A Feasibility Study to Reduce the Contamination of Photoneutrons and Photons in Organs/Tissues during Radiotherapy

Due to out-of-field effects in radiation therapy, the determination and reduction of both unwanted photon and photoneutron doses are essential for the reasonable assessment of the risks to healthy tissues.

By the application of a multilayer shield throughout the phantom and using two models for photoneutron and photon sources, doses were estimated in a 15-MV linac in tissues and organs. Different neutron moderators were used, and the best materials, such as polyethylene, polystyrene, polyvinyl chloride, paraffin, and water, were reported for shielding purpose. Boron carbide and steel were utilized as neutron and gamma absorbents. Various lengths of the shield in line with phantom stature were also assessed in this study.

Except for the target organ, with the shield throughout the phantom, both photoneutron and photon doses approximately reduced by 57-89% and 88-95%, respectively. Extra photoneutron dose in the photon source was also reported due to the shield. Then, unwanted doses, especially photon dose remarkably decreased with increasing the steel thickness. The smaller dimensions of the shield caused also a considerable reduction of the photoneutron and photon doses in the phantom.

The application of a multilayer shield reduces the photon dose remarkably in healthy tissues. Therefore, it is recommended to use shielding materials to decrease photoneutron and photon doses, which can cause a reduction in the risk of secondary cancer. Due to the relatively high mass of the shield, it is necessary to design a proper device to maintain and move the structure.