ESTRO 37 Abstract book

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ESTRO 37

Purpose or Objective Magnetic resonance imaging (MRI)-guided stereotactic body radiation therapy (SBRT) has been established as a safe and effective treatment for prostate cancer. To evaluate the acute toxicity of a phase II study concerning SBRT with a dose scheme of 36.25 Gy in 5 fractions, in patients with localized prostate cancer at low/intermediate risk (according to NCCN score) and risk of lymph node involvement <17% ( Roach Index). Material and Methods This study dealt with a prospective analysis of the 30 patients treated with SBRT using VMAT technique. The GTV (prostate with the 1/3 proximal seminal vesicles) was delineated using the MR T2-weighted (T2w) and FLAIR images, co-registered with planning-CT images. Our local rigid registration method employed the 3 intraprostatic gold fiducial markers (GFM) as landmarks. The GTV was delineated also on CT-images and compared to MR-based GTV, by using the DICE metric. All patients were contoured under the supervision of the same physician (AM). For the planning, an isotropic margin of 6 mm around the MR-based GTV is added to obtain the PTV. To study the potential association to the bladder trigone on long-term urinary function, the bladder trigone was MR-based contoured too. An indwelling catheter was inserted to aid in the delineation of the urethra. The request for dose prescription is that V100%>95% of the PTV. The rectal dose-volume goals were V50% <50%, V80% <20%, V90<10%, V100%<5%. The bladder dose-volume goals were V50% <40%, V92.4% < 12.7%, V100% <10%. Concerning the urethra and the trigone, the constraints for the maximum dose (Dmax) were Dmax<39.9 Gy and Dmax<38.9 Gy, respectively. The IGRT protocol is based on 3 intraprostatic fiducial markers, with daily online checks by CBCT. The acute toxicity gastrointestinal (GI) and genitourinary (GU) were recorded using the RTOG/EORTC scale. Additional data are collected by means of I-PSS (International Prostate Symptom Score) e IIEF-5 (International Index of Erectile Function) questionnaires. Results Between November 2015 and July 2017, 30 patients were enrolled in this prospective study, the median follow-up duration was 6 months (range, 3-18 months). The median age was 74 years (range, 60–81 years). The mean target registration error was 0.3±0.2 mm (1SD). The CT- contoured prostate resulted on average 35% larger that the true gland (MR-based volume) with a mean DICE coefficient of 0.79 (range: 0.61–0.90), such that posterior portions were always missed and anterior normal tissue always included. For the dose distributions of the organs at risk, all the dose constraints were largely satisfied. Acute GU and GE toxicity of grade 2 were observed in 26% and 10% patients respectively. The quality of life assessment showed a complete recovery of obstructive symptoms 3 months from the end of the radiotherapy. Conclusion Our study showed an important reduction in volume of MR-guided prostate, in agreement with literature data. This allows safe high dose per fraction prostate with limited acute toxicities.

EP-1600 Comparison of patterns of implantation for the 3 spacers for radiation therapy for prostate cancer. I. Latorzeff 1 , D. Marre 2 , G. Jimenez 1 , J. Mazurier 2 , P. Dudouet 1 , F. Jonca 3 , P. Seguin 4 , P. Labarthe 5 , J. Guillotreau 6 , G. Ploussard 7 , E. Bruguiere 8 1 Oncorad Garonne- Clinique Pasteur, Radiotherapy- Oncology, Toulouse, France 2 Oncorad Garonne- Clinique Pasteur, Physics, Toulouse, France 3 Clinique Ambroise Paré, Urology, Toulouse, France 4 Clinique Occitanie, Urology, Muret, France 5 Clinique des Cèdres, Urology, Cornebarrieu, France 6 Clinique Pasteur, Urology, toulouse, France 7 Clinique Saint Jean Languedoc, Urology, Toulouse, France 8 Clinique Pasteur, Radiology, Toulouse, France Purpose or Objective To study, after implant of a spacer, the anatomical and radiological parameters with MRI of installation between the 3 spacers Material and Methods Since September 2013, 46 patients, mean age 75 years, were implanted with a spacer by a single radiologist, in a space created between the prostate and the rectum (biodegradable balloon (BB), acid hyaluronique gel (AH) and polyethylene glycol gel(PEG)), before starting irradiation of the prostate cancer with low risk or intermediate stage. MRI evaluation was realized in sagittal and axial plans with T2 slides of 2,5 mm thickness. Results 16 pts were implanted with the balloon, 19 pts with the AH gel and 11 with PEG gel. Hydro-dissection was performed in every procedure before implant. MRI exams were assessed between 1 week and one month after implantation. There was no failure of pose. The 3D implanted volume measured, on the MRI ,of physiological salt solution withinthe balloon or gel was 13,5 cc on average, variable according to the technique (12.21 cc balloon, 14,69 cc AH gel and 12 cc PEG gel). Analysis of the anterio-posterior distance generated between prostate gland and rectum following implant was conducted at 3 levels (base, mid gland and apex) : mean distance was 14,2 mm, 11,3 mm et 10 mm with the 3 spacers respectively. With BB mean distances were 16,3 mmat the base, 12,1 mm at mid gland and 8,9 mm at apex. For AH and PEG gels, same measures were 11,97 mm and 14,94 mm at the base, 9,4 mm and 13,18 mm at mid gland and 10,37 mm and12,34 mm at apex respectively. In this study BB showed a better space volume created after implant whereas better distance at apex was assessed with gels than balloon, with statistical significance. As a result of good conditions of implantation there was no noteworthy toxicity in this study. Leakage of gel on péri rectal laterallly was noticed for 4 pts with AH du to specific implant procedure (use of 3 needles with AH instead of one with PEG). For EBRT treatment with IGRT using CBCT, BB was easier for patient repositioning than gels (less visible on CBCT). Conclusion This MRI study confirms the opportunity of creating a good space between prostate gland and rectum whatever system used, with a excellent implantation profile. Nevertheless differencies exist between these 3 spacers in terms of operating implantation, size of space generated post implantation and IGRT conditions for EBRT for prostate cancer treatment. A learning curve is necessary for each system.