ESTRO 38 Abstract book

S143 ESTRO 38

V100, V150 and V200 were 97% (IQR 96-97), 36% (IQR 33- 39) and 12% (IQR 10-14), respectively. After a median PSA follow-up time of 4.3 years (IQR 2.4-6.0), 44 (8%) patients experienced biochemical failure. After a median overall follow-up time of 4.9 years (IQR 2.6-6.4), 20 patients (4%) went on to receive ADT. Kaplan-Meier estimated bRFS rates were 91% (CI 88-94) and 82% (CI 77-87) at 5 and 7 years, respectively (Figure 1). The cumulative incidence of ADT use was 7% (CI 3-10) at 7 years (Figure 2). Figure 1- Kaplan-Meier estimate of biochemical relapse- free survival post 15Gy HDR-BT and EBRT

fractions in separate implants. DIL location was classified as peripheral, central or anterior. Directed biopsies of DILs were performed at the time of brachytherapy for pathologic confirmation of mpMRI results. A student t-test compared DIL D90 between modalities and DIL locations. Results Of 60 patients, 31 underwent LDR and 29 HDR brachytherapy. Up to 3 DILs were identified per patient (100 total) with 74 peripheral, 6 central, and 20 anterior DILs. Mean DIL volume was 1.9cc (SD: 1.7cc) for LDR and 1.6 cc (SD 1.3cc) for HDR (p=0.279). Mean DIL D90 was 151% (SD 30%) for LDR (217 Gy) and 132% (SD 13%) for HDR (17.8 Gy/fraction x 2). 88% of patients had biopsy confirmation of at least one DIL. DIL D90 for peripheral lesions was higher than anterior and central (p<0.001).The mean DIL D90 per location for LDR was 159% for peripheral lesions, but only 122% and 124% for central and anterior lesions, reflecting the planning algorithm which delivers 150% to the peripheral zone but limits the urethra to < 130%. The mean D90 per location for HDR was 137% for peripheral lesions, and 123% and 118% for central and anterior DILs. DIL D90 coverage was further evaluated as a percentage of the desired peripheral zone dose coverage, which was 150% for LDR and 125% for HDR. For both central and anterior lesions, HDR DIL D90 values were significantly closer to the peripheral zone benchmark dose than for LDR (p=0.007 and 0.002 respectively). Conclusion DIL location affects dose escalation, particularly because of urethral proximity, such as for anterior and central DILs. HDR brachytherapy may dose escalate better when target DIL is close to critical organs. OC-0288 Long-term results of 15Gy HDRBT boost in intermediate risk-prostate cancer:Analysis of 500+ patients L. Mendez 1 , K. Martell 1 , H. Chung 1 , C. Tseng 1 , Y. Alayed 1 , P. Cheung 1 , S. Liu 1 , D. Vesprini 1 , W. Chu 1 , E. Szumacher 1 , A. Ravi 2 , A. Loblaw 1 , G. Morton 1 1 Sunnybrook Odette Cancer Centre, Radiation oncology, Toronto, Canada; 2 Sunnybrook Odette Cancer Centre, Medical Physics, Toronto, Canada Purpose or Objective To report long-term biochemical control associated with single-fraction 15Gy high-dose-rate (HDR) brachytherapy boost followed by external beam radiation (EBRT) in intermediate-risk prostate cancer patients. Material and Methods All patients with intermediate-risk prostate adenocarcinoma that received treatment with 15Gy HDR brachytherapy (BT) boost followed by EBRT between 2009 and 2016 were retrospectively reviewed. Descriptive statistics were used to characterize the cohort. Biochemical failure was defined based on the Phoenix criteria. Kaplan-Meier method estimated biochemical relapse free survival (bRFS) at 5 and 7 years. Cumulative incidence of ADT use at 7 years was also calculated. R statistical software package v3.2.2 (www.r-project.org) was used for this analysis. Results 545 patients met the inclusion criteria for this study. Median age at HDR BT was 67 years (IQR 61-72). Median baseline PSA was 7.4 ng/mL (IQR 5.4-9.9). 302 (55%) and 243 (45%) patients had cT1 and cT2 stage disease. 9 (2%), 346 (67%) and 164 (32%) had ISUP Gleason Grade Group 1, 2 and 3, respectively. A median of 5 (IQR 3-7) out of 12 (IQR 10-12) biopsied cores were positive. A total of 90 (18%) patients received neoadjuvant ADT for a median time of 6 months (IQR 3-6). The most frequent EBRT regimen was 37.5Gy in 15 fractions with 508 patients (93%) receiving this fractionation. All patients received 15Gy in a single HDR brachytherapy fraction. Median HDR prostate

Figure 2- Cumulative incidence of ADT use post 15Gy HDR- BT and EBRT

Conclusion In this large cohort of intermediate-risk prostate cancer patients, 15 Gy HDR BT boost plus EBRT results in durable long-term biochemical control and low rates of ADT salvage.

Proffered Papers: PH 5: Proffered paper: Innovations in dosimetry and dose measurements

OC-0289 A water calorimeter as a primary standard for absorbed dose in magnetic fields L. De Prez 1 , J. De Pooter 1 , B. Jansen 1 , S. Woodings 2 , J. Wolthaus 2 , B. Van Asselen 2 , T. Van Soest 2 , J. Kok 2 , B. Raaymakers 2 1 VSL, Ionizing Radiation Standards, Delft, The Netherlands; 2 University Medical Center Utrecht, Department of Radiotherapy, Utrecht, The Netherlands Purpose or Objective MRI guided radiotherapy devices are currently in clinical use. Detector responses can be significantly affected by the magnetic field. Therefore, there is a need to characterize detectors in terms of absorbed dose to water, D w , against primary standards that are operated under the magnetic field condition. The aim of this study was to commission a water calorimeter, accepted as the Dutch national standard for D w and validate its claimed