CT treatment planning based on a simulator CT-option: comparison between calculation and measurements using TLDs in a humanoid phantom

Computer programs are widely used in radiotherapy to calculate the dose distribution in patients for different field configurations. The aim of this study was to investigate the dosimetry over the complete radiotherapy procedure. For this purpose, CT slices of a humanoid phantom (Alderson) were first taken with a simulator CT-option and the images then transferred to the treatment planning system. Irradiations were performed using linear accelerators. Measurements in the phantom were carried out with thermoluminescence dosimeters (TLD) and analysed using an automatic reader. Several common treatment configurations were investigated. Measurements within the target volume (low dose gradient) agreed with the calculation to within 3%. The differences in the penumbra region (high dose gradient) were in part much larger. This can be attributed to two factors: the uncertainties in the TLD localisation and the calculation chain. In conclusion, it can be stated that the agreement between calculations and measurements was sufficient for most clinical considerations. However, further checking is advisable in case sensitive organs lie in the vicinity of the field edge.