Modulated electron beams using multi-segmented multileaf collimation.

Abstract

Conformal radiation therapy by modulation of photon and electron beams has been described for many techniques. Modulation of electron beams with the use of either bolus or altered scanned beams is reported. Our institution previously investigated the use of MLC for fixed static electron fields. The results were dismal due to the wide penumbra associated with multileaf collimated electrons scattered from foils. The purpose of this study was to decipher whether multi-segmented electron beams using MLC would provide a useful and practical modulated beam by taking advantage of matching the diffuse beam edges. We started by planning simple target shapes, such as wedged and parabolic shapes. We optimized the segment widths, weights, gaps between segments and energies. We then irradiated phantoms using film to confirm the calculations. The study was limited to single-plane irradiation. We also planned and measured isodoses for a parotid target volume that possessed varying depths from surface to medial aspect. We found that we could optimize distributions by using the treatment depth and points of inflection to derive optimal segments. The planned treatments using a simple CET (coefficient of equivalent thickness) algorithm were confirmed with reasonable success. A 24 cm wide target was treated with nine segments in under 5 min, while most cases, including the parotid volume, were treated in under 3 min. We have developed a technique to complement a photon modulation program by treating lesions close to the surface. The debate as to whether this technique is optimal and more efficient than a bolus technique continues. We are currently devising methods for optimizing modulated electron MLC beams in multiple planes.