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To create a magnetic resonance image (MRI)-based atlas of prostate bed recurrence for prostate cancer patients undergoing salvage radiotherapy (sRT) after radical prostatectomy and to evaluate the consistency of contouring guidelines for the prostatic fossa clinical target volume (PF-CTV) delineation. Patients with biochemical recurrence and prostate bed lesions at MRI before sRT were included. A reference patient was identified and the lesion(s) of each patient was mapped by a deformable co-registration among T2 weighted (T2w) images. The structure sets were combined to create a 3D recurrence incidence map, overlapped on T2w reference set, then transferred to the planning CT. The volume and the location of the lesions were extracted. On the reference CT, 5 PF-CTV contouring strategies were implemented: Radiation Therapy Oncology Group (RTOG); European Organization for Research and Treatment of Cancer (EORTC); Francophone Group of Urological Radiotherapy (GFRU); Faculty of Radiation Oncology Genito-urinary Group (FROGG) and Princess Margaret Hospital (PMH). Coverage was assessed after identifying the centroid of each lesion: if the centroid was included in the PF-CTV, the coverage was considered complete (fully covered-FC), otherwise uncovered (UC). The coverage was evaluated for each contouring strategy and compared with the chi-squared test; the strength of the correlation was assessed with the phi coefficient. A total of 112 patients with 124 recurrences were identified. The lesions were located at the vesicourethral anastomosis (VUA), around the bladder neck (BN) and beyond the bladder (RV) in 63, 36 and 25 cases. Regardless the contouring strategy, 89.1% of the lesions were FC. Coverage was higher (p<0.001) for BN (95.6%) over RV (83.2%) and VUA (73.7%) lesions. FC/UC rates were 66.1%/33.9%, 83.9%/16.1%, 89.5%/10.5%, 83.1%/16.9%, 87.1%/12.9% for EORTC, FROGG, GFRU, RTOG and PMH, respectively. At pair-wise comparison, the EORTC definition had lower coverage (p<0.001) and was poorly correlated to the other strategies (phi values ranging 0.385-0.540, p <0.001). Conversely, the correlations between the other definitions were >0.700, the highest between PMH-FROG (0.812, p<0.001). After selecting VUA lesions only (N = 63), the probability of FC was 60.3%, 74.6%, 82.5%, 71.4% and 79.4% for EORTC, FROGG, GFRU, RTOG and PMH, respectively. The EORTC definition performed poorly than GFRU (p = 0.006) and PMH (p = 0.020). When considering lesions above the VUA (N = 61), the performance improved (FC: 72.1%, 93.4%, 96.7%, 95.1% and 95.1% for EORTC, FROGG, GFRU, RTOG and PMH, respectively) but the EORTC was significantly less covering and its correlation with the others was ≤ 0.1. According to the criterium of coverage our data do not support the use of the EORTC definition. Here we provide an MRI-based atlas of prostatic bed recurrence that can optimize sRT contours. [ABSTRACT FROM AUTHOR]

Poster: Physics track: Intra-fraction motion management PO-1601: electromagnetic localization and tracking for prostate cancer radiotherapy: impact of hip prostheses M. Bottero, G. Dipasquale, A. Lancia, R. Miralbell, M. Jaccard, T. Zilli. [Extracted from the article]

To investigate predictors of biochemical failure after salvage radiotherapy (sRT) in the context of a presumed local failure at dynamic contrast-enhancement-magnetic resonance imaging (DCE-MRI) post radical prostatectomy (RP). All patients referred for sRT at our Institution with a biochemical failure after RP have been consistently offered restaging with both PET/CT and multiparametric MRI since January 2014. Those with a presumed local failure at DCE-MRI as well as no regional/distant disease at PET/CT (either choline and/or PSMA) were selected for the present study. Exclusion criteria were history of androgen deprivation (AD) before sRT and positive nodes at RP. All patients underwent 3T DCE-MRI without endorectal coil and the lesion(s) transferred to the planning CT after co-registration. sRT consisted in 73.5 Gy to the presumed local lesion and 66-69 Gy to the prostatic bed in 30 fxs. Pelvic nodes (PN) were covered to 54 Gy/30 fxs in selected patients. The endpoint of the study was the development of a biochemical failure after sRT defined as a 0.2 ng/ml PSA rise above the nadir. Various covariates (age, pre-RP PSA, pT and pN stages at RP, margins status at RP, ISUP grade group, time from RP to sRT, PSA doubling time, PSA detectability after RP, PSA at sRT, the location, number and volume of the detected recurrence(s), AD use, PN coverage, EUA risk category) were investigated at univariate analysis (UVA) on the time to biochemical failure (bNED-survival). Covariates with a p value <0.2 at UVA were entered a Cox proportional hazards regression analysis. Up to June 2020, 146 patients satisfying all selection criteria have been treated with sRT. Median (IQR) PSA at sRT was 0.60 ng/ml (0.38-1.05 ng/ml) and only 17 patients (11.6%) received AD along with sRT. A total of 168 local lesions have been detected, 92 (54.8%), 40 (23.8%) and 36 (21.4%) at the vesicourethral anastomosis (VUA), the bladder neck and the retrovesical space, respectively. At the median (IQR) follow-up of 48.1 months (31.3-60.6 months), 22 biochemical failures have been observed for a 4-yr bNED survival of 84.4% (95%CI: 77.9-90.9%). On UVA, bNED-survival after sRT was significantly more likely for patients with VUA-only lesions (VUA-only vs others, HR=0.307, 95%CI: 0.120-0.784, p=0.014) and with smaller lesions (for every cc, HR: 1.071, 95%CI: 1.025-1.119, p=0.002). These associations remained significant (p< 0.01) on multivariate analysis as well. For patients with VUA-only disease or with lesions smaller than 0.5cc, 4-yr bNED survival rates were 90.7% (95%CI: 83.4-98.0%) and 90.6% (95%CI: 83.9-97.3%), respectively. The 46 patients with both favorable features had a 4-yr bNED rate of 94.6% (95%CI: 87.3-100%). These data support local restaging with DCE-MRI before sRT in the setting of a biochemical failure after RP. Patients with VUA-only and/or small volume lesions have an excellent outcome after dose-escalated sRT. [ABSTRACT FROM AUTHOR]