Iranian Journal of Medical Physics | 2019
A Feasibility Study to Reduce the Contamination of Photoneutrons and Photons in Organs/Tissues during Radiotherapy
Purpose: Due to out-of-field effects in radiation therapy, determination and reducing of both unwanted photons and photoneutrons dose are essential for reasonable assessment of the risks to the healthy tissues.Materials and methods:Based on the ICRP recommendation ALARA, by applying multilayer shield (neutron moderators, neutron absorbents and gamma absorbents) 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 as Polyethylene, Polystyrene, PVC, Paraffin and Water were reported. Boron carbide and steel were used as neutron and gamma absorbent. Various lengths of the shield in line with phantom stature were also studied.Results: Percentages of photoneutrons and photons dose reduction have been reported due to the presence of shields. Except for target organ, with the shield throughout the phantom both photoneutron and photon dose reduced approximately 57-89 % and 88-95 % respectively. Extra photoneutron dose in photon source due to shield were also reported. Afterwards, with increasing steel thickness unwanted doses, and especially photon dose, are remarkably reduced. The smaller dimensions of the shield have also a considerable reduction of the photoneutron and photon in the phantom.Conclusion: The application of multilayer shield reduces photon dose remarkably in healthy tissues. Hence, using the shielding material to reduce photoneutron and photon dose which can cause reduction in secondary cancer risk is recommended. Due to relatively high mass of the shield, design a proper device to maintain and move the structure is necessary.