The purpose of this work is to investigate the role of dynamic therapy techniques in electron arc therapy. A several year experience with electron arc therapy to the chest wall of women following mastectomy for breast cancer has led to a series of technical and operational modifications in the technique. The experience in over 50 patients has demonstrated the desirability for further optimization of dose distributions across the chest wall, beyond that which we have been able to perform using fixed shape electron collimators. We propose to accomplish this through dynamic movement of the individual vanes of a multi- vane electron collimator during electron arc. The aperture width of each section of the multi-vane collimator will be varied by computer control as a function of accelerator gantry angle. The desired width for each section of the collimator vs gantry angle will be determined by computer optimization, which will examine the influence of variable aperture width vs gantry angle on the chest wall dose distribution and will pick the width for each arc segment which optimizes the dose uniformity across the entire chest wall. The depth of electron penetration will be adjusted through use of multiple electron energies and through individualized bolus thickness in order to minimize the dose to the underlying lung. Electron beam characteristics for the clinically available energies will be tabulated for field widths from 3cm to 7cm. An existing routine which optimizes dose by adjusting electron field weight will be extended to optimize dose by adjusting electron field width. Computer simulations of optimized and unoptimized dose distributions will be performed for a selected group of previously treated electron arc patient contours in which dose inhomogeneity was seen in clinical treatment plans. A prototype variable vane electron collimator will be built and tested in phantom studies. A computer interface will be constructed to vary the individual vane widths as a function of gantry angle, based on the results of the computer optimization. Phantoms representing the extremes of patient anatomy seen clinically will be constructed. Using these specially constructed phantoms, thermoluminescent dosimetry and film densitometry will be used to verify the dose distributions. Review and evaluation of the optimized dose distributions will be accomplished in conjunction with a radiation oncologist experienced in photon and electron beam techniques for treatment of the post mastectomy chest wall. The assessment will quantitate the differences seen and evaluate their potential clinical significance. In a second phase (beyond this request for support), the optimized system will be tested clinically utilizing the patient base established over the past few years during our preliminary electron arc studies.
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