ESTRO 2021 Abstract Book

S641

ESTRO 2021

7.2-26.8). Of the patients treated with prostate-only EBRT, 52% had a component of pelvic nodal failure. Overall, recurrence pattern was locoregional in 34%, oligometastatic in 16%, and polymetastatic in 39%, with median PSA being 3.7 ng/mL (IQR 2.4-7.1), 4.1 ng/mL (IQR 2.4-7.2), and 8.2 ng/mL (IQR 4.3-18.7) respectively (Figure 2). Rate of polymetastatic recurrence at PSA < 10.0 ng/mL was higher for patients with Gleason 8-10 than Gleason 6-7 (38% vs 18%, p=0.05).

Serum PSA at time of Ga68-PSMA PETCT (ng/mL) < 2.0 (n=15) 2.0-5.0 (n=48) 5.0-10.0 (n=21)

Pattern of recurrence

>10.0 (n=27)

No lesion

33%

15%

5%

0%

Locoregiona

20%

48%

24%

29%

Oligometastatic

27%

17%

24%

4%

Polymetastatic

20%

21%

47%

68%

Figure 1: Patterns of recurrence in Ga68-PSMA PETCT (n=114)

Figure 2: Patterns of recurrence grouped according to Gleason score and PSA at PETCT

Conclusion Early Ga68-PSMA PETCT for rising serum PSA levels after radical radiotherapy for locally advanced prostate cancer can identify non-metastatic or oligometastatic recurrence, which may be amenable to non-systemic salvage treatment. PD-0809 4-year PSA to predict relapse risk after low dose rate brachytherapy for prostate cancer D. Noble 1 , E. Doyle 1 , G. Tramonti 2 , A. Law 1 , A. Sundaramurthy 1 , J. Brush 3 , J. Keanie 3 , C. Wood 4 , P. Drewell 4 , W. Keough 4 , D. McLaren 1 1 Edinburgh Cancer Centre, Department of Clinical Oncology, Edinburgh, United Kingdom; 2 Edinburgh Cancer Research Centre, MRC Institute of Genetics and Molecular Medicine, Edinburgh, United Kingdom; 3 Western General Hospital, Department of Radiology, Edinburgh, United Kingdom; 4 Western General Hospital, Department of Oncology Physics, Edinburgh, United Kingdom Purpose or Objective Low-dose-rate brachytherapy (LDR-BT) is an excellent treatment option for low- and intermediate-risk localised prostate cancer (PCa), and prostate specific antigen (PSA) is a biomarker of response post therapy. Uncertainty exists however concerning post-therapy PSA thresholds that constitute both relapse and cure. This