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Small Dynamic Field Dosimetry by Gfachromic Film (EBT3) and 2D-Array Diode


A Chu

A Chu1*, W Feng2, H Lincoln1, F Su1, R Nath3, (1) Yale New Haven Hospital, New Haven, CT, (2) New York Presbyterian Hospital, Tenafly, NJ, (3) Yale Univ School of Medicine, New Haven, CT

SU-E-T-118 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:Small dynamic multi-leaves collimator (MLC) dosimetry presents challenges to clinical radiological measurements. This study was to evaluate the small dynamic field dosimetry using one-scan film algorithm (FilmQA Pro/Ashland Inc.) and MapCheck 2D-array (Sun Nuclear). Commercial diode-array would provide practical needs for dynamic field QA (e.g. IMRT or VMAT). Alternatively Chromic film dosimetry serves the same purposes as diode array with much better spatial resolution, wider dose range and less energy dependency. One-scan algorithm minimizes the problems of conventional film dosimetry, which suffered time-consuming process, numerous non-radiation artifacts.

Methods: Both dosimeters along with chamber measurements (TN31010/PTW, 0.125cm3) at central-axis (CAX) as absolute dose comparisons were tested by 6MeV photon with 120HD-MLC under a Varian iX LINAC using dynamic MLC (with gaps of 1, 5, 10, 20, 50, 100mm width) sweeping through a 5.0cm jaws-opening size. The experiments were arranged by a 2cm solid-water buildup above tested films, and the 2cm water-equivalent intrinsic buildup over the MapCheck diode array at 100cm SAD setup. Each film-measurement was calibrated by 4 dose-points in the same film by one-scan algorithm.

Results:CAX absolute doses from both dosimeters showed very close results (+/-0.3% deviation) from the according chamber measurements for relatively large dynamic MLC fields (gap size > 20mm). Otherwise each dosimeter presented different problems with mixed deviations of 4~8% from CAX chamber measurements. Energy-dependence (from scatters) and low-dose bias had been well-known for diode performance, which was enhanced by small gap-width (< 1cm). Film dosimetry suffered from systematic noises, especially in low-dose (8~10 cGy), even with the one-scan algorithm corrections. The problem of one-scan protocol may be improved by the cares of the choice for calibration dose values to avoid over-correction in low-dose.

Conclusion:Film provided much better spatial details (0.35mm/pixel), e.g. MLC interleaved leakage, than 2D diode-array, and similar overall dose accuracy compared to diode array.

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