Treatment of moving lesions with Helical Intensity-Modulated Radiotherapy (HIMR): a phantom study

Objective: Treatments performed with HIMR on moving lung lesions might be inaccurate due to the simultaneous gantry rotation, patient couch translation and lesion movement. Moreover depending from the technique used to acquire the planning CT data (Free-Breathing (FB), Breath-Hold (BH) or Average Intensity Projection (AIP) from 4DCT) different result are obtained during HIMR planning. In this study the outcome of how these factors interplay and their effects on dose distribution are explored using a moving phantom and GafChromic EBT3 films. Methods: A phantom made of two pieces of tissue equivalent material embedded in two adjacent polystyrene slabs, slid in a cranio-caudal direction using a motor-driven motion platform (3cm and 16cycle/min movement extension and rate). A Philips BigBore CT scanner was used to acquire and reconstruct FB and AIP images of the moving phantom and BH images of the static phantom. The three data sets were sent to the HIMR planning station where three plans for delivering 2Gy to the central tissue equivalent material were prepared. The phantom was put on the patient couch, a GafChromic EBT3 film was inserted between the two pieces of tissue equivalent material and the three treatments were delivered. In order to compare planning and delivered dose distributions GafChromic films were also fixed to an external support that enable the phantom to slit while films stand still. Results: A significant difference between planned and delivered dose distributions was observed in the moving phantom. Gamma index passing rates (3%, 3mm 10% threshold) changed from 89,48% for the static phantom and BH CT data set to 63,89% and 61.38% for the moving phantom using AIP and FB CT data set respectively. Dose delivered by treatments planned on FB and AIP CT data sets shows under dosages (up to 4%) and over dosages (up to 50%) in the target region and surrounding regions, respectively compared to that delivered to the static target. The over dosage effect is more pronounced in caudal direction. The most evident effect on dose distribution was dose blurring. This effect also impacted dose gradients which tended to decrease in the peripheral region. Conclusions: Treatments performed with HIMR on moving targets showed a significant difference between planned and delivered dose distributions. The use of Average Intensity Projection (AIP) CT images, adequate for dose calculation in most circumstances seems not providing better results. Contouring strategies that keep into account these effects should be explored.