Determination of photon fluence spectra from a 60Co therapy unit based on PENELOPE and MCNP simulations

Photon fluence spectra of the Seibersdorf Labor/BEV Picker 60Co therapy unit were calculated using two generally recognised Monte Carlo codes, PENELOPE-2006 and MCNP5. The complexity of the simulation model was increased in three steps (from a pure source capsule and a simplified model using rotational symmetry to a realistic model of the facility). Photon fluence spectra of both codes generally agree within their statistical standard uncertainties for the case of identical geometry set-up and particle transport parameter settings. Resulting total fluence values were about 0.3% higher for MCNP as compared to PENELOPE. The verification of the simulated photon fluence spectra was based upon depth–dose measurements in water performed with a PTW 31003 ionisation chamber and a thick-walled chamber type CC01. The depth–dose curve calculated with PENELOPE agreed with the curve obtained from measurements within 0.4% across the available depth region in the 30 cm × 30 cm × 30 cm water phantom. The comparison of measured and simulated beam quality indices (TPR20,10) revealed deviations of less than 0.2%.