title: Incorporating partial shining effects in proton pencil-beam dose calculation.
abstract: A range modulator wheel (RMW) is an essential component in passively scattered proton therapy. We have observed that a proton beam spot may shine on multiple steps of the RMW. Proton dose calculation algorithms normally do not consider the partial shining effect, and thus overestimate the dose at the proximal shoulder of spread-out Bragg peak (SOBP) compared with the measurement. If the SOBP is adjusted to better fit the plateau region, the entrance dose is likely to be underestimated. In this work, we developed an algorithm that can be used to model this effect and to allow for dose calculations that better fit the measured SOBP. First, a set of apparent modulator weights was calculated without considering partial shining. Next, protons spilled from the accelerator reaching the modulator wheel were simplified as a circular spot of uniform intensity. A weight-splitting process was then performed to generate a set of effective modulator weights with the partial shining effect incorporated. The SOBPs of eight options, which are used to label different combinations of proton-beam energy and scattering devices, were calculated with the generated effective weights. Our algorithm fitted the measured SOBP at the proximal and entrance regions much better than the ones without considering partial shining effect for all SOBPs of the eight options. In a prostate patient, we found that dose calculation without considering partial shining effect underestimated the femoral head and skin dose.
authors.affiliation: Department of Radiation Physics, University of Texas, MD Anderson Cancer Center, Houston, TX 77030, USA. yupli@mdanderson.org
authors.forename: Yupeng
authors.initials: Y
authors.lastname: Li
authors.forename: Xiaodong
authors.initials: X
authors.lastname: Zhang
authors.forename: Mingfwu
authors.initials: M
authors.lastname: Lii
authors.forename: Narayan
authors.initials: N
authors.lastname: Sahoo
authors.forename: Ron X
authors.initials: RX
authors.lastname: Zhu
authors.forename: Michael
authors.initials: M
authors.lastname: Gillin
authors.forename: Radhe
authors.initials: R
authors.lastname: Mohan
mesh_terms: D000465:Algorithms; D008954:Models, Biological; D061766:Proton Therapy; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D012542:Scattering, Radiation; D011522:Protons
chemical_list: D011522:Protons