Secondary radiation doses in IMRT techniques

Abstract

Introduction The probability for developing the secondary cancer related to low peripheral doses during teleradiotherapy is nowadays of increasing interest. Among the components of the secondary dose photoneutrons and activated radionuclides should be discussed. Purpose Investigation of the secondary neutron and induced gamma radiation doses during intensity modulated radiation therapy employing Step&Shoot, sliding window and VMAT techniques. Materials and methods Elekta Synergy linacs with Agility MLC, working in 10 MV and 15 MV modes were used. Six plans with a 15 Gy fractional dose to PTV in mediastinum were studied and realized at anthropomorphic male phantom. Neutron flux was measured by the activation of indium 115 In foils. Neutron dosemeter LB 123N was also used. The induced gamma-activity of linac head was investigated using high-purity germanium spectrometer. Doses were assessed using fluence-to dose conversion coefficients. Results The dominant activation dose comes from 28 Al, 56 Mn and 187 W isotopes and may reach 2 μSv/h 10 cm from the linac head. Neutron flux densities are of the order of 10 3 –10 4 cm −2 s −1 for slow and 10 4 –10 5 cm −2 s −1 for fast neutrons. Additional doses connected with neutrons are ∼0.5 mSv/MU for 10 MV beam and within the range of 0.8–1.7 mSv/MU for 15 MV beam. Variation of neutron doses across the patient body is within 13%, for studied plans. Conclusion The neutron flux can be even 3.5 times lower whereas gamma-ray doses 5 times lower for 10 MV than for 15 MV beams in IMRT techniques. Disclosure No conflict of interest.