MO-D-BRB-11: Out-Of-Field Dose Measurements in Radiotherapy Using Photons and Particles

Within the European project ALLEGRO (grant agreement no. 231965), the out-of-field dose delivered to a patient when treated with different radiotherapy modalities was investigated. The study compared the dose distribution during photon and particle irradiations both in a water and an anthropomorphic phantom to evaluate the risk of inducing secondary malignancies.

Two sets of experiments with standardized conditions were used for a systematic comparison. In the former, a water phantom was irradiated with a 2D squared field to characterize the lateral dose fall-off with high spatial resolution. The latter employed an anthropomorphic phantom treated for a target volume placed at the center of its head to simulate a brain tumor. The dose was measured in several planes along the phantom main axis. For both types of experiments the dose was measured with a PTW diamond detector. Additionally, the use of TLDs and bubble detectors provided some information on the secondary neutron field produced both in the accelerator structure and the target itself. In total, experiments were conducted at six facilities using photons, protons and carbon ions; the ion irradiations were performed with passive delivery and the scanning technique.

A significant difference among the out-of-field dose profiles is observed for distances larger than 3 cm to the target. The distribution delivered by photons is a factor 10 to 400 higher than the values of charged particles. Scanning ions reduces the out-of-field dose more than passive delivery at distances larger than 10 cm.

The study emphasizes the physical advantage of using charged particles for tumor therapy. Together with the favorable depth dose deposition, ions spare the normal tissue surrounding the target more efficiently than photons. These results imply a lower risk of long-term effects, such as the induction of secondary malignancies, following treatments with particles compared to photons. This work was funded by the European ALLEGRO project (Grant Agreement No. 231965).