Range and modulation dependencies for proton beam dose per monitor unit calculations

Abstract

Calculations of dose per monitor unit ͑D/MU͒ are required in addition to measurements to increase patient safety in the clinical practice of proton radiotherapy. As in conventional photon and electron therapy, the D/MU depends on several factors. This study focused on obtaining range and modulation dependence factors used in D/MU calculations for the double scattered proton beam line at the Midwest Proton Radiotherapy Institute. Three dependencies on range and one dependency on modulation were found. A carefully selected set of measurements was performed to discern these individual dependencies. Dependencies on range were due to: ͑1͒ the stopping power of the protons passing through the monitor chamber; ͑2͒ the reduction of proton fluence due to nuclear interactions within the patient; and ͑3͒ the variation of proton fluence passing through the monitor chamber due to different source-to-axis distances ͑SADs͒ for different beam ranges. Different SADs are produced by reconfigurations of beamline elements to provide different field sizes and ranges. The SAD effect on the D/MU varies smoothly as the beam range is varied, except at the beam range for which the first scatterers are exchanged and relocated to accommodate low and high beam ranges. A geometry factor was devised to model the SAD variation effect on the D/MU. The measured D/MU variation as a function of range can be predicted within 1% using the three modeled dependencies on range. Investigation of modulated beams showed that an analytical formula can predict the D/MU dependency as a function of modulation to within 1.5%. Special attention must be applied when measuring the D/MU dependence on modulation to avoid interplay between range and SAD effects.

Clinical evaluation of the iterative metal artifact reduction algorithm for CT simulation in radiotherapy

“…We have used the definition of R80 as the proton range, which corresponds to the position where the dose has decreased to 80% of the maximum dose. 22,23 Similar trends were observed when the patient CT data were utilized to compare dose distributions. While generally the photon dose distributions were similar, the maximum global difference was encountered in scenarios with most metal and most metal artifacts in the beam path (bilateral hip implant patient).…”

MR‐based CT metal artifact reduction for head‐and‐neck photon, electron, and proton radiotherapy

“…For the dose calculation experiments, we investigated the impact of the MAR algorithms on the planned maximum dose (photons) and particle range (electrons and protons). For the electrons, we quantified the particle range by the distal depth at 90% of maximum dose (the therapeutic range , denoted R 90 ), and similarly for the protons by R 80 [see Fig. (b)].…”

Evaluation of an empirical monitor output estimation in carbon ion radiotherapy

“…There have been many studies regarding the dose output calculation method based on empirical models for a proton beam. [9][10][11][12][13][14] These studies have yielded successful estimations of the dose per monitor unit (d/MU), with errors of the order of 3.0%. More specifically, the field aperture dependence for carbon ion beams has been studied by introducing pencillike beams or Clarkson integration technique.…”