The Impact of Neutrons in Clinical Proton Therapy

Abstract

In proton therapy, high-energy proton beams cause the production of secondary neutrons. This leads to an unwanted dose contribution, which can be considerable for tissues outside of the target volume regarding the long-term health of cancer patients. Due to the high biological effectiveness of neutrons with regard to cancer induction, small neutron doses can be important. Published comparisons of neutron dose measurements and the corresponding estimates of cancer risk between different treatment modalities differ over orders of magnitude. In this report, the controversy about the impact of the neutron dose in proton therapy is critically discussed and viewed in the light of new epidemiological studies. In summary, the impact of neutron dose on cancer risk can be determined correctly only if the dose distributions are carefully measured or computed. It is important to include not only the neutron component into comparisons but also the complete deposition of energy as precisely as possible. Cancer risk comparisons between different radiation qualities, treatment machines, and techniques have to be performed under similar conditions. It seems that in the past, the uncertainty in the models which lead from dose to risk were overestimated when compared with erroneous dose comparisons. Current risk models used with carefully obtained dose distributions predict a second cancer risk reduction for active protons vs. photons and a more or less constant risk of passive protons vs. photons. Those findings are in general agreement with newly obtained epidemiologically results.

Keywords: neutrons; proton therapy; second cancer.

Figure 1

(A) Eric Hall’s comparison (12) of neutron dose equivalent per treatment Gray as a function from the distance of the field edge. Measurements of several researchers were combined by scaling the neutron dose. (B) Comparison of neutron dose equivalent by Hälg et al. (13). The measurements were performed under the same conditions for each treatment modality.

Figure 2

Modeled second cancer risk after radiotherapy of the prostate relative to a historic radiation treatment (four-field-box in blue). The data were taken from Ref. (14) and updated with better dose measurements (15). Prostate cancer was chosen for comparison with the epidemiological study of Chung et al. (21) as their patient cohort consisted mainly of prostate patients.