title: Analysing collimator structure effects in head-scatter calculations for IMRT class fields using scatter raytracing.
abstract: The frequent blocking of the irradiated volume in intensity modulated radiation therapy (IMRT) makes the head-scatter fraction of the incident photon fluence more significant than that in conventional therapy with open fields. On the other hand. certain collimator configurations block scatter photons directed to a given observation point while allowing primary photons to be transmitted. The 'anomalous blocking' makes the primary field a poor indicator of the scatter fluence. Since large MU-to-cGy ratios in IMRT can magnify head-scatter uncertainties, it becomes necessary to accurately model both the effective scatter source and the collimator structure that limits the scatter reaching the irradiated volume. First we obtain a dual-source model, using a Taylor series expansion to derive the effective scatter source distribution from the data measured for the Elekta SL20 linac equipped with a multi-leaf collimator (MLC). Then, using a raytracing algorithm, we calculate the transmission of scatter rays from the effective scatter source plane to points in the patient plane. The method can account for the anomalous blocking of scatter by the MLC leaves and the backup diaphragms. For a variety of collimator settings tested, the calculations agree with measurements to an accuracy of 0.002psi10 x 10, where psi10 x 10 is the total (primary + scatter) photon fluence of an open 10 x 10 cm2 field for the same MU delivered. Although the significance of collimator structure in IMRT depends strongly on fields shapes employed for the delivery, potential cumulative errors on the order of a few per cent can be avoided in fluence calculations if the proposed method is used.
authors.affiliation: Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, USA. snaqvi@sun2.ummc.umaryland.edu
authors.forename: S A
authors.initials: SA
authors.lastname: Naqvi
authors.forename: M
authors.initials: M
authors.lastname: Sarfaraz
authors.forename: T
authors.initials: T
authors.lastname: Holmes
authors.forename: C X
authors.initials: CX
authors.lastname: Yu
authors.forename: X A
authors.initials: XA
authors.lastname: Li
mesh_terms: D000465:Algorithms; D007091:Image Processing, Computer-Assisted; D015233:Models, Statistical; D008962:Models, Theoretical; D017785:Photons; D020266:Radiotherapy, Conformal / Q000379:methods*; D012542:Scattering, Radiation