DCE-MRI-Based Atlas for Prostate Bed Recurrence after Radical Prostatectomy: Consistency of CTV Delineation with the Available Contouring Guidelines.

PURPOSE/OBJECTIVE(S): 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.

MATERIALS/METHODS: 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.

RESULTS: 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.

CONCLUSION: 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.