SU‐E‐T‐90: Determination of Orthovoltage Beam‐Hardening Filters Generating the NIST‐Traceable Beam Quality

Purpose: We performed Monte Carlo simulation using EGSnrc for simulating an X‐RAD 320 Biological Irradiator (Precision X‐Ray Inc., North Branford, CT) which generates orthovoltage x‐rays. Monte‐Carlo simulation was used to determine an appropriate filter to generate the NIST‐traceable beam quality so that a calibration factor necessary for a secondary chamber can be obtained by comparing to a NIST primary standard. Methods: The dimensions and positions of various components including a target within the X‐RAD 320 irradiator were implemented in BEAMnrc. The Monte Carlo simulation was validated by comparing with measurement of x‐ray transmission with a tube potential of 200, 250, and 300 kVp. Using the validated Monte‐Carlo codes, appropriate beam‐hardening filters were investigated to produce the NIST‐traceable beam qualities such as M200, M250, and M300. Results: The simulated transmission of x‐rays with a tube potential of 200, 250, and 300 kVp agreed with the measured ones with a difference of −0.1 ± 2.0%, 1.7 ± 2.1%, and 1.6 ± 2.3%, respectively. The statistical uncertainty in the Monte Carlo simulations was less than 0.5%. The Monte Carlo simulations verified that the NIST‐listed filters for M200 (4.35 mmAl + 1.12 mmCu), M250 (5.25 mmAl + 3.2 mmCu), and M300 (4.25 mmAl + 6.5 mmSn) were adequate for our X‐RAD 320 irradiator. The simulated half‐value layers and homogeneity coefficients using the above filters were 1.64 mmCu and 67.5% for 200 kVp, 3.20 mmCu and 86.1% for 250 kVp, and 5.3 mmCu and 97.1% for 300 kVp, which are in good agreement with the NIST‐traceable beam quality. Conclusions: Monte Carlo simulations of an X‐RAD 320 irradiator were performed to determined beam‐hardening filters for producing the NIST‐traceable beam quality. The NIST‐listed filters were satisfactory for the X‐RAD 320 irradiator to generate the NIST‐traceable beam quality. This research was supported by the Intramural Research Program of the NIH, NCI.