Impact of simple tissue inhomogeneity correction algorithms on conformal radiotherapy of lung tumours

Background and purpose: Conformal radiotherapy requires accurate dose calcu lation at the dose specification point, at other points in the planning tar get volume (PTV) and in organs at risk. To assess the limitations of treatm ent planning of lung tumours, errors in dose values, calculated by some sim ple tissue inhomogeneity correction algorithms available in a number of cur rently applied treatment planning systems, have been quantified. Materials and methods: Single multileaf collimator-shaped photon beams of 6 , 8, 15 and 18 MV nominal energy were used to irradiate a 50 mm diameter sp herical solid tumour, simulated by polystyrene, which was located centrally inside lung tissue, simulated by cork. The planned dose distribution was m ade conformal to the PTV, which was a 15 mm three-dimensional expansion of the tumour. Values of both the absolute dose at the International Commissio n on Radiation Units and Measurement (ICRU) reference point and relative do se distributions inside the PTV and in the lung were calculated using three inhomogeneity correction algorithms. The algorithms investigated in this s tudy are the pencil beam algorithm with one-dimensional corrections, the mo dified Batho algorithm and the equivalent path length algorithm. The calcul ated data were compared with measurements for a simple beam set-up using ra diographic film and ionization chambers. Results: For this specific configuration, deviations of up to 3.5% between calculated and measured values of the dose at the ICRU reference point were found. Discrepancies between measured and calculated beam fringe values (d istance between the 50 and 90% isodose lines) of up to 14 mm have been obse rved. The differences in beam fringe and penumbra width (20-80%) increase w ith increasing beam energy. Our results demonstrate that an underdosage of the PTV up to 20% may occur if calculated dose values are used for treatmen t planning. The three algorithms predict a considerably higher dose in the lung, both along the central beam axis and in the lateral direction, compar ed-with the actual delivered dose values. Conclusions: The dose at the ICRU reference point of such a tumour in lung geometry is calculated with acceptable accuracy. Differences between calcul ated and measured dose distributions are primarily due to changes in electr on transport in the lung, which are not adequately taken into account by th e simple tissue inhomogeneity correction algorithms investigated in this st udy. Particularly for high photon beam energies, clinically unacceptable er rors will be introduced in the choice of field sizes employed for conformal treatments, leading to underdosage of the PTV. In addition, the dose to th e lung will be wrongly predicted which may influence the choice of the pres cribed dose level in dose-escalation studies. (C) 2001 Elsevier Science Ire land Ltd. All rights reserved.