title: Influence of focal spot on characteristics of very small diameter radiosurgical beams.
abstract: Percentage depth dose (PDD) distributions and beam profiles of very small diameter (1.5-5 mm) megavoltage radiosurgical beams calculated with Monte Carlo (MC) technique critically depend on the diameter of the circular focal spot used in the simulation: The smaller is the field diameter, the larger is the effect. Thus, in simulations of radiosurgical fields that have diameters of the order of the focal spot size, an accurate focal spot geometry should be used. We used a simplified moving slit technique in conjunction with a diode detector for evaluation of the focal spot size and shape of a megavoltage 6 MV linac as well as for determination of the equivalent focal spot diameter of the linac for use in MC simulations. The measured total diode signal contains three components: A direct focal spot signal, a background signal, and an extra-focal radiation signal. A single profile scan of the focal spot signal is Gaussian like in shape, and its full width at half maximum is used to define the focal spot dimension for this scan. The focal spot of our 6 MV linac is approximated with a Gaussian circle, and when the geometry of the effective focal spot circle is used in MC simulations, the agreement between MC-calculated and measured PDD distributions as well as beam profiles is good even for radiosurgical fields as small as 1.5 mm in diameter. Our results also confirm that matching the penumbral areas of accurately measured large-field beam profiles to the same areas of the MC-calculated beam profiles reliably leads to a realistic effective focal spot size for use in MC simulations of very small diameter beams.
authors.affiliation: Department of Medical Physics, McGill University Health Centre, 1650 avenue Cedar Montreal, Quebec H3G 1A4, Canada. esham@medphys.mcgill.ca
authors.forename: Edwin
authors.initials: E
authors.lastname: Sham
authors.forename: Jan
authors.initials: J
authors.lastname: Seuntjens
authors.forename: Slobodan
authors.initials: S
authors.lastname: Devic
authors.forename: Ervin B
authors.initials: EB
authors.lastname: Podgorsak
mesh_terms: D000388:Air; D003198:Computer Simulation; D004867:Equipment Design; D006801:Humans; D015233:Models, Statistical; D008962:Models, Theoretical; D009010:Monte Carlo Method; D016011:Normal Distribution; D010315:Particle Accelerators; D016634:Radiosurgery / Q000295:instrumentation* / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D013997:Time Factors; D014965:X-Rays