title: Physical aspects of dynamic stereotactic radiosurgery with very small photon beams (1.5 and 3 mm in diameter).
abstract: Stereotactic radiosurgery is often used for treating functional disorders. For some of these disorders, the size of the target can be on the order of a millimeter and the radiation dose required for treatment on the order of 80 Gy. The very small radiation field and high prescribed dose present a difficult challenge in beam calibration, dose distribution calculation, and dose delivery. In this work the dose distribution for dynamic stereotactic radiosurgery, carried out with 1.5 and 3 mm circular fields, was studied. A 10 MV beam from a Clinac-18 linac (Varian, Palo Alto, CA) was used as the radiation source. The BEAM/EGS4 Monte Carlo code was used to model the treatment head of the machine along with the small-field collimators. The models were validated with the EGSnrc code, first through a calculation of percent depth doses (PDD) and dose profiles in a water phantom for the two small stationary circular beams and then through a comparison of the calculated with measured PDD and profile data. The three-dimensional (3-D) dose distributions for the dynamic rotation with the two small radiosurgical fields were calculated in a spherical water phantom using a modified version of the fast XVMC Monte Carlo code and the validated models of the machine. The dose distributions in a horizontal plane at the isocenter of the linac were measured with low-speed radiographic film. The maximum sizes of the Monte Carlo-calculated 50% isodose surfaces in this horizontal plane were 2.3 mm for the 1.5 mm diameter beam and 3.8 mm for the 3 mm diameter beam. The maximum discrepancies between the 50% isodose surface on the film and the 50% Monte Carlo-calculated isodose surfaces were 0.3 mm for both the 1.5 and 3 mm beams. In addition, the displacement of the delivered dose distributions with respect to the laser-defined isocenter of the machine was studied. The results showed that dynamic radiosurgery with very small beams has a potential for clinical use.
authors.affiliation: McGill University Health Centre, Montreal General Hospital, 1650 Avenue Cedar, Montréal, Québec H3G 1A4, Canada.
authors.forename: Kamen A
authors.initials: KA
authors.lastname: Paskalev
authors.forename: Jan P
authors.initials: JP
authors.lastname: Seuntjens
authors.forename: Horacio J
authors.initials: HJ
authors.lastname: Patrocinio
authors.forename: Ervin B
authors.initials: EB
authors.lastname: Podgorsak
mesh_terms: D004307:Dose-Response Relationship, Radiation; D005373:Film Dosimetry; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D011786:Quality Control; D011874:Radiometry / Q000379:methods*; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity