Feasibility study of multileaf collimated electrons with a scattering foil based accelerator

Background and purpose: There is an ever evolving process to improve the technical aspects of electron beam delivery. Both the foil/applicator and scanning electron beam systems have gone through recent upheavals. Concomitantly, multileaf collimators are now a staple method for collimating photons. We undertook a study of multileaf collimated electron beam (MLCEB) using a dual scattering foil system. Materials and methods: We compared MLCEB with applicator collimated electron beams (AEB) by examining the dosimetric aspects of the two systems using 70 and 80 cm SSDs for the MLCEB, the minimum practical SSDs achievable. Percent depth dose, isodose profiles, output factors, leakage, surface dose, bremstrahlung, effective SSDs, etc. were measured using film and/or ion chamber. Clinical fields, such as posterior neck node (PNN), were compared. We also investigated the use of MLCEB for are therapy using segments. Results: In all cases, the MLCEB performed inferior, as judged by isodoses, uniformity index (UI) and penumbra analysis. The 80 cm SSD (minimum for PNN), low energy, small fields, was the worst case. For a 6 MeV beam, the UI/penumbra was 0.823/10 mm for the AEB, and 0.561/29 mm for the MLCEB at 80 cm SSD. The PNN multileaf fields exhibited narrow 90% and 80% isodose lines, and wide 20% and 10% lines. Conclusions: We conclude that multileaf for PNN fields could not be matched to adjacent off-cord photon fields. The 'stair-stepping' effect associated with MLC photons was absent for electrons.