ABSTRACT
Objective To study the accuracy of collapsed cone convolution ( CCC) and anisotropic analytical algorithm ( AAA) in dosimetric calculation on the air cavity interface. Methods A BEAMnrc/EGSnrc Monte Carlo ( MC ) simulation was performed on a Varian Trilogy linear accelerator. The IBA Dosimetry “blue phantom” 3D scanning system was used to verify the accuracy and reliability of the MC simulation. Central axis depth dose distribution and lateral dose profile in a water?equivalent phantom with variously sized air cavities were calculated by CCC and AAA. The obtained depth dose distribution and lateral dose profile were compared with those by MC simulation and EBT2 film, respectively. Results Both CCC and AAA overestimated the dose on the air cavity interface. In spite of some errors, CCC had a higher accuracy than AAA. The errors were mainly related to computational grid, field size, photon energy, cavity size, and the number of fields. Conclusion Electronic disequilibrium on the air cavity interface should be taken into account when CCC and AAA are used for dosimetric calculation in treatment planning system.
ABSTRACT
Objective To introduce and evaluate the dosimetric characteristics of a GyroKnife cobalt-60 system for radiosurgery.Methods0.015 cm3 and 0.600 cm3 ionization chamber,EDR2 film and semiconductor dosimeter were used to measure the dose rate of center point for the four collimators.The diameter of the four collimators were 5 mm,12 mm,30 mm and 50 mm,respectively.0.015 cm3 ionization chamber was used to test the dose-time linear relationship and dose stability.0.015 cm3 ionization chamber and semiconductor was used to measure the dose error of phantom between treatment planning system (TPS)calculation and measurement.The film was used to measure the error of TPS calculation isodose lines width.ResultsThe results from the four measurement methods have no significant difference for the collimator 50 mm.But for collimator 5 mm great discrepancy appeared.The error between calculation by TPS and the measurement by semiconductor was biggest and the value was 4.8％.Most of the error was within 3.0％.The error of the 50％ isodose line along x-axis was biggest and the value was 4.9 mm,others are all within 2.0 mm.ConclusionThe dosimetric character of this system is suitable for stereotactic radiotherapy.
ABSTRACT
Objective To introduce and evaluate the characteristics of a GyroKnife Digital Cobalt-60 System for stereotactic radiotherapy and radiosurgery in mechanical aspect.Methods To test the radiation safety,the dose rate required for radiation protection was measured by using an ionization survey meter ( Inovision Model 451 B,Cleveland,OH ).Micrometer was used to verify the accuracy of the table in movement and protractor to verify the rotation movement range.The 160 mm diameter polystyrene spherical phantom and film were used in measurement of the radiation Full Width at Half Maximum.And the difference between radiation isocenter and mechanical isocenter was also verified.Results Clinical requirement in radiation protection was met.The maximum error of table movement accuracy was 0.2 cm at Z axis; the maximum error of gantry movement accuracy was 0.1°.For all the collimators,radiation Full Width at Half Maximum ( FWHM ) was 2.55,5.40,10.50,and 18.55 mm and average penumbra was 5.0,12.4,30.1,and 51.5 mm,respectively.The maximum difference between mechanical center and radiation center was 1.41 mm.Conclusions Stereotactic radiotherapy and radiosurgery has good mechanical characteristics and is suitable clinic applications.