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

By Wen C. Hsi, Andries N. Schreuder, Michael F. Moyers, Chris E. Allgower, Jonathan B. Farr and Anthony E. Mascia


Calculations of dose per monitor unit (D∕MU) 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: (1) the stopping power of the protons passing through the monitor chamber; (2) the reduction of proton fluence due to nuclear interactions within the patient; and (3) the variation of proton fluence passing through the monitor chamber due to different source-to-axis distances (SADs) 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

Topics: Radiation Therapy Physics
Publisher: American Association of Physicists in Medicine
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Provided by: PubMed Central
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