The study and analysis of heterogeneity correction algorithms used in the delivery of step and shoot and sliding window IMRT
Schmitter, Mary K.
MetadataShow full item record
The accuracy of tissue heterogeneity dose correction algorithms for IMRT delivery using step and shoot and sliding window techniques is studied. A solid water phantom incorporating a lung equivalent heterogeneity layer was constructed. The phantom was CT scanned and contouring of targets was done. A commercial treatment planning system was used to evaluate the optimal fluence for two IMRT field setups. Each fluence was then run through a leaf motion calculator to provide deliverable fluences using both step and shoot and sliding window techniques. Heterogeneity correction algorithms were used in both the dose optimization process and the final dose calculations. GafChromic EBT film was strategically placed within the phantom. The phantom was irradiated with each set of IMRT treatment fields and each film was then processed. The measured dose distribution was compared to the predicated distributions. Preliminary studies with static fields (1×10 cm 2 through 10×10 cm 2 ) showed that heterogeneity algorithms became less accurate with decreasing field width. The largest difference between the predicted and the delivered dose on the beam axis inside the lung equivalent material was 27% found for the EqTAR algorithm for the 1×10 cm 2 field size. Current heterogeneity correction algorithms were found to become less accurate with decreasing field widths. Since sliding window techniques typically irradiate with narrower fields, heterogeneity algorithms are more accurate for step and shoot delivery where fewer but larger fields are typically used. The accuracy of IMRT dose delivery in regions of tissue heterogeneity is therefore dependent on the delivery technique.