reports of practical oncology and radiotherapy 1 7 ( 2 0 1 2 ) 339–346 View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Available online at www.sciencedirect.com jou rnal homepage: http://www.elsevier.com/locate/rpor Thermal and resonance neutrons generated by various electron and X-ray therapeutic beams from medical linacs installed in polish oncological centers a,∗ b a,d c b Adam Konefał , Andrzej Orlef , Marcin Łaciak , Aleksander Ciba , Marek Szewczuk a Department of Nuclear Physics and its Applications, Institute of Physics, Silesian University, Katowice, Poland b Department of Medical Physics, Center of Oncology, Gliwice Branch, Gliwice, Poland c Radiotherapy Department of the Stanisław Leszczynski´ Memorial Hospital, Katowice, Poland d Institute of Occupational Medicine and Environmental Health, Sosnowiec, Poland a r t i c l e i n f o a b s t r a c t Article history: Background: High-energy photon and electron therapeutic beams generated in medical lin- Received 5 October 2011 ear accelerators can cause the electronuclear and photonuclear reactions in which neutrons Received in revised form with a broad energy spectrum are produced. A low-energy component of this neutron radi- 24 April 2012 ation induces simple capture reactions from which various radioisotopes originate and in Accepted 14 June 2012 which the radioactivity of a linac head and various objects in the treatment room appear. Aim: The aim of this paper is to present the results of the thermal/resonance neutron flu- Keywords: ence measurements during therapeutic beam emission and exemplary spectra of gamma Thermal/resonance neutrons radiation emitted by medical linac components activated in neutron reactions for four X- Induce radioactivity ray beams and for four electron beams generated by various manufacturers’ accelerators Medical linacs installed in typical concrete bunkers in Polish oncological centers. Materials and methods: The measurements of neutron fluence were performed with the use of the induced activity method, whereas the spectra of gamma radiation from decays of the resulting radioisotopes were measured by means of a portable high-purity germanium detector set for field spectroscopy. Results: The fluence of thermal neutrons as well as resonance neutrons connected with ∼ 6 2 the emission of a 20 MV X-ray beam is 10 neutrons/cm per 1 Gy of a dose in water at a reference depth. It is about one order of magnitude greater than that for the 15 MV X-ray beams and about two orders of magnitude greater than for the 18–22 MeV electron beams regardless of the type of an accelerator. Conclusion: The thermal as well as resonance neutron fluence depends strongly on the type and the nominal potential of a therapeutic beam. It is greater for X-ray beams than for elec- trons. The accelerator accessories and other large objects should not be stored in a treatment room during high-energy therapeutic beam emission to avoid their activation caused by thermal and resonance neutrons. Half-lives of the radioisotopes originating from the simple capture reaction (n,) (from minutes to hours) are long enough to accumulate radioactivity of components of the accelerator head. The radiation emitted by induced radioisotopes causes the additional doses to staff operating the accelerators. © 2012 Greater Poland Cancer Centre. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. ∗ Corresponding author. Tel.: +48 503830026. E-mail address: [email protected] (A. Konefał). 1507-1367/$ – see front matter © 2012 Greater Poland Cancer Centre. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.rpor.2012.06.004 340 reports of practical oncology and radiotherapy 1 7 ( 2 0 1 2 ) 339–346 1. Background High-energy photons and electrons used in radiotherapy can cause nuclear reactions (i.e. electronuclear and photonuclear 1–4 reactions ) in which neutron radiation is produced. This 5–19 problem was investigated by many authors. The resulting neutrons have a broad energy spectrum with mean energy 5–9 of several hundred keV. Significant contribution to the 10–12 spectrum is given by thermal and resonance neutrons. These low-energy neutrons come into being as a result of a slow-down processes (elastic scattering, mainly) of neutrons produced in the above mentioned reactions. The neutrons are slowed down mainly in walls, floor and ceiling of a treatment facility. The thermal and resonance neutrons can cause simple capture reactions from which various radioisotopes originate Fig. 1 – The view of the detection system with a high-purity and in which the radioactivity of air, linac head and vari- germanium detector by ORTEC (1 – the aluminum shield of ous objects in the treatment room appear. The main sources the germanium crystal, 2 – container with liquid nitrogen, 3 13 of neutrons are large components of linac head i.e. colli- – electronic modules – multichannel analyzer, amplifier, mators of the beam, a flattening filter, a target in the case etc., 4 – laptop). of X-ray beams and, additionally, scattered foils instead of a flattening filter in the case of electron beams. The resulting 8 neutrons can travel through a treatment room and a maze. of a circle with the radius of 0.75 cm. The foil superficial den- Thus, simple capture reactions can take place in the whole 2 115 sity was in the range from 98 to 110 mg/cm . Isotope In treatment facility. A comprehensive knowledge of the nuclear 22 is characterized by a relatively high cross section of 160 b reactions and radioisotopes induced during emission of ther- for a simple thermal neutron capture reaction. It also has a apeutic beams generated by medical linacs was presented in 23 14–21 high resonance of about 28,000 b at 1.45 eV and a number many papers. of lower resonances in the 3.5 eV to 1 keV range for the above- mentioned reaction, yielding the resonance activation integral 24 of 2700 b. 2. Aim The measurement of the thermal/resonance fluence consisted of two stages. In the first stage, the indium foil was The main aim of this paper is to present the results of mea- placed at a chosen location in the neutron field induced by a surements of the thermal and resonance neutron fluence in therapeutic beam inside a treatment room. The indium foil the vicinity of a medical linac during emission of various high- was activated by a capture of neutrons. The thermal and reso- energy therapeutic beams. Additionally, exemplary spectra of nance neutrons activate the indium according to the following gamma radiation emitted by medical linac components acti- reaction: vated in neutron reactions are shown. The purpose of this part of the study was to identify gamma emitters induced in 115 1 116m + n → , T = . In 0 In (unstable half-life 1/2 54 21 m) neutron reactions in linac accessory during emission of high- 49 49 energy photon/electron therapeutic beams. The presented 116m followed by radioactive decay. The unstable state In disin- investigations were carried out for four various therapeutic − 116 ˇ tegrates by decay into the exited state Sn*. Deexcitation electron beams and for four X-ray beams generated by three of this state gives gammas. In the second stage, the activated various manufacturers’ medical linacs installed in typical con- foil was removed to the operator room after the therapeutic crete bunkers in three Polish oncological centers: the Center beam emission and it was located on the surface of the germa- of Oncology in Gliwice, the Stanisław Leszczynski´ Memorial nium detector shield to measure the indium foil activity. In the Hospital in Katowice and the Greater Poland Cancer Centre in presented investigation, the detection system with the high- Poznan.´ purity germanium detector by ORTEC (Fig. 1) was applied to measure the activity of the foil on the basis of a measurement 3. Materials and methods of a spectrum of gammas emitted by the activated indium. In the presented investigation, the foil activity was determined 3.1. Measurements of the thermal and resonance by a net count area NCA of the peak at 1293.54 keV (Fig. 3), using neutron fluence the calibration factor K. The factor K was defined as a ratio of 60 the activity of the calibration source Co to net count area of The measurements of the thermal and resonance neutron the peak at 1332.50 keV in the gamma energy spectrum emit- 60 fluence were performed with the use of the induced activ- ted by the Co source used for the calibration. The energy ity method. This method uses the dependence between the of 1332.50 keV is close to 1293.54 keV from gammas emitted indium foil activity caused by thermal/resonance neutrons by the activated indium foil. The calibration source was in the and the thermal/resonance neutron fluence. The neutron acti- same shape and it had the same surface as the applied indium 115 vation detectors were indium foils ( In isotope) in the shape foils. reports of practical oncology and radiotherapy 1 7 ( 2 0 1 2 ) 339–346 341 The cadmium cover method was applied to separate the thermal neutron fluence and the resonance neutron fluence. Each measurement of the neutron fluence was performed twice: first by the use of the indium foil placed in a 1 mm thick cadmium shield, and then after the decay of the excited nuclei but without the shield. The cadmium shield absorbs almost all thermal neutrons but it lets through most of resonance ones. Thus, the total activity of the indium foil covered by cadmium is induced by resonance neutrons. However, a part of the resonance neutrons is absorbed in the cadmium layer.