Room: ePoster Forums
Purpose: It is a common practice to force electron densities (ED) of oral contrast or air cavity regions to water on planning CT images during reference plan generation, considering that these regions are either not present or changed during treatment. This work investigates the dosimetric impact of forced ED over the course of RT delivery.
Methods: Clinical reference plans for four pancreatic cancer patients were generated by forcing ED of oral contrast and air cavity containing organ contours to water. Daily CT-on-rails IGRT images from 28 fractions of six patients were co-registered and a mean IGRT CT was constructed. Reference plans were then reoptimized by forcing ED of oral contrast and air cavity containing contours to the mean ED value of these contours obtained from the mean IGRT CT. Finally, the reference and reoptimized plans were recalculated on each daily IGRT image without forcing ED of any structures. Differences in commonly used dose-volume parameters were evaluated for all plans.
Results: Air cavity regions were found to be present on daily CT images throughout the treatment course. The average ED values for small bowel, colon, stomach, and duodenum were found to be 0.928, 0.900, 0.942, 0.984, respectively. Dmax increased up to 3.2% for plans recalculated on daily CT images with no forced ED. The volume of the prescribed isodose line (V100%) increased up to 43% for plans recalculated on daily CT images.
Conclusion: Electron densities of oral contrast media should be forced to water, given the absence of oral contrast media during treatment. However, ED forcing of air cavity regions on planning CT images to water is not necessary and may not be advisable given that reference plan dose distributions with forced ED may not be representative of the actual dose delivered in patients.