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On Dose Error Sensitivities for Hypofractionated Helical Tomotherapy Treatments

M Kissick

M Kissick*, D Campos, Y Yue, University of Wisconsin Madison, Madison, WI

SU-E-T-304 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Tour purpose is to explore the efficacy of optimizing and adapting hypofractionated treatments for oxygen dynamics, and with Helical Tomotherapy (HT) in particular. Hypofractionated treatments such as SBRT induce a higher sensitivity to any dose error that would have been averaged away with a typical number of fractions. The greatest uncertainty relates to the effectiveness of the dose and that aspect is dominated by the short term oxygen dynamics induced by the radiation.

Methods: This work builds on a suite of HT studies related to 1. the interplay or lack thereof for respiratory motion, 2. Ill-posed optimizations that can lead to cold spots, 3. Proper choices for pitch, but the largest issue is 4. Reoxygenation dynamics for which we have started to use a new optical device.

Results: For each of the first three items mentioned above, simple conditions to enhance robustness can be specified as follows: 1. Control motion on the order of the couch speed relative to the beam width, and use probabilistic optimization, 2. Do not prescribe gradients sharper than the beam profile can accomplish, 3. Use a pitch value of 0.86/integer, and 4. Find a way to robustly track the tumor oxygen dynamics in real-time per patient and find robust adaptations for these dynamics.

Conclusion: For hypofractionated treatments, there will be an increased sensitivity to critical physics based dose error issues. However, the more severe issues are biological, and in particular, reoxygenation which is more complex for fewer fractions because the dynamics prior to cell death make the of the linear-quadratic equation difficult. Physicists need to extend their work into new directions for these new types of treatments.

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