Iterative reconstruction of volumetric modulated arc radiotherapy plans using control point basis vectors
2014
Volumetric Modulated Arc Radiotherapy is an innovative technique currently utilized to efficiently deliver complex
treatments. Dose rate, speed of rotation, and field shape are continuously varied as the radiation source rotates about the
patient. Patient specific quality assurance is performed to verify that the delivered dose distribution is consistent with the
plan formulated in a treatment planning system. The purpose of this work is to present novel methodology using a
Gafchromic EBT3 film image of a patient plan in a cylindrical phantom and calculating the delivered MU per control
point. Images of two dimensional plan dose matrices and film scans are analyzed using MATLAB with the imaging
toolbox. Dose profiles in a ring corresponding to the film position are extracted from the plan matrices for comparison
with the corresponding measured film dose. The plan is made up of a series of individual static Control Points. If we
consider these Control Points a set of basis vectors, then variations in the plan can be represented as the weighted sum of
the basis. The weighing coefficients representing the actual delivered MU can be determined by any available
optimization tool, such as downhill simplex or non-linear programming. In essence we reconstruct an image of the
delivered dose. Clinical quality assurance is performed with this technique by computing a patient plan with the
measured monitor units and standard plan evaluation tools such as Dose Volume Histograms. Testing of the algorithm
with known changes in the reference images indicated a correlation coefficient greater than 0.99.
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