SU‐E‐J‐178: Patient‐Specific Bulk Electron Density Overridden Planning for Prostate and Brain Cancer

Purpose: To evaluate the use of patient‐specific bulk electron density overridden plans for prostate and brain IMRT dose calculations. Methods: CT images and IMRT treatment plans for 10 prostate cancer patients and 10 brain cancer patients were retrospectively reviewed and analyzed. For each patient, a region of interest(ROI) was selected to cover all radiation beam paths plus 1cm margins in both superior and inferior directions. Patient anatomy within the ROI was segmented into fat, non‐fat soft tissue, bone and air based on CT number ranges of [−,‐50], [−,49,200], [201,4000], [− 1200,−800], respectively. Average electron densities for each tissue type were obtained from CT values for each patient, and all plans were recalculated on patient‐specific density overridden datasets to generate patient‐specific bulk density overridden plans. These overridden plans were compared to the clinically approved CT‐based plan using dose‐volume metrics for PTV coverage and OAR sparing. Results: Dose deviations between the patient‐specific density overridden plan and the clinically approved plan were minimal. Most parameters used to describe planning target volume coverage and OAR sparing were within 0.3% of approved CT based plans and all parameters were within 1% of the approved CT based plans. Conclusion: This study demonstrates that by assigning patient‐specific bulk electron densities to different tissue types, the dosimetric error of the patient‐specific bulk density overridden plans is within 1% of the approved CT based plan. This, patient‐specific bulk density override technique may potentially be used to map the CT values from CT images to MR images.