SU-FF-T-304: Characteristics of Neutron Equivalent Dose Around Medical Linear Accelerators

Abstract

Purpose: To investigate the characteristics of neutron equivalent dose (NED) around medical linear accelerators (linac). Method and Materials: Two types of bubble detectors (BD-100R for fast neutrons and BDT for thermal neutrons) were used to measure the neutron equivalent dose. Most experiments were carried out using a Varian Clinac 2300C/D linac, 18 MV photon beam at two points of measurement: machine isocenter (point I) and on the isocenter axis 1 m off the isocenter (point C). Results: The NED at point I is 12.7 mSvn/100 MU (10% is from thermal neutron) and this dose decreases to 1.7 mSvn/100 MU (13%) at point C. The NED at point I increases with the increasing field size, whereas the NED at point C exhibits the maximum value for a 10×10 cm2 field. The use of a multileaf collimator (MLC) increases the NED at point C but does not show any significant effects for the NED at point I. In order to facilitate conversion of NED in air to NED in tissue so that the patient photoneutron dose can be estimated, a new quantity NTAR (neutron tissue-air ratio) is introduced and measured in this work. As well, a NED depth dose curve is determined. Inverse square law can not be applied to the NED results measured at different positions along the central beam axis. Conclusion: Photoneutrons produced by a high energy photon beam delivers to the radiotherapy patient a equivalent dose of about 1% of the treatment dose inside the treatment field and 0.1∼0.3% outside of the treatment field. The dose inside the field increases with the increasing field size, while the dose outside the field decreases with the increasing field size. NTAR provides an easy method for the conversion of NED in air to NED in tissue.