ESTRO 2020 Abstract Book

S1087 ESTRO 2020

Purpose or Objective To report results of in-vivo 2D dosimetry in HDR surface brachytherapy for patients treated with the Freiburg-flap (Elekta Brachytherapy) and custom-made surface applicators. To quantify the need for more accurate dose calculation engines in comparison to TG43-based planning. To highlight the advantage of radiochromic film 2D dosimetry in the assessment of delivered dose to the targets and organs-at-risk, and introduce adaptive planning based on the measurements. Material and Methods In this study, data were analyzed from 87 film measurements taken during the delivery of 53 fractions of 21 HDR surface brachytherapy cases. Delivered doses were measured with pre-cut EBT3 film pieces (Figure 1(a)). Films were placed on patients’ skin (clinical target) or out- of-field and all cases were planned at the 3 mm depth from the skin surface. The prescription dose per fraction ranged from 250 cGy to 600 cGy depending on the diagnosis and treatment site. Treatment plans were calculated using TG43 formalism in Oncentra Brachy™ treatment planning system (Elekta Brachytherapy) and dose was also recalculated using advanced collapsed cone engine (ACE). For each fraction, measurements were compared to calculations at the skin surface to evaluate inter-fractional dosimetric variability and the need for planning adaptation. A histogram of all measurements normalized to the prescription dose was created to evaluate the percent increase in skin dose as a surrogate for dose delivered at the prescription depth. Results A representative case (Figure 1(b)) shows histograms of measured doses recorded during five fractions of a partial scalp treatment and compared to expectations from TG43- based planning. Another representative two-fraction case is shown in Figure 1(c) to highlight differences between measurements and calculations. On average (for all cases) these differences were: -7.1% [range: -3% to -15%] w.r.t. TG43 and 2.4% [range: -2% to 6%] w.r.t. ACE. Dose adaptation was shown to be necessary in some complex cases and an example is shown in Figure 1(d) of a breast case (200 cGy × 23 fractions) with significant folds and air gaps that was adapted by increasing selected dwell times following the in-vivo dosimetry results of the first fraction. Figure 2 shows a histogram of all measured data in this study summarizing the dosimetric evaluation of skin dose in HDR surface brachytherapy (yellow highlighted area).

of clinical data supporting improved outcomes. HDR boost may be a convenient means of overcoming radioresistance that needs further investigation. A clinically validated multigene expression model of tumour radiosensitivity (RSI) was developed by Torres-Roca and colleagues and validated in multiple cohorts and disease types. In this study, the RSI gene signature was validated in two prostate cancer radiotherapy cohorts. Material and Methods A total of 386 D’Amico classified high-risk patients treated from 2008-2014 were identified for this analysis: 218 patients received intensity modulated radiotherapy (IMRT) to the prostate only (60 Gy in 20# or 74 Gy in 37# ) and 168 patients received IMRT (37.5 Gy in 15#) and a high dose rate (HDR) brachytherapy boost (15 Gy). This equates to a BED α/β 1.5-3Gy of 120 – 180 Gy for IMRT only and 159 – 265 Gy for IMRT and HDR boost. Androgen deprivation was given to all patients with duration ranging from 3-36 months. Biochemical failure was defined as prostate-specific antigen (PSA) rise of ≥2 ng/ml above nadir post radiotherapy. The clinical end-point was progression-free survival (PFS). Gene expression data were generated from diagnostic needle core biopsies using Affymetrix Clariom S arrays. RSI scores were calculated using a published rank- based linear regression algorithm. The RSI score cut-off was the upper quartile to dichotomise patients into radioresistant (RSI-R) and radiosensitive (RSI-S). Kaplan- Meier statistics were used for survival outcomes. Results The mean follow-up for the entire cohort was 55 months (95% CI 56 – 61 months). The upper quartile cut-off for the RSI-R score was 0.41 (range 0.14 – 0.56). The 5-year PFS for radioresistant (RSI-R) vs radiosensitive (RSI-S) patients in the IMRT cohort was 54.9 % vs. 74.9% ( p = 0.024). The 5- year PFS for RSI-R vs RSI-S in the HDR boost cohort was 76.2 % vs 71.4% ( p = 0.71).

Conclusion Our study validates for the first time use of the RSI in prostate cancer patients undergoing definitive (without surgery) radiotherapy. The RSI signature should be explored further to select patients with high-risk prostate cancer who should benefit from dose escalation with a HDR brachytherapy boost. Session: Imaging and dosimetry OC-1032 In-vivo film dosimetry indicates a role for model-based algorithms in HDR surface brachytherapy S. Aldelaijan 1,2,3,4,5 , D.A. O'Farrell 1 , T.C. Harris 1 , R.A. Cormack 1 , J.P. Seuntjens 3 , S. Devic 4 , P.M. Devlin 1 , I.M. Buzurovic 1 1 Dana-Farber/Brigham and Women's Cancer Center, Department of Radiation Oncology, Boston, USA ; 2 McGill University, Department of Biomedical Engineering, Montreal, Canada ; 3 McGill University, Medical Physics Unit, Montreal, Canada ; 4 Jewish General Hospital, Department of Radiation Oncology, Montreal, Canada ; 5 King Faisal Specialist Hospital & Research Centre, Biomedical Physics Department, Riyadh, Saudi Arabia