The influence of breathing motion on intensity modulated radiotherapy in the step-and-shoot technique:: phantom measurements for irradiation of superficial target volumes

For intensity modulated radiotherapy (IMRT) of deep-seated tumours, dosimetric variations of the original static dose profiles due to breathing motion can be primarily considered as blurring effects known from conventional radiotherapy. The purpose of this dosimetric study was to clarify whether these results are transferable to superficial targets and to quantify the additional effect of fractionation. A solid polystyrene phantom and an anthropomorphic phantom were used for film and ion chamber dose measurements. The phantoms were installed on an electric driven device and moved with a frequency of 6 or 12 cycles per minute and an amplitude of 4 mm or 10 mm. A split beam geometry of two adjacent asymmetric fields and an IMRT treatment plan with 12 fields for irradiation of the breast were investigated. For the split beam geometry the dose modifications due to unintended superposition of partial fields were reduced by fractionation and completely smoothed out after 20 fractions. IMRT applied to the moving phantom led to a more homogeneous dose distribution compared to the static phantom. The standard deviation of the target dose which is a measure of the dose homogeneity was 10.3 cGy for the static phantom and 7.7 cGy for a 10 mm amplitude. The absolute dose values, measured with ionization chambers, remained unaffected. Irradiation of superficial targets by IMRT in the step-and-shoot technique did not result in unexpected dose perturbations due to breathing motion. We conclude that regular breathing motion does not jeopardize IMRT of superficial target volumes.