DOSIMETRIC VERIFICATION OF OPEN ASYMMETRIC PHOTON FIELDS CALCULATED WITH A TREATMENT PLANNING SYSTEM BASED ON DOSE-TO-ENERGY-FLUENCE CONCEPTS

Output normalized dose profiles for asymmetric open photon fields have been calculated using a commercial treatment planning system (TPS) ba sed on a dose-to-energy-fluence concept. The model does not require an y additional measurements for off-axis fields. Calculations are compar ed with measurements for quadratic fields of 5 cm x 5 cm up to 20 cm x 20 cm, with their geometric field centre positioned 10 cm off-axis in the in-plane direction. The measurements include depth doses and prof iles in-plane as well as cross-plane for nominal photon energies of 4, 6 and 18 MV x-rays. Both calculated and measured doses are normalized with respect to a 10 cm x 10 cm reference field, therefore making it possible to compare not only the relative distributions but also the a bsolute dose levels; that is, calculation of monitor units is included . The calculated depth-dose curves are generally in good agreement wit h measured data with an accuracy at the absolute dose level of 2% at d epths beyond the dose maximum. The cross-plane profiles are calculated with an accuracy better than 3% within the field. The 'tilt' towards the collimator central axis of the in-plane profiles is predicted by t he model, but is somewhat overestimated at large depths. The system pr ovides the possibility to separate the primary and scattered parts of the dose and the cause of this tilting was studied by comparing calcul ated phantom-scattering and head-scattering dose profiles for a symmet ric 40 cm x 20 cm field to dose profiles for an asymmetric 20 cm x 20 cm field. The tilting is shown to originate from a change both in phan tom scattering and in head scattering compared to the case of symmetri cal fields. The results indicate that the investigated TPS can calcula te dose distributions in open asymmetric fields with a high degree of accuracy, typically better than 2-3%.