In‐phantom dosimetry and spectrometry of photoneutrons from an 18 MV linear accelerator

Abstract

A combination of three superheated drop detectors with different neutron energy responses was developed to evaluate dose-equivalent and energy distributions of photoneutrons in a phantom irradiated by radiotherapy high-energy x-ray beams. One of the three detectors measures the total neutron dose equivalent and the other two measure the contributions from fast neutrons above 1 and 5.5 MeV, respectively. In order to test the new method, the neutron field produced by the 10 cm X 10 cm x-ray beam of an 18 MV radiotherapy accelerator was studied. Measurements were performed inside a tissue-equivalent liquid phantom, at depths of 1, 5, 10 and 15 cm and at lateral distances of 0, 10, and 20 cm from the central axis. These data were used to calculate the average integral dose to the radiotherapy patient from direct neutrons as well as from neutrons transmitted through the accelerator head. The characteristics of the dosimeters were confirmed by results in excellent agreement with those of prior studies. Track etch detectors were also used and provided an independent verification of the validity of this new technique. Within the primary beam, we measured a neutron entrance dose equivalent of 4.5 mSv per Gy of photons. It was observed that fast neutrons above 1 MeV deliver most of the total neutron dose along the beam axis. Their relative contribution increases with depth, from about 60% at the entrance to over 90% at a depth of 10 cm. Thus, the average energy increases with depth in the phantom as neutron spectra harden.

Neutron Dose Equivalent in Tissue Due to Linacs of Clinical Use

“…15 compares experimental measurements of H, made with bubble detectors and detectors based on the CR-39 polymer by [21] and [8,9] [21] in surface are similar to the results of the calculations for the Varian Clinac of 15 MV in this work; but with increasing depth, these experimental results show much higher values than those obtained by [8,9], which are more consistent with Monte Carlo calculations performed in this work, especially for 15 MV. This is the general situation for all employees measurement systems for neutrons, without having to date, the ideal detector that can respond at any desired energy range.…”

“…Neutron fluence and spectra in water have been measured using bubble detectors and superheated drop detectors [8,9,21], 197 Au-based Bonner spheres [12] and thermoluminescent dosemeters [11,35,40]. [10] measured the neutron fluence at the patient plane for various linacs using gold-foil activation.…”

Characterisation of Silicon Diode Arrays for Dosimetry in External Beam Radiation Therapy

“…When a linac with photon energy higher than 10 MV is used, neutrons are produced by photons interacting with high-Z material located in the linac head (primary collimators, jaws, flattening filter) [25][26] [27] and any other material surrounding the beam. Neutrons affect the silicon detector response differently than photons, by producing different types of radiation damage, such as displacing of silicon atoms from the crystal lattice and cluster defects [1] [27] [28]. According to Howell et al, the model and linac manufacturer does not play a role in changing the spectrum and intensity of photo-neutrons [29].…”

“…According to Howell et al, the model and linac manufacturer does not play a role in changing the spectrum and intensity of photo-neutrons [29]. Linacs with energy higher than 10 MV generate primary neutrons with average kinetic energy ranging between 1 and 2 MeV [30] [31], with an equivalent dose of 4.5 mSv·Gy -1 of photon dose delivered at surface of a solid water phantom [28]. The effect of photo-neutron radiation was investigated using an 18 MV medical Clinac (Varian, USA) at St. George Cancer Centre (Sydney, Australia).…”

Measurement of the secondary neutron dose distribution from the LET spectrum of recoils using the CR-39 plastic nuclear track detector in 10 MV X-ray medical radiation fields

“…As for measurement under the treatment beam, the primary photon beam is predominantly too intense to identify a secondary neutron in a humanoid phantom because the high dose transfer by the photon masks any signals from secondary neutrons in conventional electronic detectors. Passive-type dosimeters such as the thermoluminescent dosimeter (TLD) [3], activation foil [31], bubble detector [32], and plastic nuclear track detector (PNTD) [6,33,34] have been utilized for this application. For in-field neutron measurement, TLD and activation foil are also sensitive to photons; therefore, it is difficult to discriminate between photons and neutrons, whereas the bubble detector and PNTD are considered more suitable detectors because they are insensitive to photons.…”