Room: Stars at Night Ballroom 4
Purpose: The effectiveness of arc-based deliveries (such as 4pi and VMAT) derives from its ability to create sharp dose gradients, separating the high dose to the tumor from the low dose nearby structures. In this work, we focus on the limitation of VMAT to create sharper dose distributions. An argument will be presented that energy-modulation that includes lower energy x-ray spectrums (i.e. <6MV), will be a key optimization parameter in the next generation of radiation therapy, so called energy-modulated arc radiotherapy (EMAT).
Methods: To demonstrate the limitations of VMAT and the potential of EMAT, pencil beams for 100keV, 400keV and 2MeV x-rays of 1mm width were generated with TOPAS MC. A rotational treatment delivery was simulated for each energy with varying number of treatment angles. The pencil beams were focused at the center of a 7.5cm radius cylinder. The tradeoff between x-rays energy, dose gradient, skin dose toxicity vs. number of beam orientations were analyzed.
Results: (1) Dose gradient and skin dose converges when the number of beams reaches 360. Â Beyond this number, no further effect was observed. (2) Dose gradient measured as the distance from 80% to 10% isodose lines is 0.5cm for 2MeV, 0.38cm for 400keV, and 0.25cm for 100keV. (3) Skin dose (as a percent of maximum dose) is 1% for 2MeV, 1.6% for 400keV,Â and 3.7% for 100keV when number of beam orientation reaches 360. Thus the skin dose toxicity for lower energy x-rays can be overcome by arc therapy.
Conclusion: The use of low and high energy spectrums has opposing benefits and tradeoffs that lend themselves as another tool in delivery optimization. The next generation of radiation therapy combines all these aspects in energy modulated arc delivery, where optimization parameters will now control beam penetration and penumbra.