UCLA Journal of Radiation Oncology Fall and Winter 2022 - Flipbook - Page 30
UCLA RADIATION ONCOLOGY JOURNAL
Process and Challenges
Figure 1 Sim-free workflow
We begin by importing an existing diagnostic CT into the Ethos planning system. Thus
far, we’ve found the attenuation correction (AC) CT from PET/CT studies to be the most
useful study for this application due to the field of view routinely encompassing the entire
body. Our physicians will contour on this CT image and the planner will quickly generate
a treatment plan. Crucially, we will never deliver this initial plan. The patient will be in a
substantially different position when we set them up on the Ethos for their treatment as
compared to their diagnostic CT. For example, they may have lost weight since the image
was acquired. Additionally, the Ethos’ flat treatment couch differs from the round couch
tops found on diagnostic scanners and this discrepancy can create differences in tissue
path lengths that cause mismatches in the calculated vs. the delivered dose. Without
Ethos’ online adaptive capabilities, we would be forced to try to physically match the
patient position in the diagnostic image and simply accept the remaining unmanageable
inaccuracies.
Figure 2 Differences in patient anatomy due to weight loss and the effects of a flat vs round
Fortunately, we can adapt each fraction in order to account for the differences in
patient size and position. Online adaptation based on daily CBCT eliminates the need
for difficult, potentially painful attempts to match diagnostic imaging positioning
and lets us achieve dosimetric accuracy that would not otherwise be possible. Ethos
online adaptive planning is highly streamlined and we’re able to complete the process
in under fifteen minutes so the impact on treatment times would be minimal.
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