ESTRO 37 Abstract book

S129

ESTRO 37

Bladder point, trigone (D 2cm3

multi-resolution scheme, better results are obtained after 5 minutes. After one hour, the same results are obtained as when using 20,000 points from the start. After 5 minutes, for all patients, the DVIs of the plans found with the multi-resolution scheme are already better than the DVIs of the clinical plan and close to plans found using 20,000 points after 1 hour (Fig. 1).

, D 0.1cm3

), bladder neck

), and urethra (D 0.1cm3

, D50). Furthermore, dose to

(D 0.1cm3

Posterior-Inferior points (PIBS/PIBS+2cm) was extracted as surrogate of the external urethral sphincter. Finally, the vaginal reference length (VRL), defined as the distance from PIBS to vaginal sources, was measured. 20 LACC [FIGO Stage: IB(1)- IIB(18)-IIIA(1)] patients treated with External Beam Radiation Therapy (EBRT) and IGABT (2 PDR fractions) according to the EMBRACE protocol were selected. Results The reported values represent the cumulative EBRT+BT dose converted to EQD2. Median D 2cm3 values were 72.0[59.3-82.4] and 54.4[48.8-69.9] Gy for bladder wall and trigone, respectively. Bladder wall dose was systematically higher, and hotspots often placed outside the trigone (r=.70). ICRU point dose, with a median value of 65.0[49.8-80.5] Gy, has not a better correlation (r=.72) (Figure a). Median D 0.1cm3 values for bladder wall, trigone, bladder neck and urethra were 88.0[68.2-100.3], 70.7[54.9-96.5], 53.0[46.5-64.5], and 50.1[45.7-54.4] Gy, respectively. Urethra D50 correlated with PIBS dose (r=.80). Median values of PIBS and PIBS+2cm were 21.7[4.3-55.2] and 50.9[44.5-89.2] Gy, respectively. Median VLR was 5.5[2.2-7.6] cm. ICRU point dose and hotspots in trigone and bladder neck correlated with VRL (Figure b). Border of Symphysis

Conclusion By combining our novel bi-objective optimization approach for BT planning with a multi-resolution scheme for the number of DC points, many high-quality plans that insightfully identify trade-offs between target coverage and organ sparing can be obtained within a few minutes, ensuring clinical usability. PV-0257 Functional sub-structures of lower urinary tract in cervix cancer: contouring and dose distribution S. Spampinato 1 , L. Fokdal 1 , E. Marinovskij 2 , S. Axelsen 3 , E.M. Pedersen 4 , R. Pötter 5 , J. Lindegaard 1 , K. Tanderup 1 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 2 Aarhus University Hospital, The MR Research Center, Aarhus, Denmark 3 Aarhus University Hospital, Department of Obstetrics and Gynaecology, Aarhus, Denmark 4 Aarhus University Hospital, Røntgen og Skanning, Aarhus, Denmark 5 Vienna General Hospital- Medical Univerisy of Vienna, Department of Radiotherapy, Vienna, Austria Purpose or Objective Radiotherapy related urinary morbidity is a complex phenomenon consisting of various clinical endpoints (i.e. frequency, cystitis, incontinence, bleeding, fistula) that may be related to various anatomical sub-structures. However, dose to the bladder and correlation with morbidity is currently mainly evaluated through contouring of the outer wall. The aim of this study is to investigate contouring and dose evaluation in sub- structures potentially responsible for urinary morbidity in Locally Advanced Cervical Cancer (LACC) after radiochemotherapy with Image Guided Adaptive Assuming that structures related to the bladder base are essential for frequency and incontinence, a methodology for contouring bladder subvolumes (trigone, bladder neck, urethra) was established. Structures were contoured for each BT fraction and DVH parameters were extracted: outer bladder wall (D 2cm3 , D0 .1cm3 ), ICRU Brachytherapy (IGABT). Material and Methods

Conclusion This study showed that parameters currently used for IGABT bladder dose reporting (D 2cm3 , ICRU point) are not sufficient for describing the dose distribution in sub- structures of the lower urinary tract. In particular, D 2cm3 for the outer bladder wall is not representative of trigone dose and is higher than dose to bladder neck and urethra. PIBS dose correlates with urethra D50 and may be a good surrogate for the external sphincter. Finally, the study showed that the applicator position (indicated by VRL) is important for dose sparing of bladder base. Further understanding of dose-effect relationships may be gained by systematic delineation of bladder sub-structures. PV-0258 Dose to the Bladder Neck: Impact on Urinary Toxicity after MRI-guided HDR Prostate Brachytherapy N. Sanmamed 1 , P. Chung 1 , A. Berlin 1 , J. Borg 1 , B. Lao 1 , R. Weersink 1 , A. Simeonov 1 , A. Rink 1 , C. Menard 2 , J. Helou 1 1 Princess Margaret Cancer Centre, RADIATION ONCOLOGY, Toronto, Canada 2 Centre Hospitalier de L'Universite du Montreal, Radiation Oncology, Montreal, Canada