issue 10
The Beamlet
After a holiday break, The Beamlet picks up where it left off—with a closer look at the four elements of CTrue™ imaging. Last issue, we considered true position. Here, we review true density and dose.
Like your diagnostic CT scanner or CT Simulator, daily CT images from the Hi·Art System® are acquired helically using a fan-beam with a conventional CT detector mounted on a ring gantry. Fan-beam CT images have significantly less scatter than CT images acquired using a cone beam with a broad flat panel detector. The result is pixel values that can be accurately and consistently converted to density maps, just like a diagnostic CT scanner. This means CTrue images are valuable for more than just guiding patient position.
With the added advantage of an MV beam, CTrue images yield accurate CT numbers regardless of patient size and shape, and even in the presence of high atomic number materials. Furthermore, imaging dose is consistently low (1–3 cGy, depending on slice thickness). With these properties, the images are not only consistent in quality, but are usable for accurate, heterogeneous dose calculation with each fraction*. This opens the door to dose reconstruction and adaptive planning.
DVHs showing first day of treatment (left) and three weeks into treatment (right). Solid lines = orgininal planned dose; dashed lines = dose calculated on daily MVCT.
Planned vs. delivered dose.
Increase in delivered dose (10%)
due to weight loss over
three weeks of treatment.
As a TomoTherapy® user with our new Planned Adaptive™ software, you can calculate the dose delivered to the patient quickly using any of the daily CTs used for IGRT. This can serve as a dosimetric quality assurance measure of the patient’s anatomic changes, and allows for efficient adaptive planning should those changes be significant enough to warrant a new treatment plan.
*The use of megavoltage CT (MVCT) images for dose recomputations, K. Langen, et al, Phys. Med. Biol. 50 (2005) 4259–4276
Note: This is the second of three Beamlets that will address key points from our ASTRO introduction of CTrue. The last issue in the series will address true delivery.
