[Electron fields in clinical application. A comparison of pencil beam and Monte Carlo algorithm].

BACKGROUND AND PURPOSE For several years three-dimensional treatment-planning systems have used pencil beam algorithms in the calculation of electron fields. Nowadays, exact Monte Carlo methods are commercially available, showing good correspondence to experimental results. Clinical examples are investigated to find differences in the dose distribution of treatment plans, which are calculated with both pencil beam and Monte Carlo algorithm. MATERIAL AND METHODS Two different clinical applications are regarded: (1) an irradiation of the chest wall, and (2) an electron field to the vertebral column. The dose distributions are calculated by Oncentra MasterPlan on the one hand, using the Monte Carlo code VMC++, and by Helax TMS on the other hand (both Nucletron B.V., Veenendaal, The Netherlands). Profiles and depth dose curves are evaluated by the Verisoft program of PTW (Freiburg, Germany). RESULTS In the case of chest wall irradiation, the depth dose curves for the three investigated energies, 9, 15 and 21 MeV, agree rather well, also in lung tissue. The mean value for the lung differs only by 4% related to the dose maximum. In the case of vertebral column irradiation, however, the dose difference is more pronounced and, in the prevertebral region, is 56% lower for the VMC++ plan than in the pencil beam calculation. CONCLUSION For irradiations of the chest wall, dose distribution calculations by means of pencil beam algorithm may be applied. Calculating electron dose distributions in cases of larger bone inhomogeneities, the more exact Monte Carlo algorithm should be preferred.