Modulated electron beams using multi-segmented multileaf collimation

Background and purpose: Conformal radiation therapy by modulation of photon and electron beams has been described for many techniques. Modulation of e lectron beams with the use of either bolus or altered scanned beams is repo rted. Our institution previously investigated the use of MLC for fixed stat ic electron fields. The results were dismal due to the wide penumbra associ ated 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 laking advantage of matching the diffuse beam edges. Materials and methods: We started by planning simple target shapes, such as wedged and parabolic shapes. We optimized the segment widths, weights, gap s 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 po ssessed varying depths from surface to medial aspect. Results: We found that we could optimize distributions by using the treatme nt depth and points of inflection to derive optimal segments. The planned t reatments using a simple CET (coefficient of equivalent thickness) algorith m were confirmed with reasonable success. A 24 cm wide target was treated w ith nine segments in under 5 min, while most cases, including the parotid v olume, were treated in under 3 min. Conclusions: We have developed a technique to complement a photon modulatio n program by treating lesions close to the surface. The debate as to whethe r this technique is optimal and more efficient than a bolus technique conti nues. We are currently devising methods for optimizing modulated electron M LC beams in multiple planes. (C) 1998 Elsevier Science ireland Ltd. All rig hts reserved.