Alec M. Block MD, Presenter: Nothing to Disclose

Jason Luce, Abstract Co-Author: Nothing to Disclose

Elizabeth Loo MA, Abstract Co-Author: Nothing to Disclose

Jeffrey Lin, Abstract Co-Author: Nothing to Disclose

Mark A. Hoggarth, Abstract Co-Author: Nothing to Disclose

John C. Roeske, Abstract Co-Author: Research Grant, Varian Medical Systems, Inc

A previous retrospective study has shown that the cumulative skin dose from kV planar IGRT may be as large as ~2 Gy, depending on disease site, such as in the pelvis. In a phantom study, we demonstrated that by lowering mAs, this dose can be greatly reduced without sacrificing image quality. The purpose of the current study is to determine if mAs can be reduced on actual patient images while maintaining adequate image quality to be used for daily IGRT positioning.

Anterior and lateral planar kV images were obtained with the manufacturer-recommended parameters (kVp, mAs) on pelvic, thoracic, and abdominal RANDO phantoms. Separately, anterior and lateral images of these phantoms were produced by using the same kVp, but systematically reducing the mAs settings. Contrast-to-noise ratios (CNR) were calculated for both the standard phantom images and reduced mAs images. For each body site, CNR was plotted against dose. The mAs at the point where CNR begins to decrease was determined. From each plot, mAs values were selected that were slightly above that which yielded the initial decrease in CNR. Daily kV anterior and lateral images were obtained using these reduced mAs settings for 7 patients currently being treated with IGRT (3 thoracic, 2 abdominal, 2 pelvic). For these 7 patients, CNR were calculated for both the reduced mAs settings and the standard settings which were used to image the patients one day prior.

The largest kV planar imaging dose was seen in pelvic cases. For example, using the standard mAs, the average lateral pelvic dose was 212.0 cGy. When the mAs was reduced by 49.2% (from 126 to 64 mAs), there was only a 7.2% reduction in CNR. For all images, the average CNR using standard parameters was 5.18 ± 3.22; with the reduced parameters, CNR was 5.24 ± 3.12. A paired t-test showed no statistical difference in CNR using standard vs. reduced mAs (p=0.79).

Since the absorbed cumulative skin dose from kV planar IGRT may be as large as ~2 Gy, imaging dose reduction should be considered. Using actual patient images, this study shows that, by altering mAs, the absorbed imaging dose can be greatly reduced without sacrificing image quality (CNR).

The absorbed cumulative skin dose from kV planar IGRT may be as large as ~2 Gy.  By altering mAs, the absorbed imaging dose can be greatly reduced without sacrificing image quality.

Block, A, Luce, J, Loo, E, Lin, J, Hoggarth, M, Roeske, J, Quantitative Analysis of Image Quality Following Dose Reduction for kV Planar Radiographs Used for Image Guidance.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12029169.html