title: Random and systematic beam modulator errors in dynamic intensity modulated radiotherapy.
abstract: This paper reports on the dosimetric effects of random and systematic modulator errors in delivery of dynamic intensity modulated beams. A sliding-widow type delivery that utilizes a combination of multileaf collimators (MLCs) and backup diaphragms was examined. Gaussian functions with standard deviations ranging from 0.5 to 1.5 mm were used to simulate random positioning errors. A clinical example involving a clival meningioma was chosen with optic chiasm and brain stem as limiting critical structures in the vicinity of the tumour. Dose calculations for different modulator fluctuations were performed, and a quantitative analysis was carried out based on cumulative and differential dose volume histograms for the gross target volume and surrounding critical structures. The study indicated that random modulator errors have a strong tendency to reduce minimum target dose and homogeneity. Furthermore, it was shown that random perturbation of both MLCs and backup diaphragms in the order of sigma = 1 mm can lead to 5% errors in prescribed dose. In comparison, when MLCs or backup diaphragms alone was perturbed, the system was more robust and modulator errors of at least sigma = 1.5 mm were required to cause dose discrepancies greater than 5%. For systematic perturbation, even errors in the order of +/- 0.5 mm were shown to result in significant dosimetric deviations.
authors.affiliation: Department of Radiation Oncology, University of Washington, Box 356043, Seattle, WA 98195, USA. hparsai@u.washington.edu
authors.forename: Homayon
authors.initials: H
authors.lastname: Parsai
authors.forename: Paul S
authors.initials: PS
authors.lastname: Cho
authors.forename: Mark H
authors.initials: MH
authors.lastname: Phillips
authors.forename: Robert S
authors.initials: RS
authors.lastname: Giansiracusa
authors.forename: David
authors.initials: D
authors.lastname: Axen
mesh_terms: D001932:Brain Neoplasms / Q000532:radiotherapy; D003198:Computer Simulation; D019544:Equipment Failure Analysis / Q000379:methods* / Q000592:standards; D006801:Humans; D008579:Meningioma / Q000532:radiotherapy; D008954:Models, Biological; D015233:Models, Statistical; D011786:Quality Control; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods* / Q000592:standards; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity