Depth Dose-Equivalent and Effective Energies of Photoneutrons Generated by 6–18 Mv X-Ray Beams for Radiotherapy

Abstract

Photoneutron production was investigated on Siemens KD 2 and Varian Clinac accelerators operating in the 6-18 MV range. Neutron dose equivalent rates were measured on the surface of a water phantom at the isocenter of the accelerators and also inside the phantom at depths of 1, 5, and 10 cm and off-axis distances of 0, 20, and 50 cm. Superheated drop detectors based on dichlorofluoromethane and etched-track detectors with boronated converters were employed in this study. The energy response of these detectors permits a direct measurement of dose equivalent without prior knowledge of the neutron energy spectra. Dose equivalent rates were assessed using the Q(L) relationship from ICRP publication 60, as well as using earlier data from ICRP publication 21. This permitted both a comparison with previously published data and an assessment of the impact of the recent ICRP recommendations--which were found to increase the dose equivalent levels by about 30%. In addition, the depth corresponding to 50% of maximum dose equivalent, dH50, was determined along the central axis of the beams and at 50 cm off-axis. Monte Carlo neutron transport calculations were performed to determine the depth-dose equivalent distributions in a phantom irradiated with monoenergetic neutrons. Effective energies of the photoneutron spectra were then estimated by comparing our measured dH50 values to those calculated for monoenergetic neutrons. It was found that the effective photoneutron energy is 1.8-2.1 MeV within the 10-18 MV x-ray beams, and it is 0.5-0.8 MeV for photoneutrons transmitted through the accelerator head. Data from this work cover most of the x-ray beam energies in clinical use and permit an assessment of integral dose values as well as specific organ doses to a radiotherapy patient.

Neutron Dose Equivalent in Tissue Due to Linacs of Clinical Use

“…This difference in the energy of the initial electron, produce variations in the absorbed dose due to photons, and more specifically, its value at the isocenter, which, as already seen, can lead to differences in the values of H. Figure 13. Comparison of the dose equivalent normalized to the maximum due to neutrons calculated in this study, with published results by [8] and [19].…”

“…It is concluded that phantom behaves as a moderador of neutrons due to its high content of 1 H, just 11.1% of total the ICRU phantom, with a threshold of thermal production of only 2.2 MeV [8]. …”

“…13 compares the dose equivalent in depth obtained for three Varian accelerators operating at 18 MV: The Clinac 2100 C/D studied by us, the Clinac 21EX considered by [19] and a Varian generic simulated by [8] These differences may be related to the following motives: The accelerator studied by [19] is a Varian Clinac 21EX, equipped with a Millennium MLC of 120 sheets. In our case, the MLC is one of 80 sheets.…”

“…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.…”

Estimate of Photonuclear Reaction in a Medical Linear Accelerator Using a Water-Equivalent Phantom

“…[4][5][6] In addition, secondary radiation may lead to radioactivation in the treatment room. Therefore, the evaluation of neutron dose is important in the medical field.…”