THE ACCURACY OF CT-BASED INHOMOGENEITY CORRECTIONS AND IN-VIVO DOSIMETRY FOR THE TREATMENT OF LUNG-CANCER

Purpose: To determine the accuracy of dose calculations based on CT-de nsities for lung cancer patients irradiated with an anterioposterior p arallel-opposed treatment technique and to evaluate, for this techniqu e, the use of diodes and an Electronic Portal Imaging Device (EPID) fo r absolute exit dose and relative transmission dose verification, resp ectively. Materials and methods: Dose calculations were performed usin g a 3-dimensional treatment planning system, using CT-densities or ass uming the patient to be water-equivalent. A simple inhomogeneity corre ction model was used to take CT-densities into account. For 22 patient s, entrance and exit dose calculations at the central beam axis and at several off-axis positions were compared with diode measurements. For 12 patients, diode exit dose measurements and exit dose calculations were compared with EPID transmission dose values. Results: Using water -equivalent calculations, the actual exit dose value under lung was, o n average, underestimated by 30%, with an overall spread of 10% (1 SD) in the ratio of measurement and calculation. Using inhomogeneity corr ections, the exit dose was, on average, overestimated by 4%, with an o verall spread of 6% (1 SD). Only 2% of the average deviation was due t o the inhomogeneity correction model. The other 2% resulted from a sma ll inaccuracy in beam fit parameters and the fact that lack of backsca tter is not taken into account by the calculation model. Organ motion, resulting from the ventilatory or cardiac cycle, caused an estimated uncertainty in calculated exit dose of 2.5% (1 SD). The most important reason for the large overall spread was, however, the inaccuracy invo lved in point measurements, of about 4% (1 SD), which resulted from th e systematic and random deviation in patient set-up and therefore in t he diode position with respect to patient anatomy. Transmission and ex it dose values agreed with an average difference of 1.1%. Transmission dose profiles also showed good agreement with calculated exit dose pr ofiles. Conclusions: The study shows that, for this treatment techniqu e, the dose in the thorax region is quite accurately predicted using C T-based dose calculations and a simple heterogeneity correction model. Point detectors such as diodes are not suitable for exit dose verific ation in regions with inhomogeneities. The EPID has the advantage that the dose can be measured in the entire irradiation field, thus allowi ng an accurate verification of the dose delivered to regions with larg e dose gradients.