title: Properties of unflattened photon beams shaped by a multileaf collimator.
abstract: Several studies have shown that removal of the flattening filter from the treatment head of a clinical accelerator increases the dose rate and changes the lateral profile in radiation therapy with photons. However, the multileaf collimator (MLC) used to shape the field was not taken into consideration in these studies. We therefore investigated the effect of the MLC on flattened and unflattened beams. To do this, we performed measurements on a Varian Clinac 21EX and MCNPX Monte Carlo simulations to analyze the physical properties of the photon beam. We compared lateral profiles, depth dose curves, MLC leakages, and total scatter factors for two energies (6 and 18 MV) of MLC-shaped fields and jaw-shaped fields. Our study showed that flattening filter-free beams shaped by a MLC differ from the jaw-shaped beams. Similar differences were also observed for flattened beams. Although both collimating methods produced identical depth dose curves, the penumbra size and the MLC leakage were reduced in the softer, unflattened beam and the total scatter factors showed a smaller field size dependence.
authors.affiliation: Radiation Physics, University of Texas, M. D. Anderson Cancer Center 1515 Holcombe Boulevard, Unit 94, Houston, Texas 77030, USA. falkpoenisch@yahoo.com
authors.forename: Falk
authors.initials: F
authors.lastname: Pönisch
authors.forename: Uwe
authors.initials: U
authors.lastname: Titt
authors.forename: Oleg N
authors.initials: ON
authors.lastname: Vassiliev
authors.forename: Stephen F
authors.initials: SF
authors.lastname: Kry
authors.forename: Radhe
authors.initials: R
authors.lastname: Mohan
mesh_terms: D003198:Computer Simulation; D005374:Filtration / Q000379:methods; D006801:Humans; D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D013679:Technology, Radiologic / Q000295:instrumentation / Q000379:methods*