes in treating patients with head and neck cancer. Whenever possible, he uses image-guided radiation therapy to target a tumor more precisely while reducing damage to surrounding tissue.

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