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Use of a Commercial Ion Chamber Detector Array for the Measurement of High Spatial Resolution Photon Beam Profiles

V Karimnia

V Karimnia1*, M Belley123 , R Rodgers123 , M Price123 , (1) University of Rhode Island, Kingston, RI, (2) Rhode Island Hospital, Providence, RI, (3) Warren Alpert Medical Medical School of Brown University, Providence, RI


SU-I-GPD-T-553 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Required linear accelerator commissioning and quality control measurements are time consuming tasks, often requiring a water tank scanning system. Alternatively, this work describes a method to acquire variable resolution, photon beam profile data, equivalent to water tank scans, utilizing a commercially available ion chamber detector array (Sun Nuclear, IC-Profiler™).

Methods: Data was acquired using a Varian TrueBeam™ LINAC, 6MV photons. An IC-Profiler™ (0.5cm physical detector separation) was placed on the couch atop 10.0cm of solid water for backscatter and under 4.1cm of solid water to achieve a source to detector distance of 100cm and 5cm water equivalent depth. The detector array was aligned with CAX and 200 MU was delivered for field sizes of 5ₓ5, 10ₓ10, and 15ₓ15 cm2. Next, the couch was shifted laterally 0.4cm, in increments of 0.1cm, delivering 200 MU following each shift. Python script was used to sum and re-normalize the individual measurements, considering each measurement’s displacement from the CAX, and return a composite high resolution scan profile (δ = 0.1cm). The procedure was repeated for the inline direction. The composite profile data was compared to water tank measured ion chamber data at the equivalent depth and field sizes using gamma (2%/2mm criteria).

Results: Percentage of points satisfying gamma with 2%/2mm criteria was 81.3% and 75.0% for 5ₓ5; 90.9% and 90.1% for 10ₓ10; and 93.5%, and 93.6% for 15ₓ15cm2 fields; crossline and inline, respectively. All gamma values were <0.5 in profile regions where dose>20% dmax.

Conclusion: Photon beam profiles with 0.1cm resolution were acquired with an IC-Profiler™ (0.5cm physical detector resolution) and shown to agree with equivalent water-tank ion chamber measurements. Coupled with scripted couch movements and automated beam delivery (e.g.via Varian Developer Mode), this technique opens up new possibilities for rapid acquisition of high resolution, 2D-3D dosimetric data mitigating the need for in-water measurements.

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