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Background: High-energy photon beams in radiotherapy can induce photoneutron production in linear accelerators (LINACs) operating above 10 MV, leading to secondary neutron contamination (Králík et al., 2008). These neutrons contribute to unwanted dose deposition in patients, making their characterization crucial for optimizing patient safety.
Materials and Methods: Monte Carlo simulations using MCNP5 were performed to model neutron production, transport, and interactions within a simulated patient phantom. A LINAC operating at 12, 15, 18, and 25 MV was considered to evaluate secondary neutron dose equivalent and fluence. The distributions of these quantities were analyzed as functions of beam energy and depth.
Results: As photon beam energy increases, both the secondary neutron dose equivalent and fluence rise within the patient phantom. However, the thermal and fast neutron fluence decrease with depth for all photon beam energies. From 0.25 cm to 18 cm depth, the thermal neutron fluence decreases by approximately a factor of 3, while the fast neutron fluence decreases by approximately a factor of 5. The peak of fast neutron fluence becomes more pronounced as photon beam energy increases from 12 MV to 25 MV. Additionally, the neutron dose equivalent decreases with depth, reducing by factors of 60, 53, 42, and 35 for 12, 15, 18, and 25 MV, respectively, from 0.25 cm to 18 cm depth.
Conclusion: This study highlights the significant impact of increasing beam energy on neutron production. The findings underscore the importance of accounting for secondary radiation in high-energy radiotherapy to minimize unnecessary patient exposure and enhance treatment safety.
Keywords: radiotherapy, photoneutron, spectra, dose equivalent, Monte Carlo simulations, phantom tissue
| Abstract Category | Medical Physics |
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