MONTE-CARLO DOSIMETRY STUDY OF A 6 MV STEREOTACTIC RADIOSURGERY UNIT

Small-field and stereotactic radiosurgery (SRS) dosimetry with radiati on detectors, used for clinical practice, have often been questioned d ue to the lack of lateral electron equilibrium and uncertainty in beam energy. A dosimetry study was performed for a dedicated 6 MV SRS unit , capable of generating circular radiation fields with diameters of 1. 25-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. With this c ode the accelerator was modelled for radiation fields with a diameter as small as 0.5 cm. The radiation fields and dosimetric characteristic s (photon spectra, depth doses, lateral dose profiles and cone factors ) in a water phantom were evaluated. The cone factor (S-t) for a speci fic cone c at depth d is defined as S-t (d, c) = D(d, c)/D(d, c(ref)), where c(ref) is the reference cone. To verify the Monte Carlo calcula tions, measurements were performed with detectors commonly used in SRS such as small-volume ion chambers, a diamond detector, TLDs and films . Results show that beam energies vary with cone diameter. For a 6 MV beam, the mean energies in water at the point of maximum dose for a 0. 5 cm cone and a 5 cm cone are 2.05 MeV and 1.65 MeV respectively. The values of S-t obtained by the simulations are in good agreement with t he results of the measurements for most detectors. When the lateral re solution of the detectors is taken into account, the results agree wit hin a few per cent for most fields and detectors. The calculations sho wed a variation of S-t with depth in the water. Based on calculated el ectron spectra in water, the validity of the assumption that measured dose ratios are equal to measured detector readings was verified.