SU‐E‐T‐689: Effects of Coordinate System Orientation On ICRU Bladder Point Dose

Purpose: To quantify the displacement and dosimetric errors on position of the ICRU bladder point due to coordinate system orientation in gynecological HDR procedures. Methods: Eight patients (23 treatment fractions) receiving HDR brachytherapy for gynecological malignancies were evaluated. Bladder points were placed for each of two coordinate systems (ICRU and applicator) for each patient fraction. The ICRU coordinate system refers to true anatomical orientations (e.g., ant/post) while the applicator coordinate system is transformed (translated and rotated) based on the location and orientation treatment applicator and is used to define prescription points and optimize planned dose distributions. In both cases, the ICRU bladder point definition was used to place the point (most posterior point of foley balloon on lateral image and center of foley balloon on AP image) with differences resulting from definition of coordinate system orientation. The displacement in three dimensions and point dose differences were calculated for each patient fraction. Results: Point displacement of the applicator coordinate system point (Bappl) relative to the ICRU point (BICRU) was 6.3 ± 0.7 mm (mean ± std err) with a range of 1.5 to 12.6 mm, a statistically significant Result. Dosimetrically, dose to Bappl was 7.3 ± 1.6% (mean ± std err) of the prescribed dose higher than BICRU with a range of −7.9 to 24.7%, a statistically and clinically significant Result. This results in an overestimation of bladder dose when using the applicator coordinate system and using ICRU point dose criteria for plan evaluation. Conclusion: Failure to use a true anatomical coordinate system can lead to statistically and clinically significant dosimetric errors in dose to the defined bladder point. Use of the applicator coordinate system to define the bladder point can potentially cause compromises in plan quality in attempting to meet normal tissue dose tolerances evaluated with incorrectly reported doses.