Response of local disease at magnetic resonance after salvage radiotherapy for prostate cancer

ABSTRACT

Purpose or Objective To assess the pattern of response on dynamic contrast enhancement magnetic resonance imaging (DCE-MRI) of presumed local lesions in the setting of salvage radiotherapy (sRT) after radical prostatectomy (RP). Materials and Methods The present prospective study (NCT04703543) was conducted at a single Institution between August 2017 and June 2020. Eligibility criteria were undetectable prostate specific antigen (PSA) after RP;biochemical recurrence (2 consecutive PSA rises to 0.2 ng/ml or greater);a presumed local failure at DCE-MRI (early/fast enhancing discrete lesion on DCE sequences);no distant metastases at choline-PET/CT;no previous history of androgen deprivation therapy and/or RT. Accrued patients underwent sRT as it follows 66-69 Gy/30 fractions to the prostatic bed, 73.5 Gy/30 fractions to the local failure at DCE-MRI, 54 Gy/30 fractions to the pelvic nodes (when treated). All patients were offered DCE-MRI 3 months after sRT, and repeated at 3-month intervals until complete disappearance or a maximum of 4 scans. The endpoint of the study, complete response (CR), was defined as the complete disappearance of the target lesion at DCE-MRI. In case of misses before CR, the observation was considered as a persisting partial response (PR). Results 62 patients with 72 nodules at DCE-MRI were accrued. All patients underwent the 1st DCE-MRI at a median of 3.3 months (IQR 3.1-4.1) after sRT, 33 patients (53.2%) presented a CR, 27 (43,5%) a PR, 2 (3.2%) no response. One patient, implanted with a cardiac device, did not undergo further MRI. Three more patients declined further testing after the 1st (N=2) or the 2nd (N=1) re-evaluation due to the COVID-19 pandemic. Twenty-eight patients underwent a 2nd DCE-MRI after a median of 6.8 months (IQR 6.5-7.6) from sRT, 20 had a CR, 8 had a PR. After a median time of 10.7 months (IQR 10.6-12.6), 6 patients were scanned for a 3nd DCE-MRI 4 CR, 2 PR. The last patient reported a CR after 16.7 months. The majority (94.3%, 95%CI 88.0-100.0%) of lesions had completely disappeared by the 3rd re-evaluation or a median time of 10.7 months from the end of sRT (Figure).(Figure Presented) Independent predictors of CR at 1st re-evaluation on multivariable analysis were the volume of the lesion at pre-sRT DCEMRI (OR 0.076, 95%CI 0.009-0.667;p=0.02), the time of re-evaluation from treatment (OR 3.39, 95%CI 1.156-9.993;p=0.026) and the PSA percent decrease at the 5th week of sRT (OR 1.02, 95%CI 0.999-1.050;p= 0.058) (Table). (Table Presented) Receiver-operating characteristic curve (ROC) analysis identified the best cut-off on CR for baseline volume at 0.545 cc, AUC 0.683 (95%CI 0.548-0.818, p=0.014). The probability of a CR for lesions larger than the cut-off identified at ROC analysis was only around 75% at 10.7 months. Conclusion The vast majority of local lesions disappears at DCE-MRI after sRT, though larger lesions may require more than 10 months from treatment end.