The effect of different lung densities on the accuracy of various radiotherapy dose calculation methods: Implications for tumour coverage

Abstract

Purpose To evaluate against Monte-Carlo the performance of various dose calculations algorithms regarding lung tumour coverage in stereotactic body radiotherapy (SBRT) conditions. Materials and methods Dose distributions in virtual lung phantoms have been calculated using four commercial Treatment Planning System (TPS) algorithms and one Monte Carlo (MC) system (EGSnrc). We compared the performance of the algorithms in calculating the target dose for different degrees of lung inflation. The phantoms had a cubic ‘body’ and ‘lung’ and a central 2-cm diameter spherical ‘tumour’ (the body and tumour have unit density). The lung tissue was assigned five densities ( ρ lung ): 0.01, 0.1, 0.2, 0.4 and 1 g / cm 3 . Four-field treatment plans were calculated with 6- and 18 MV narrow beams for each value of ρ lung . We considered the Pencil Beam Convolution ( PBC Ecl ) and the Analytical Anisotropic Algorithm ( AAA Ecl ) from Varian Eclipse and the Pencil Beam Convolution ( PBC OMP ) and the Collapsed Cone Convolution ( CCC OMP ) algorithms from Oncentra MasterPlan. Results When changing ρ lung from 0.4 to 0.1 g / cm 3 , the MC median target dose decreased from 89.2% to 74.9% for 6 MV and from 83.3% to 61.6% for 18 MV (of dose maximum in the homogenous case at both energies), while for both PB algorithms the median target dose was virtually independent of lung density. Conclusions Both PB algorithms overestimated the target dose, the overestimation increasing as ρ lung decreased. Concerning target dose, the AAA Ecl and CCC OMP algorithms appear to be adequate alternatives to MC.