[OA039] Contouring and dose reporting for lower urinary tract sub-structures in cervix cancer

Purpose Radiotherapy related bladder morbidity include various clinical endpoints (i.e. frequency, cystitis, incontinence, bleeding, fistula) that may be related to various anatomical sub-structures. Evaluation and reporting of bladder dose is currently based on the total bladder volume with contouring of the outer wall. This study aims to investigate contouring and dose evaluation of several bladder sub-structures potentially responsible for urinary morbidity after radiochemotherapy and Image Guided Adaptive Brachytherapy (IGABT) in locally advanced cervical cancer (LACC). Methods The study hypothesis is that different bladder structures are related to different morbidity endpoints. Therefore, a methodology for contouring subvolumes (trigone, bladder neck, urethra) was established. Structures were contoured for all BT fraction extracting DVH parameters: outer bladder wall (D2cm3, D0.1cm3), ICRU Bladder point, trigone (D2cm3, D0.1cm3), bladder neck (D0.1cm3), and urethra (volume, D0.1cm3, D2cm3. A total of 40 LACC [FIGO Stage: IB(3), IIB(30), IIIA(1), IIIB(1), IVB(5)] patients according to the EMBRACE protocol was selected. Results The reported values represent the cumulative EBRT+BT dose in EQD2. Median D2cm3 values were 71.7 [59.2–81.8] and 56.2 [47.8–69.3] Gy for bladder wall and trigone, respectively. Bladder wall dose was systematically higher, and hotspots often placed outside the trigone. Median ICRU point dose was 63.0 [49.8–80.5] Gy, Median D0.1cm3 values for bladder wall, trigone, bladder neck and urethra were 85.6 [68.2–109.8], 70.9 [48.3–105.6], 61.7 [46.5–76.9], and 50.6 [45.6–64.7] Gy, respectively. Median urethra volume and D2cm3 dose were 3.2 [1.4–6.5] cm3 and 47.9 [44.8–58.7] Gy, respectively. Conclusion The study showed that the parameters currently used for IGABT bladder dose reporting (D2cm3, ICRU point) do not fully and accurately describe the dose distribution in the lower urinary tract sub-structures. D2cm3 for the outer bladder wall is often higher than trigone dose and in many cases the ICRU bladder point is not a good indicator. It is interesting to note that urethra volume may be relevant for endpoints such as incontinence varied across patients. Further understanding of dose-effect relationships for the bladder may be gained by future systematic delineation of bladder sub-structures.