ESTRO 37 Abstract book

ESTRO 37

S577

sparing devices. Results with and without hydrogel spacer implantation were as follows: D 1cc was 7.96±0.22 Gy and 8.43±0.40 Gy, respectively; and the maximum dose to the rectum was 13.50±0.30 Gy and 11.90±0.55 Gy, respectively. Reduction in maximum dose (p < 0.001) and D 1cc (p = 0.046)) to the rectum is observed in cases implementing the hydrogel rectal spacer. Considering a typical HDR prostate brachytherapy dose prescription of 27.0 Gy (two 13.5 Gy fractions) and an α/β = 3.0 for rectum, the average maximum EQD2 point dose for patients receiving hydrogel implants was 59.74±2.79 Gy compared to 71.92±2.36 Gy (p < 0.001) for those treated without the device. Conclusion DVH analysis for the patient sampling with hydrogel implantation demonstrated statistically significant reduction in D max and D 1cc metrics. When compared to plans without hydrogel implants, the average EQD2 of the max point dose to rectum for patients receiving hydrogel implants was lower and fell within the TD5/5 of 60 Gy, as recommended by Emami et al. Future clinical evaluation of these patients may allow for a statistical relationship between the observed dose reduction and clinical toxicity. PO-1028 Acute urinary retention and hematuria in prostate cancer patients treated with HDR-BT L.C. Mendez 1 , A. Ravi 2 , C.L. Tseng 1 , H. Chung 1 , M. Paudel 2 , M. Wronski 2 , M. Davidson 2 , A. Loblaw 1 , G. Morton 1 1 Odette Cancer Centre - University of Toronto, Radiation Oncology, Toronto, Canada 2 Odette Cancer Centre - University of Toronto, Medical Physics, Toronto, Canada Purpose or Objective To study first month post-procedural urinary retention and hematuria in prostate cancer patients treated with 15Gy high dose rate brachytherapy (HDR-BT). Material and Methods Patients treated with 15Gy HDR-BT followed by external beam radiation therapy (EBRT) were evaluated in this unmatched case-control study. All patients had intermediate or high-risk prostate cancer and received HDR-BT and EBRT at Sunnybrook-Odette Cancer Centre (Toronto, Canada) with over one month of post- procedure follow up time. Urinary retention and hematuria requiring catheterization were retrospectively collected together with time to event (from procedure date) and length of Foley catheter use. Clinical characteristics (age, baseline PSA, clinical stage, Gleason score, International Prostate Symptom Score (IPSS), use of androgen deprivation therapy (ADT), alpha-blocker, 5α-reductase inhibitor) and treatment specifications (number of afterloading catheters, prostate V100, V150, V200, D90, urethra DMax and D10) were also obtained. Variables were described with appropriate measures of central tendency and dispersion. Univariate and multivariate binary logistic regression analysis were used to determine predictors of post-BT complications. Statistics was performed in STATA 13.0 and a p-value < 0.05 was considered statistically significant. Results 400 patients treated with 15Gy HDR-BT were collected, but only 224 were included in this analysis as 176 received EBRT outside of the institution. Patients clinical and treatment characteristics are shown in Table 1. In total, 22 patients (9.8%) required catheter placement due to acute urinary retention in a median time to catheter

Conclusion The proposed method provides robust inverse planning with improved dose-volume response compared to standard clinical state-of-the-art treatment planning systems. The pre-calculation of MC dose kernels, using GPU, allows to consider tissue heterogeneity in intraoperative dose calculations. The implant’s optimization based on the DVH metrics minimizes the system’s learning curve for the user, since optimization and treatment quality criteria are identical. PO-1027 Dosimetric Improvement in HDR Prostate Brachytherapy Patients using Hydrogel Spacer Implantation S. Cavanaugh 1 , S. Crawford 2 , J. Dick 2 , P. Schantz 2 , T. Tsui 2 , K. Harpool 2 , W. Snyder 2 , J. Swanson 2 1 Cancer Treatment Centers of America, Radiation Oncology, Newnan GA, USA 2 Landauer Medical Physics, Radiation Oncology, Newnan GA, USA Purpose or Objective A commercially available bioabsorbable hydrogel system— a mixture of a precursor (trilysine buffer solution and polyethylene glycol powder) and accelerator (salt buffer)—is implanted between the prostate and rectum of men undergoing radiation therapy for prostate cancer. Use of this device provides additional separation between the prostate and rectum, ostensibly reducing rectal injury due to proximity of the high dose target. This study investigates rectal sparing achieved in high dose rate (HDR) prostate brachytherapy patients with hydrogel Two hundred and five monotherapy HDR prostate brachytherapy cases were selected from the patient population treated from July 2015 to October 10, 2017. Patients were selected based on a treatment regimen of 13.5 Gy per fraction. Of these cases, 173 fractions were delivered to patients with implanted hydrogel rectal spacers while 32 fractions were treated without the use of this or any other rectal sparing device. For this patient cohort, the dose volume histogram (DVH) from the previously calculated plan was used to obtain D 1cc , D 2cc , and maximum rectal dose statistics based on rectum contours drawn on the planning CT dataset by the dosimetrist at the time of treatment planning. To compare to conventionally fractionated external beam radiation therapy (EBRT), the equivalent dose in 2 Gy fractions (EQD2) was calculated for each rectal dose statistic. Statistical analysis software was employed for data analysis. Results The D 1cc and maximum dose to the rectum for cases in which the hydrogel spacer was implanted prior to treatment were compared to cases treated without rectal spacer implantation. Material and Methods