Abstract Book

S131

ESTRO 37

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

The dose to the bladder neck (BN) has been suggested to be a predictor of urinary toxicity. Magnetic resonance imaging (MRI) is one of the best imaging modalities to delineate the BN. We aim to assess the impact of the dose to the BN on physician and patient-reported urinary toxicity after MRI-guided high dose-rate brachytherapy (HDR-BT) boost. Material and Methods Fifty-one patients were treated with a single 15-Gy MRI- guided HDR-BT implant followed by external beam radiotherapy (EBRT) as part of a prospective phase II clinical trial. MRI-based treatment planning was used. The clinical target volume (CTV) was defined as the prostate and a 2mm craniocaudal extension was added to generate the planning target volume (PTV). Urethra, bladder, rectum and penile bulb were contoured as organs at risk (OARs) and dosimetric parameters collected prospectively. The BN was delineated in retrospect on T2-weighted images by the same radiation oncologist and reviewed by an independent physician. Acute (≤3 months) toxicity and health-related quality of life (HRQoL) data were collected prospectively using CTCAE v.4 and the expanded prostate index composite (EPIC) respectively. A minimally important difference (MID) was defined as a deterioration of HRQoL scores at 3 months compared to baseline ≥ 0.5 standard deviation of baseline score. Linear and logistic regression models were used as appropriate to assess the impact of BN dose on urinary toxicity and HRQoL. A two-tailed p -value ≤0.05 was considered statistically significant. Results The median BN volume was 0.6 cc [interquartile range (IQR): 0.4-0.7]. The median maximum dose to the BN (BNDmax) and urethra (UDmax) was 20.5 Gy (IQR 17.9- 26.1) and 19.8 Gy (IQR 18.8- 21.0) respectively. The median dose to 0.5cc of the urethra (UD0.5) was 17.3 Gy (IQR:16.6-17.3). On univariate linear regression analysis, BNDmax was not significantly associated with any of the urethral dose parameters. In addition only 5.9% of the total amount of variation in BNDmax was explained by the UDmax (R 2 =0.059, p =0.09). Acute grade 2+ urinary toxicity was observed in 30% of patients. Among those, 2 patients had an acute urinary retention. No grade 4+ toxicity was reported. Furthermore 44% of patients reported a MID in EPIC urinary domain score at 3 months. None of the dosimetric parameters including BNDmax was associated with acute grade 2+ urinary toxicity or MID. However, the 2 patients with urinary retention had a BND max in the highest quartile; 28.3 and 26.4 Gy (>175% of prescription dose). Conclusion While a high BN dose was observed in patients who had an acute urinary retention in our cohort, the predictive value of this parameter is yet to be determined in a larger cohort of patients. Meanwhile, with the increased use of MRI in brachytherapy treatment planning, it is worthwhile delineating the BN and paying appropriate caution to doses delivered to this anatomical structure. PV-0259 Impact of an additional chemotherapy cycle during brachytherapy in cervical cancer patients A. Escande 1,2 , S. Bockel 2 , M. Khettab 2 , E. Manea 2 , I. Dumas 2 , R. Mazeron† 2 , A. Schernberg 2 , E. Deutsch 2 , P. Morice 3 , C. Haie-Meder 2 , C. Chargari 4,5 1 Centre Oscar Lambret, Academic Department of Radiation Oncology, Lille, France 2 Gustave Roussy, Brachytherapy Unit-Radiotherapy Department, Villejuif, France 3 Gustave Roussy, Surgery Department, Villejuif, France 4 Institut de Recherche Biomédicale des Armées, Bioradiology, Bretigny-sur-Orge, France 5 French Military Health Academy, Ecole du Val-de-Grâce, Paris, France

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

Purpose or Objective