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Physical aspects of dynamic stereotactic radiosurgery with very small photon beams (1.5 and 3 mm in diameter)
Author(s) -
Paskalev Kamen A.,
Seuntjens Jan P.,
Patrocinio Horacio J.,
Podgorsak Ervin B.
Publication year - 2003
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1536290
Subject(s) - isocenter , imaging phantom , radiosurgery , monte carlo method , linear particle accelerator , dosimetry , physics , nuclear medicine , beam (structure) , collimator , optics , radiation therapy , medicine , mathematics , radiology , statistics
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 EGS nrc 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.