SU‐E‐T‐209: Deconvolution of An Integral Film Exposure in a Cylindrical Phantom for VMAT QA

Abstract

Purpose: Present an algorithm that is capable of deconvoluting a single radiochromic film exposed in a cylindrical phantom into the basic control point components. Changes noted in the measured control points can be incorporated into a delivered treatment plan using a commercial treatment planning system to compare the delivered dose with the planned dose using DVH tools. Methods: A 180 degree VMAT verification plan was subdivided into control points every 5 degrees. Dose matrices for the individual control points were exported for analysis to a PC with MATLAB software. The dose in the center ring of the cylindrical phantom represents the simulated film measurement. The plan dose is equal to the sum of the control points. Mathematically, the control points are the basis vectors of thedelivered plan. The deconvolution algorithm utilizes linear optimization to first determine the coefficients of the vectors and then to check for changes in the individual control points. Changes in the control points were introduced to test the deconvolution algorithm. For development purposes only a single plane was investigated. Measurement of both the entrance and exit doses improves the accuracy and is also used to compute the control point angle. Results: (1) Measured film has superior resolution compared to electronic detectors, (2) the deconvolution algorithm can determine the individual delivered control point parameters, (3) the measured control points can be used to calculate the delivered dose distribution, and (4) the treatment planning system can be used to compare the plan and delivered dose distribution within the patient using its DVH tools. Conclusion: Deconvolution of an integral film in a cylindrical phantom for a VMAT plan represents the delivered control points that can be entered into the treatment planning system for patient specific VMAT QA utilizing DVH tools.