ESTRO 2020 Abstract Book

S634 ESTRO 2020

Moderate hypofractionation for localized prostate cancer has become a standard of care in many radiotherapy centers worldwide. Several fractionation and planning protocols exist, with CHHiP and PROFIT (60 Gy in 20 fractions) being two of the most commonly used. We retrospectively compared the doses received by organs at risk using these 2 protocols. Material and Methods We retrospectively reviewed the charts of 15 randomly selected de-identified patients treated with intensity modulated radiation therapy (IMRT) for prostate cancer in a single tertiary care center. Each of these patients had a planning CT simulation with full bladder and empty rectum. For each patient, we generated 2 sets of contours for target volumes and organs at risk in accordance with both CHHiP and PROFIT protocols. The CHHiP protocol requires the contouring of 3 different target volumes planned to different doses utilizing simultaneous integrated boost technique, whereas the PROFIT protocol requires only a single target volume.Using Panther software (Prowess Inc, Concorde, CA) version 5.10. IMRT plans were generated (total 30 plans), using the respective planning targets and normal tissue constraints. Dependent t-tests were used to evaluate average mean dose as well as V60, V50 and V40 of the rectum, bladder and penile bulb. Results Patients had a mean age of 72.6, average PSA of 11.9, and mostly a Gleason score 7. Average mean dose,and V40 of the bladder and rectum were not significantly different between the two protocols. However, both bladder and rectum V60 and V50 were significantly lower in CHiPP as compared to PROFIT. The dose to the penile bulb was also significantly higher in terms of mean dose, V50 and V40 in the PROFIT arm. (Table 1)

(PCa). MR-depicted GTV regions have been correlated with the most common site of disease recurrence after RT. SABR with simultaneous integrated boost (SIB) to intraprostatic GTV is dosimetrically feasible without significantly increasing the dose to other OARs with the exception of the urethra. Herein, we hypothesized that rectal spacer may increase the degrees of freedom during SABR planning, thereby enabling a selective high-precision multiparametric MR (mpMR)-directed SIB SABR without incremental dose to the urethra. Material and Methods Ten randomly selected patients from a prospective study of whole-gland SABR plus focal brachytherapy dose- escalation to mpMR-defined GTV. Patients had intermediate/high-risk PCa, with a PI-RADS 4/5 lesion on MR (>5mm and < 1/3 of gland involved). Patients underwent insertion of 3 fiducial markers and rectal spacer. Simulation imaging: mpMR and computed tomography (CT), all co-registered using the fiducial markers. GTV and prostate CTV were contoured on mpMR, mapped onto planning CT using deformable image registration (Raystation v 6.1). 5mm isotropic expansion except laterally (3mm) was used to generate the PTVs. A dose of 36.25Gy/5 and 50Gy/5 were delivered using VMAT to the PTV_prostate and PTV_gtv, respectively. A second distribution was then generated with higher priority placed in reducing the urethra dose by allowing increased dose of up to 3Gy to the rectum and keeping the other OARs within planning constraints. Differences between planned and accumulated delivered doses (D Acc ) were assessed by dose accumulation analyses on treatment cone-beam CT images, assuming geometric relationship of urethra did not change relative to fiducial markers. Planning objectives were PTV D98% ≥ 34.4Gy and CTV D95% ≥40Gy; additional objective of PTV_boost D99% ≥ 47.5Gy for SIB; rectum D20% ≤ 29Gy and D1cc ≤ 36Gy; penile bulb D50 <29.5Gy, bladder D40 <18.1Gy and D10cc <37GY, and urethra D50 < 42Gy. Results The median (range) of GTV and CTV volumes (cc) were 3.5 (0.3-5.1) and 46.4 (21.2 – 85.7). Table 1 summarizes the mean and range of planned and D Acc to the urethra. Difference between planned and D Acc urethra was non significant. D Acc to 50% and 1% of urethra (Gy) were 43.9 (40.6 - 48.5) and 50.7 (45.3 - 52.3). After replanning to spare the urethra dose, D Acc was reduced to 41.4 (39.5 – 47.0) and D1 48.7 (43.1-52.7). The planned median rectal D50 and D1 (Gy) changed from 3.8 (1.9-8.6) to 7.7 (3.3- 12.6) and 31.7 (24.4-37.9) to 33.6 (26.2-39.2) respectively. Conclusion Urethral sparing is feasible with dose-escalation by MR- directed SIB to 50 Gy in this cohort of patients with rectal spacers without exceeding any other OAR dose constraints. This approach may allow selected dose escalation in the ultra-hypofractionated setting while minimizing urinary toxicity.

Conclusion In this retrospective dosimetric study comparing doses to organs at risk using CHHiP and PROFIT protocols, we observed statistically significant higher V60 and V50 to the bladder and rectum when using the PROFIT protocol. For the penile bulb,V40, V50 and mean dose were also significantly higher using PROFIT protocol. While contouring and planning using CHHiP protocol are usually more demanding, our results show that doing so, decreases the radiation dose to the organs at risk, specifically the penile bulb, and potentially minimizes the risk of impotence. These results need to be validated in a larger cohort of prospectively treated patients. PO-1203 Urethral sparing prostate SABR with integrated boost: novel by-stander effect of hydrogel spacer A. Billfalk Kelly 1 , V. Kong 1 , P. Bettoli 1 , J. Helou 1 , P. Chung 1 , A. Berlin 1 1 Princess Margaret Cancer Centre, Radiation Oncology, Toronto, Canada

PO-1204 Clinical outcomes and prognostic factors in patients with localized prostate cancer treated HDR BT D. Delishaj 1 , C.P. Soatti 1 , C. Frigerio 2 , R. D'amico 1 , F. Bonsignore 2 , I.C. Fumagalli 1 , G. De Nobili 1 , A. Cocchi 1 , A. Vola 1 , G. Sangalli 2 , F. Declich 2 , A. Colombo 1 1 Hospital Alessandro Manzoni-Radiation Oncology Department- ASST LECCO- Lecco- Italy, Radiotherapy, Lecco, Italy ; 2 Hospital Alessandro Manzoni-Medical Physics Unit- ASST LECCO- Lecco- Italy, Radiotherapy, Lecco, Italy

Purpose or Objective Background:

Genitourinary toxicities are a dose-limiting factor for SABR dose-escalation schedules in localized prostate cancer