Secondary neutron spectra from modern Varian, Siemens, and Elekta linacs with multileaf collimators

Abstract

Neutrons are a by-product of high-energy x-ray radiation therapy ͑threshold for ͓␥ , n͔ reactions in high-Z material ϳ7 MeV͒. Neutron production varies depending on photon beam energy as well as on the manufacturer of the accelerator. Neutron production from modern linear accelerators ͑linacs͒ has not been extensively compared, particularly in terms of the differences in the strategies that various manufacturers have used to implement multileaf collimators ͑MLCs͒ into their linac designs. However, such information is necessary to determine neutron dose equivalents for different linacs and to calculate vault shielding requirements. The purpose of the current study, therefore, was to measure the neutron spectra from the most up-to-date linacs from three manufacturers: Varian 21EX operating at 15, 18, and 20 MV, Siemens ONCOR operating at 15 and 18 MV, and Elekta Precise operating at 15 and 18 MV. Neutron production was measured by means of gold foil activation in Bonner spheres. Based on the measurements, the authors determined neutron spectra and calculated the average energy, total neutron fluence, ambient dose equivalent, and neutron source strength. The shapes of the neutron spectra did not change significantly between accelerators or even as a function of treatment energy. However, the neutron fluence, and therefore the ambient dose equivalent, did vary, increasing with increasing treatment energy. For a given nominal treatment energy, these values were always highest for the Varian linac. The current study thus offers medical physicists extensive information about the neutron production of MLC-equipped linacs currently in operation and provides them information vital for accurate comparison and prediction of neutron dose equivalents and calculation of vault shielding requirements.

Out‐of‐field doses and neutron dose equivalents for electron beams from modern Varian and Elekta linear accelerators

“…Followill et al (23) published a compilation of neutron source strength values for high‐energy photon beams on several linear accelerators. Our Q value for the 18 MV photon beam measured on the Varian 21iX is in agreement with the 18 MV values for Varian linear accelerators found in the literature, 22 , 23 whereas our Q values for the electron beams are the first to be published. Neutron dose equivalent measurements have been made by Nath et al, (9) Lin et al, (10) and Biltekin et al (11) for high‐energy electron beams.…”

“…(6), the weighting factors ${\mathrm{H}}_{\text{th}}false(\mathrm{normaln}/{\text{cm}}^2/\text{mSv}false)=3.74\times {10}^7$ and $H_{dir,sc}false(n/c{m}^2/mSvfalse)=1.89\times {10}^6/{\stackrel{true¯}{\mathrm{normalE}}}_{\text{normaldir},\text{normalsc}}^{0.72}$, where $\overline{\mathrm{E}}$ is the average neutron energy and ${\stackrel{true¯}{\mathrm{normalE}}}_{\text{sc}}=0.24\hspace{0.17em}{\stackrel{true¯}{\mathrm{normalE}}}_{\text{dir}}$ . For each linear accelerator manufacturer, we assumed the same average direct neutron energy as has been previously measured for photon therapy (22) . Uncertainty in these type of measurements are typically around 20%‐30% 21 , 23 …”

Dosimetric assessment of brass mesh bolus for postmastectomy photon radiotherapy

“…Based on data from Howell et al, (19) the thermal neutron fluence per Gy for a Varian 21EX linac in a room with a surface area of 210 m 2 is approximately $1\times {10}^5\text{neutrons}/{\text{cm}}^2$ for 15 MV photons. The activation products and their corresponding activities of a brass mesh exposed to 500 MU of 15 MV X‐rays at a dose rate of 600 MU/min are presented in Table 1.…”

“…Yellow brass is an alloy composed of approximately 60% copper and 40% zinc, which are two elements with nuclides having considerable thermal neutron capture cross sections. Howell et al (19) determined the photoneutron spectra and thermal neutron flux for a Varian 21EX at various nominal energies. The NIST Center for Neutron Research hosts a neutron activation and scattering calculator that will calculate the activity of activation products given some thermal neutron flux (20) .…”

Estimation of neutron and gamma radiation doses inside the concrete shield wall for 10 and 15 MV medical linear accelerators

“…According to Howell et. al [14], the neutron average energies of 15 MV linacs are 0.27 MeV at Varian, 0.20 MeV at Siemens, and 0.23 MeV at Elekta. Energy were almost the same though the manufactures were different.…”