FEASIBILITY STUDY OF MULTILEAF COLLIMATED ELECTRONS WITH A SCATTERINGFOIL BASED ACCELERATOR

Background and purpose: There is an ever evolving process to improve t he technical aspects of electron beam delivery. Both the foil/applicat or and scanning electron beam systems have gone through recent upheava ls. Concomitantly, multileaf collimators are now a staple method for c ollimating photons. We undertook a study of multileaf collimated elect ron beam (MLCEB) using a dual scattering foil system. Materials and me thods: We compared MLCEB with applicator collimated electron beams (AE B) by examining the dosimetric aspects of the two systems using 70 and 80 cm SSDs for the MLCEB, the minimum practical SSDs achievable. Perc ent depth dose, isodose profiles, output factors, leakage, surface dos e, 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 jud ged by isodoses, uniformity index (UI) and penumbra analysis. The 80 c m 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 exhibit ed narrow 90% and 80% isodose lines, and wide 20% and 10% lines. Concl usions: We conclude that multileaf for PNN fields could not be matched to adjacent off-cord photon fields. The 'stair-stepping' effect assoc iated with MLC photons was absent for electrons.