Planar IGRT dose reduction: A practical approach

Purpose: To retrospectively estimate the cumulative absorbed dose (at the skin) from kilovoltage planar x-rays received by 90 patients treated on a Varian iX and to determine if that dose could be reduced without sacrificing image quality. Methods and materials: To estimate surface dose, measurements were obtained using the "in-air" method by varying the source-to-detector distance from 80 to 100 cm in steps of 5 cm. Energy was varied from 70 to 120 kVp. Using these data, a global equation was developed to estimate the cumulative skin dose by applying the imaging settings (kVp, mAs), patient-specific source-to-skin distance, and total number of images. To reduce the imaging dose, anterior and lateral images of RANDO phantoms were obtained using the same kVp; however, the mAs settings were systematically reduced. Contrast-to-noise ratios (CNRs) were calculated for both the standard phantom images and reduced mAs images. The mAs values were chosen to minimize skin dose while maintaining a similar CNR. Last, daily kV anterior and lateral images were obtained using these reduced mAs settings for 7 patients currently being treated with image guided radiation therapy. CNR was determined and compared with the values obtained on images taken 1 day before this change. Results: Average cumulative kV imaging dose was as large as 162.2 cGy for pelvic cases with standard kVp, mAs. Other doses varied by site and technique. By lowering mAs, this dose could be reduced by 49% with only a 0.9% decrease in CNR. For the 7 patients currently being treated with image guided radiation therapy, CNR values were not statistically different (P =79), whereas the skin dose was reduced by an average of approximately 50%. Conclusions: kV planar imaging dose reduction should be considered, given the large cumulative skin dose for certain disease sites. When mAs are reduced, planar dose reduction is clinically feasible without sacrificing image quality.