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UBC Theses and Dissertations

Dose verification of rotating collimator intensity modulated radiation thearpy Schmuland, Moira Leigh

Abstract

A novel method of delivering Intensity Modulated Radiation Therapy (IMRT) has been proposed in which the entire multileaf collimator (MLC) is rotated between each segment. Current linear accelerators (linacs) are not designed for IMRT delivery with collimator rotation therefore extensive quality assurance testing must be done before this technique can be used clinically. This thesis has two main objectives. The first is to evaluate whether IMRT plans incorporating collimator rotation can be delivered on a standard radiotherapy linac. The second is to evaluate whether the dose distributions produced with rotating MLC (RMLC) IMRT are comparable to IMRT plans conventionally delivered, without collimator rotation. First, the Dynamic Beam Delivery (DBD) toolbox, the interface required to independently control collimator rotation, was tested. It was verified that the DBD toolbox could rotate the collimator accurately to its intended angle within 0.5°. Also, fluence distributions of varying complexity were generated using the in-house RMLC algorithm and validated using film-based verification methods. Dosimetric analysis provided information regarding the MLC modeling and resolution limitations of the algorithm. Clinical RMLC IMRT treatment plans were generated and delivered and, after incorporating a 1.4 mm leaf-gap requirement, good agreement between the measured and calculated distributions for single fields were obtained. These test results indicated that RMLC IMRT is viable on radiotherapy linacs. To evaluate whether treatment plans optimized for RMLC IMRT delivery were comparable to plans optimized for conventional IMRT delivery, composite multi-field dose distributions delivered to a water-equivalent phantom were analyzed. Prostate, nasopharynx and c-shape structures were used as the target volumes and the treatment sites were optimized for RMLC, conventional, and rotating aperture optimization (RAO) IMRT. The measured and calculated (Varian CadPlan treatment planning system) dose distributions were compared using orthogonal profiles and a gamma factor analysis. Results indicate that RMLC and conventional IMRT plans are comparable when using a 5.0 mm width beamlet (versus the 2.5 mm width beamlet) during the optimization. In addition, it was found that the RAO IMRT plans, which also incorporated collimator rotation, yielded the best results.

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