Program Information
Limitations of Radiographic Film-Based CT Radiation Beam Profile Measurements
N Artz*, E Somasundaram, S Brady, St. Jude Children's Research Hospital, Memphis, TN
Presentations
TU-RPM-GePD-IT-3 (Tuesday, August 1, 2017) 3:45 PM - 4:15 PM Room: Imaging ePoster Theater
Purpose: Demonstrate limitations of radiation beam profile measurements using radiographic film.
Methods: Gafchromic XR-QA2 radiographic film was placed on a lead plate at isocenter of a GE Revolution and exposed by 5mm collimated beams from 50-725mAs at 120kVp. The film was digitized to 1200dpi using a flatbed scanner. For comparison, 15 MOSFETs (BestMedical) were spaced 5mm apart and exposed at 350 and 500mAs. For fine sampling, multiple exposures were taken with the MOSFETs shifting in 0.5 increments. Full-width-at-half-maximum (FWHM) values were calculated for film and MOSFET beam profiles with a custom-built MATLAB script using linear interpolation at the 50% threshold (one above, one below) for ascending and descending sides. Similarly, XR-QA2 film was exposed using 160mm collimation and FWHM was calculated at 125, 325, and 725mAs. For reference, a 19.7mm ion chamber (Radcal,10X6-0.6) was placed perpendicular to the z-axis at isocenter and sampled (±100mm in 10mm steps) with 160mm collimated beams at 125 and 325mAs.
Results: 5mm Collimation - Film-based FWHM measurements increased with mAs (6.5-9.6mm). Each of these ten measurements passed GE’s performance criteria, but only two passed ACR criteria based on nominal collimation (ie. 5mm). MOSFET-based FWHM increased from 8.2 to 9.1mm for 350 and 500mAs respectively, similar to the film (8.4 and 9.4mm). 160mm Collimation – Measured FWHM was 137.4, 163.7, and 164.9mm at 125, 375, and 725mAs for film, although it was consistent at 162.4 and 162.3mm with 125 and 375mAs for the ion chamber. The measured heel-effect varied significantly with film exposure but not with the ion chamber. The film and ion chamber profiles were similar at 375mAs.
Conclusion: Film-based beam profile measurements demonstrated a strong dependence on mAs for 5 and 160mm collimations. This data suggests that measurements at 350 to 375 mAs may provide more accurate results using film.
Funding Support, Disclosures, and Conflict of Interest: Funding Support: ALSAC, the fundraising organization for St. Jude Children's Research Hospital
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