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A Mathematical Model for PinPoint Chamber Correction in Measuring Small Fields

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T Li

T Li*, Y Zhang , X Li , D.E. Heron , M.Saiful Huq , University of Pittsburgh Medical Center, Pittsburgh, PA


SU-E-T-17 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: For small field dosimetry, such as measuring the cone output factor for stereotactic radiosurgery, ion chambers often result in underestimation of the dose, due to both the volume averaging effect and the lack of electron equilibrium. The purpose of this work is to develop a mathematical model, specifically for the pinpoint chamber, to calculate the correction factors corresponding to different type of small fields, including single coneâ‚‹based circular field and nonâ‚‹standard composite fields.

Methods:A PTW 0.015cc PinPoint chamber was used in the study. Its response in a certain field was modeled as the total contribution of many small beamlets, each with different response factor depending on the relative strength, radial distance to the chamber axis, and the beam angle. To get these factors, 12 coneâ‚‹shaped circular fields (5mm,7.5mm, 10mm,12.5mm, 15mm,20mm, 25mm,30mm, 35mm,40mm, 50mm,60mm) were irradiated and measured with the PinPoint chamber. For each field size, hundreds of readings were recorded for every 2mm chamber shift in the horizontal plane. These readings were then compared with the theoretical doses as obtained with Monte Carlo calculation. A penalizedâ‚‹leastâ‚‹square optimization algorithm was developed to find out the beamlet response factors. After the parameter fitting, the established mathematical model was validated with the same MC code for other nonâ‚‹circular fields.

Results:The optimization algorithm used for parameter fitting was stable and the resulted response factors were smooth in spatial domain. After correction with the mathematical model, the chamber reading matched with the Monte Carlo calculation for all the tested fields to within 2%.

Conclusion:A novel mathematical model has been developed for the PinPoint chamber for dosimetric measurement of small fields. The current model is applicable only when the beam axis is perpendicular to the chamber axis. It can be applied to nonâ‚‹standard composite fields. Further validation with other type of detectors is being conducted.

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