Factory quality assurance of passive radiotherapy intensity modulators for electrons using kilovoltage x‐ray imaging

PURPOSE: This work developed an x‐ray‐based method for performing factory quality assurance (QA) of Passive Radiotherapy Intensity Modulators for Electrons (PRIME) device fabrication. This method measures errors in position, diameter, and orientation of cylindrical island blocks on a hexagonal grid that comprises PRIME devices and the impact of such errors on the underlying intensity distribution. METHODS: X‐ray images were acquired of six PRIME devices, which modeled three error cases (small random, large random, and systematic errors) for two island block diameters (0.158 and 0.352 cm). Island blocks in each device, 0.6 cm long tungsten cylinders of constant diameter, were spaced 0.6 cm on a hexagonal grid over approximately 8 cm square. Using a 50 kVp x‐ray image, each island block projected a racetrack, whose perimeter was fit to a function that allowed determination of its position, diameter, and angular orientation (θ, ϕ). These measured parameters were input into a pencil beam algorithm (PBA) dose calculation performed in water (16 MeV, SSD = 103 cm) for each device. PBA calculated intensity distributions using measured and planned (exact) island block parameters were compared. RESULTS: Θ distributions for the 0.158 and 0.352 cm devices were nearly identical for each error case, with θ values for most island blocks being within 3.2°, 8.5°, and 7.5° for the small random, large random, and systematic error PRIME devices, respectively. Corresponding intensity differences between using measured and planned island block parameters were within 1.0% and 2.8% (small random), 2.2% and 4.8% (large random), and 3.2% and 6.7% (systematic) for the 0.158 and 0.352 cm devices, respectively. CONCLUSION: This approach provides a viable and economical method for factory QA of fabricated PRIME devices by determining errors in their planned intensity distribution from which their quality can be assessed prior to releasing to the customer.