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Neutron-induced electronic failures around a high-energy linear accelerator.
Stephen F Kry1, Jennifer L Johnson, R Allen White
1Department of Radiation Physics, M. D. Anderson Cancer Center, The University of Texas, 1515 Holcombe Boulevard, Houston, Texas 77030, USA. sfkry@mdanderson.org
Medical Physics
|March 3, 2011
Summary
High-energy radiotherapy beams can cause neutron radiation, leading to electronic system failures in CT scanners. Shielding offers limited protection, highlighting the need for medical physicists to assess neutron sensitivity in vault electronics.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Equipment
Background:
- In-vault electronic systems, such as CT scanners on rails, are increasingly used in radiotherapy vaults.
- These systems can malfunction when exposed to high-energy radiotherapy beams, particularly those involving high monitor units (MUs).
- The precise cause of these malfunctions, specifically the role of neutron radiation, requires investigation.
Purpose of the Study:
- To investigate the presence and impact of neutron radiation on an in-vault CT-on-rails system that repeatedly malfunctioned.
- To evaluate the effectiveness of neutron shielding options for protecting electronic systems in radiotherapy vaults.
- To determine the types of neutrons responsible for electronic failures.
Main Methods:
- Determined the CT scanner's failure rate as a function of delivered 18 MV monitor units (MUs).
- Assessed failure rates with and without borated polyethylene (BPE) shielding on the linac head (2.7 cm and 7.6 cm thick).
- Utilized Monte Carlo simulations to calculate neutron fluence and spectrum within the CT scanner bore, exploring shielding configurations.
Main Results:
- The CT scanner exhibited a 57% failure rate after 200 MUs without shielding.
- Neutron shielding on the linac head provided minimal benefit; even 7.6 cm of BPE resulted in a 29% failure rate after 200 MUs.
- Fast neutrons were identified as the primary cause of electronic failures, and shielding effectiveness was similar whether applied to the linac head or the CT scanner.
Conclusions:
- Electronic systems, like the CT-on-rails, can be highly sensitive to neutron radiation generated by high-energy radiotherapy beams.
- Medical physicists must evaluate the neutron sensitivity of electronic systems within therapy vaults, especially newer or unassessed equipment.
- Current shielding methods offer limited protection, necessitating awareness and monitoring of potential neutron-induced electronic failures.
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