Room: Exhibit Hall | Forum 4
Purpose: To measure the impact of intrafractional motion on dose homogeneity during pencil beam scanning (PBS) proton irradiation.
Methods: Treatment plans were generated by optimizing to an appropriate internal target volume expansion of a 50-mm diameter spherical clinical target volume (CTV). Treatment plans with/without layer-based repainting were created for multiple clinically relevant motion amplitudes. Dose measurements were made by exposing radiochromic film to proton irradiation from a pencil beam scanning delivery system. The film was embedded in an acrylic block at the center plane of the CTV. Using a programmable motion-stage, the acrylic block was driven through a periodic motion pattern (cosâ?´) with a 4-second period. Film measurements were obtained with the motion directed orthogonal and parallel to the primary beam scanning direction. A total of 80 film measurements were acquired from repeated measurements of the conditions described above. Dose volume histograms from the two-dimensional mid-plane of the CTV were extracted from the film, and dose homogeneity(HI) was assessed using Dâ‚…/Dâ‚‰â‚….
Results: Under static conditions, the HI of the dose delivered to the CTV was 1.06Â±0.00 (meanÂ±SD). The 16mm peak-to-peak motion measurements, aggregated over both the orthogonal and parallel directions, show 1) an increase in HI (HIáµ?áµ’áµ—â?±áµ’â?¿=1.14Â±0.03; p < 0.01) and 2) improved homogeneity for repainted deliveries (HIáµ?áµ’áµ—â?±áµ’â?¿â?ºÊ³áµ‰áµ–áµƒâ?±â?¿áµ—=1.09Â±0.02; p<0.01). Measurements aggregated over both 16mm and 12mm peak-to-peak motions show that motion orthogonal to the major beam scanning direction resulted in significantly reduced dose homogeneity within the target (HIáµ’Ê³áµ—Ê°áµ’áµ?áµ’â?¿áµƒË¡=1.11Â±0.04 vs. HIáµ–áµƒÊ³áµƒË¡Ë¡áµ‰Ë¡=1.09Â±0.03;p < 0.05).
Conclusion: We have directly measured the impact of motion on target dose homogeneity in a modern PBS system. Statistical bounds on target DVHs as a function of motion amplitude and direction can be extracted from these measurements and potentially used with future measurements to estimate the impact of interplay in clinical scenarios.