Measurements of neutron radiation and induced radioactivity for the new medical linear accelerator, the Varian TrueBeam

• The investigation of secondary radiation was performed for the new medical accelerator, the Varian TrueBeam.
• Secondary neutrons were measured using a helium chamber and the induced activity method.
• The identification of produced radioisotopes was based on measurements of the gamma spectra.
• The correlation between the neutron fluence and the mode and energy of the therapeutic beam was observed.
• 11 radioisotopes resulting from the neutron production were identified.

Contemporary linear accelerators applied in radiotherapy generate X-ray and electron beams with energies up to 20 MeV. Such high-energy therapeutic beams induce undesirable photonuclear (γ,n) and electronuclear (e,e'n) reactions in which neutrons and radioisotopes are produced. The originated neutron can also induce reactions such as simple capture, (n,γ), reactions that produce radioisotopes. In this work measurements of the non-therapeutic neutrons and the induced gamma radiation were carried out in the vicinity of a new medical accelerator, namely the Varian TrueBeam. The TrueBeam is a new generation Varian medical linac making it possible to generate the X-ray beams with a dose rate higher than in the case of the previous models by Varian. This work was performed for the X-ray beams with nominal potentials of 10 MV (flattening filter free), 15 MV and 20 MV, and for a 22 MeV electron beam. The neutron measurements were performed by means of a helium chamber and the induced activity method. The identification of radioisotopes produced during emission of the therapeutic beams was based on measurements of the energy spectra of gammas emitted in decays of the produced nuclei. The gamma energy spectra were measured with the use of the high-purity germanium detector. The correlation between the neutron field and the mode and nominal potential was observed. The strongest neutron fluence of 3.1 × 106 cm−2 Gy−1 and 2.0 × 106 cm−2 Gy−1 for the thermal and resonance energies, respectively, was measured during emission of the 20 MV X-ray beam. The thermal and resonance neutron fluence measured for the 15 MV X-rays was somewhat less, at 1.1 × 106 cm−2 Gy−1 for thermal neutrons and 6.7 × 105 cm−2 Gy−1 for resonance neutrons. The thermal and resonance neutron fluences were smallest for the 10 MV FFF beam and the 22 MeV electron beam and were around two orders of magnitude smaller than those of the 20 MV X-ray beam. This work has shown that the neutron reactions are dominant because of relatively high cross sections for many elements used in the accelerator construction. The detailed analysis of the measured spectra made it possible to identify 11 radioisotopes induced during TrueBeam delivery. In this work the following radioisotopes were identified: 56Mn, 122Sb, 124Sb, 131Ba, 82Br, 57Ni, 57Co, 51Cr, 187W, 24Na and 38Cl.