does the evidence support the hype?

High energy proton beam therapy, an advanced form of radiotherapy, made its UK debut in 2018. The NHS and private clinics started offering this treatment the same year, but they were treating patient groups. very different. While the NHS used this therapy to treat cancer in children, private clinics began treating men with prostate cancer.

The situation is similar in other countries. Patients with prostate cancer are the most frequently referred patients for proton beam therapy in the United States and make up the majority of patients treated in many proton centers.

But who sees the greatest benefit of proton beam therapy? And do these benefits justify the substantial cost of treatment (often tens of thousands of pounds)? There is strong evidence for the benefits of proton therapy in children, but it is much less clear in prostate cancer and other cancers in adults.

This apparent discrepancy between evidence and practice underlies fundamental disagreements about the standard of proof required for new treatments and the role of patient choice in health care.

Why protons?

Proton therapy is an alternative form of radiation therapy, usually administered by X-rays. Both approaches target radiation rays on the tumor, killing cancer cells when they interact with and damage the patient's DNA. But radiation can also damage healthy tissues and organs surrounding the tumor, which can lead to side effects.

Modern X-ray radiotherapy minimizes these side effects by using intelligent methods of administration. The tumor is treated in many different directions, which means that it can receive a greater dose of radiation while reducing the dose received by healthy tissue and hence the damage. However, X-ray radiotherapy continues to deliver a significant dose of radiation to healthy tissue located in front of and behind the tumor.

Unlike photons (used in X-ray radiotherapy) that cross the body, protons only travel a certain distance in the body before stopping. By carefully adjusting this range, it is possible for proton therapy to deliver almost no dose beyond the tumor. This greatly reduces the total dose to healthy tissues.

It is argued that this benefit makes proton therapy a better approach than conventional radiotherapy. And this is true for some cancers. For example, reducing the total dose received by children significantly reduces the risk of delayed development and additional cancers later in life. Protons can also facilitate the administration of specific treatments to tumors close to sensitive organs, such as the spinal cord or the optic nerve.

Protons in prostate cancer

In prostate cancer (and many other cancers in adults), the benefits are less obvious. A big advantage of protons is the reduction of the total dose delivered to the patient. However, the main side effects badociated with this phenomenon are much less important in adults: developmental delays are not a factor, and the risk of additional cancers is much lower, their development generally taking several decades.

Instead, the main side effects occur in high-dose prostate organs such as the bladder and rectum. A high dose should be administered in a region including the prostate and some extra healthy tissue to ensure complete treatment of the disease. The extra margin takes into account the variation that occurs during treatment, such as differences in the way the patient goes to bed, to ensure that the cancer receives a full dose each day. Since these patient variations are independent of the type of radiation, protons and X-rays usually deliver high doses similar to these healthy tissues.

Despite this, proton therapy has been widely adopted because the physical benefits are clear, even modest, and will become evident over time as more and more people reach the five- and ten-year survival threshold. But the results of studies on the effects of proton therapy on prostate cancer are mixed. Although protons have been shown to be effective in the treatment of prostate cancer, no consistent benefit has been observed in terms of long-term survival or quality of life.

One of the challenges in measuring the benefits of proton therapy in prostate cancer is that conventional radiotherapy is already very effective. In the early stage of prostate cancer (stages 1 to 3), nine out of ten men treated with X-ray should remain cancer free after five years. They also have relatively low long-term side-effect rates compared to many other cancers. As a result, proton therapy has limited leeway to improve results, precisely because the results are already excellent. As the technology evolves, it will further reduce side effects and improve remission rates in conventional radiotherapy, which will mean that it is increasingly difficult to demonstrate the benefits of proton beam therapy.

As a result, the NHS and the American Society of Radiation Oncology (ASTRO) do not recommend proton therapy for prostate cancer except as part of a clinical trial. Many of these major international trials are underway, but it will take about a decade before we have conclusive results. Even in this case, it is expected that protons represent, at best, an evolution rather than a revolution in prostate cancer radiotherapy.

This has not prevented a demand for proton therapy from the prostate causing a dramatic expansion in the number of proton therapy centers internationally. This is partly because prostate cancer is one of the most common types of cancer. The treatment of common cancers may actually be used to support the availability of proton therapy for other, rarer cancers. But it is crucial that the benefits for each individual are properly considered.

It is difficult for unfounded statements from organizations such as the NHS and ASTRO to compete with the dramatic promises made by proton therapy clinics. However, a balanced discussion of the benefits and costs of all types of radiation therapy is essential to ensure that patients have all the evidence before they separate their money.