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An Investigation of Patient Plan-Specific Correction Factors for An Exradin A1SL Ion Chamber in IMRT Treatments On a Medical Linear Accelerator

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V Desai

V Desai*, Z Labby , W Culberson , Univ of Wisc Madison, Madison, WI


TU-FG-205-2 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: 205

Purpose: To assess the magnitude and variability of the clinical kQ,clin correction factor, a chamber- and plan-specific quantity which translates the chamber reading to an absorbed dose to water, for an A1SL scanning chamber across a large number of composite step-and-shoot intensity modulated radiation therapy (IMRT) patient plans using Monte Carlo, and to contextualize those results using descriptive plan complexity metrics.

Methods: 85 previously treated step-and-shoot plans incorporating a wide variety of treatment sites and levels of modulation were imported and converted to EGSnrc Monte Carlo inputs using validated models of a Varian Clinac 21EX and an Exradin A1SL ionization chamber. Full treatment simulations incorporating all gantry, collimator, couch, and MLC motions were delivered to the chamber model embedded in a cylindrical phantom geometry with appropriate beam shifts to center the chamber in a high-dose, low-gradient region per clinical practice. The magnitudes of the clinical kQ,clin correction factors, along with plan-averaged descriptive metrics including aperture area and average leaf pair opening (ALPO), were determined for every plan.

Results: Six out of the 85 plans exhibited correction magnitudes greater than 0.5% from unity with simulation uncertainty for calculated factors on the order of 0.3%. The absolute minimum aperture area encountered in this data set was 23.2 cm² and the absolute minimum ALPO was 4.27 cm.

Conclusion: In certain clinical cases, the net contribution from troublesome nonstandard subfields in the composite plan can be negligible. Furthermore, common field metrics such as aperture area, equivalent square, and ALPO largely break down in being useful qualities to consider when the aperture shapes within the plans are highly irregular. More relevant metrics should be developed, investigated, and implemented to better understand the numerous nonstandard fields utilized in modern IMRT and VMAT and their ultimate impact on the clinical kQ,clin correction factor.

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