Random and systematic beam modulator errors in dynamic intensity modulated radiotherapy.
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.
Parsai
Homayon
H
Department of Radiation Oncology, University of Washington, Box 356043, Seattle, WA 98195, USA. hparsai@u.washington.edu
Cho
Paul S
PS
Phillips
Mark H
MH
Giansiracusa
Robert S
RS
Axen
David
D
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