Hadron therapy (i.e. proton or heavy ion irradiation) is becoming one of the most attractive approaches in the treatment of cancer. The reduction of the dose burden is particularly important for children because long-term survivors of childhood cancer receiving RT are at a significantly increased risk of second malignant neoplasms (SMNs) [Oeffinger, et al., N Engl J Med, 2006]. Nevertheless there is a major concern related to the carcinogenic risk from secondary neutrons, which are unavoidably present with particle treatments [Newhauser, et al., Nat Rev Cancer, 2011]. In particular neutrons are believed to be the most harmful radiation for the human body because they are highly biological effective with regard to cancer induction and thus even a very small absorbed dose might cause side effects. Therefore assessment of neutron doses in clinical proton therapy requires special attention for radiation protection and prevention of SMNs.
In the coming years cancer patients will be treated with protons in Belgium. Two new proton therapy facilities will be constructed; one in UZ Leuven and one in CHU Charleroi. These facilities will operate with high-energy proton beams up to 250 MeV, meaning patients will be subjected to unwanted absorbed doses of very high-energy secondary neutrons.
To monitor neutron doses is however a big challenge because both the biological effect of neutrons and the neutron detection efficiency of most neutron detectors are largely depending on the neutron energies and to have a complete picture of the energy spectrum there is a need for a large range of equipment hampering neutron dose monitoring in a small area. Currently in vivo neutron doses in particle therapy are not performed routinely and are limited to Monte Carlo (MC) simulations. Additionally there is a lack of measured data in anthropomorphic phantoms [Sahay, et al., J Radiol Prot, 2014]. The use of bubble detectors has been proposed to estimate the patient secondary neutron doses by irradiation in anthropomorphic phantoms. In SCK•CEN we have developed superheated droplet detectors (SSDs). The droplets undergo transition to the gas phase upon energy deposition by fast neutrons. Bubbles are formed accompanied by an acoustic shock wave which is read in an acoustic detection system.