ESTRO 2021 Abstract Book

S1087

ESTRO 2021

Relative Seriality (RS) NTCP model and the biologically equivalent uniform dose (BEUD). Results The structure os coxae upper (above acetabulum) bilateral (OCUB) showed the highest correlation, with a significant level of hematocrit drop (≥4). The values for the D 50 , γ, and s parameters of the RS model were 26.9 Gy (25.9-27.9), 1.3 (1.2-2.2) and 0.12 (0.10-0.83), respectively. The AUC of BEUD was 0.73 and patients with BEUD to UCUB ≥ 27Gy had 8.2 times higher rate of significant hematocrit drop vs. < 27Gy. The bone volume of the whole pelvis (BVWP) and the os coxae total bilateral (OCTB) also showed significant correlations for the same endpoint (AUC values of 0.72 and 0.69, respectively). In those cases, similar BEUD thresholds were found (27Gy and 28Gy, respectively) but with lower odds ratios (2.8 and 2.9, respectively). Conclusion The dose-response curve of os coxae upper bilateral for a significant drop in hematocrit could be determined by fitting the clinical data with the RS NTCP model. A threshold of BEUD < 27Gy was found to significantly reduce the risk for this endpoint. PO-1325 Factors that influence urinary incontinence correction results after prostatectomy and radiotherapy N.A. Linares Mesa 1 , M. Ruza Sarrasín 1 , Y. Ramírez Castilla 2 , J.M. Rico Pérez 1 , M.F. Lorenzo Gómez 3 1 Hospital Juan Ramón Jiménez, Oncología Radioterápica, Huelva, Spain; 2 GenesisCare, Oncología Radioterápica, Cádiz, Spain; 3 Hospital Universitario de Salamanca, Oncología Radioterápica, Salamanca, Spain Purpose or Objective Prostate cancer (PC) is the third most common neoplasia in men worldwide and the first in developed countries, except for skin cancer. The proportion of 10 years or older long-term survivors with curative treatment is around 90%, with cancer-specific survival of almost 100%. Therefore, it is essential to mitigate the treatment side effects impact on the quality of life. Stress urinary incontinence (SUI) most common cause in prostate cancer patients is radical prostatectomy (RP), which if combined with radiotherapy (RT) increases the frequency and severity. Our objective is identify the factors influence on stress urinary incontinence surgical treatment results in men who have received radiotherapy after radical prostatectomy for prostate cancer treatment. Materials and Methods Retrospective multicenter observational study in 226 patients treated with adjuvant or rescue external RT after radical prostatectomy. All were surgically intervened for urinary incontinence. Study groups: GAs1: initial success and GBf1: initial failure. Subsequently, two subgroups are distinguished after one year follow- up: success, or failure. SUI rate was measured as urine loss volume with the pad test and was classified as mild, moderate and severe. Success after surgery was defined as no type of urine loss, neither mild, nor moderate or severe. Statistical analysis with descriptive statistics, Student's t, Fisher's exact test, ANOVA, multivariate analysis. A statistical significance for p <0.05 was accepted. The study is approved by the Ethics Committee for Drug Research. Results Age in the beginning continents was lower. Age did not influence the follow-up incontinents at the follow-up year. Gleason proved to be a prognostic factor, the most aggressive histologies (G8) were associated with a worse response to surgery. All the G8 failed initially and continued incontinent one year passed. TOT and REMEEX® devices have more success rates than failures at initial time after surgery; however, there is a higher initial failure rate with ATOMS® and PHORBAS® devices. A third of patients operated by ATOMS®/PHORBAS® that initially failed were rescued in a second time. With REMEEX® device, more initial success is achieved, having a higher failure rate in a second time in prostactectomized and irradiated patients. Conclusion Suburethral anti-incontinence devices, adjustable or not, have a high success rate in urinary incontinence correction after adjuvant or rescue radiotherapy after radical prostatectomy, first time of 60.66% and in a second time they add 82.74% of patients without losses. Body mass index, PSA, TNM T stage or prostate volume are not related to treatment outcome. Older age, higher Gleason score, longer UI evolution time, lower urinary tract symptoms pre-prostatectomy and greater comorbidity are related to less chance of success and worse incontinence correction maintenance evolution. PO-1326 Dosimetric and clinical impact of rectal spacers in patients undergoing VMAT-IGRT in prostate cancer E. O'Neill 1 , M. Samuji 1 , P.J. Kelly 1 1 Bon Secours Cork in association with UPMC Hillman Cancer Centre, Radiotherapy, Cork, Ireland Purpose or Objective Since the publication of prospective randomised studies there has been increasing use of perirectal hydrogel prior to radiation for prostate cancer. The goal of this intervention is to reduce the risk of rectal side effects and improve patient quality of life. The use of gel spacers has not been universally adopted for several reasons, among them cost issues, concerns regarding potential toxicity of this intervention particularly in the setting low rates of rectal side effects with modern radiation techniques, and, also whether gel spacers truly impact on patient-reported quality of life in a clinically meaningful way. The utilisation of a hydrogel rectal spacer in addition to fiducial marker implantation was recently adopted at our institution following completion of the appliers training programme. We examined two sequential cohorts of patients treated at our centre pre- and post-gel spacer implementation to assess the a) the dosimetric impact of this intervention and b) acute GI toxicity of this recent change in practice at our institution. Materials and Methods To date 25 patients have completed radiation following gel spacer placement, 20 receiving 60Gy in 20