Proton Radiation Appears to Lower Risk for Secondary Cancers
September 25, 2008 — A specialized new form of radiation therapy that uses protons allows for greater focusing of the beam on the cancer target than standard photon-radiation therapy. However, during the process of delivering proton radiation, nuclear interactions lead to a scattering of neutrons, and there has been some concern that this scattered radiation might lead to an increase in secondary malignancies among cancer survivors.
New results, presented at the American Society for Therapeutic Radiology and Oncology (ASTRO) 50th Annual Meeting, in Boston, Massachusetts, go some way in alleviating those fears, said Dr. Anthony Zietman, MD, from Massachusetts General Hospital, in Boston, and the incoming president of ASTRO.
A follow-up of patients treated with proton-radiation therapy showed a 2-fold decrease in secondary malignancies, compared with patients treated with standard photon-radiation therapy. Because this study was retrospective, more data and studies are needed to prove this hypothesis, the authors cautioned.
The results were presented by senior author Nancy Tarbell, MD, also from the Massachusetts General Hospital. "Since cancer patients are surviving for longer periods of time, side effects of therapy are becoming increasingly important for doctors to consider when developing treatment plans," she commented in a statement.
Two-Fold Reduction in Risk of Secondary Malignancies
Dr. Tarbell and colleagues followed 503 patients who underwent proton-radiation therapy at the Harvard Cyclotron Laboratory for a median of 7.7 years. They were matched with 591 patients who received standard photon-radiation therapy from the Surveillance, Epidemiology and End Results (SEER) cancer registry, who had a median follow-up of 6.1 years. Patients were matched by age at radiation treatment, year of treatment, cancer histology, and site of treatment, the researchers noted.
Secondary cancer was reported in 6.4% (32 of 503) of patients who received proton radiation and in 12.8% (203 of 1591) of those who received standard photon radiation.
The median age of the patients who received proton-radiation therapy was 56 years and the median age of the those who received standard photon-radiation therapy was 59 years. After adjustment for age and sex, treatment with standard photon radiation was significantly associated with a risk for a secondary malignancy (adjusted hazard ratio, 2.73; 95% confidence interval, 1.87–3.98; P < .0001).
A major limitation of this study is the short follow-up time, Dr. Zietman commented in an interview. "We would not expect to see secondary malignancies from radiation for about 10 to 15 years, so at the follow-up in this study (6 to 7 years), we would not have expected to see any difference between the 2 groups," he said. The fact that the risk for secondary malignancies was 2-fold lower in the proton-radiation group is "interesting," and to some extent it is reassuring, he commented. "At least the proton-radiation group did not show a higher rate, which has been a worry." He emphasized the need for longer follow-up, adding: "This is not the answer yet."
Proton-radiation therapy is a "huge deal" in the United States, Dr. Zietman said. There are 6 centers in operation, but they are "fantastically expensive, in the region of $150 million each." So far, there has been little evidence that there are benefits from this specialized radiation therapy. This is the first study that has attempted to document this, and it was an excellent "good-faith attempt," he said.
Dr. Zietman was not involved in the study reported at the meeting, but he does use proton-radiation therapy at the Massachusetts General Hospital as an experimental protocol for patients with prostate cancer.
The researchers have disclosed no relevant financial relationships.
American Society for Therapeutic Radiology and Oncology (ASTRO) 50th Annual Meeting: Abstract 17. Presented September 22, 2008.