AbstractID: 8626 Title: Verification of Measured Beam Characteristics for Small Stereotactic Radiosurgery
Fields using Various Ionization Chambers and Radiochromic film with Monte Carlo Technique
In Stereotactic Radiosurgery (SRS) the empirical dose method comprises the use of measured data, such as tissue maximum ratio, off
axis ratio, and output factor. However, this dosimetric approach fails because of lateral electronic disequilibrium in small field sizes,
consequently, leading to inaccurate dose distribution to the target. The use of Monte Carlo (MC) technique for computation of dose in
radiotherapy has proven to overcome this inadequacy.
This paper will attempt to verify the efficacy of Monte Carlo technique in dose calculation for small field SRS beams. We utilized the
OMEGA BEAM code to model a 6MV photon beam using collimator sizes of 7.5 mm, 1 cm, and 4 cm in diameter. Calculated beam
characteristics (%DD, lateral profiles, and output factors) were subsequently compared to the measured data. Quantitative
measurements were made using a cylindrical ionization chamber with a collecting volume of 0.015 cm3, a parallel plate ionization
chamber with a collecting volume of and 0.002 cm3, and a diamond ionization chamber as well as radiochromic film dosimetry.
Our results showed a favorable agreement between calculated and measured percent depth dose for 7.5 mm, 1 cm, and 4 cm field size
collimators. Calculated beam profiles of 1 cm and 4 cm field size at 1.5 cm, 5 cm, and 10 cm depths were within 3% difference of the
measured data.
The output factors showed a significant dependency on the collecting volume size of the ionization chamber used. The difference
between the measured and calculated output factors were less than 4%.