Page 180 - Quantitative Imaging of Small Tumours with Positron Emission Tomography

detected more lesions, resulting in their re-classification as non-oligometastatic (23). In our study, 11/40 patients (27.5%) had no initial PSA response after SBRT (presence of PSA nadir: HR 0.23; p<0.001), which may reflect suboptimal metastasis detection rates for [18F]- fluoromethylcholine PET/CT in this setting. However, whether PSMA-guided management will improve patient outcomes remains to be shown. Limitations of this study are its retrospective nature and relatively small sample size. A strength is its clinical relevance - and PET/CT imaging, SBRT, and PSA follow-up (tested at least every 3 months) were all performed according to institutional protocols used in clinical practice. The role of post-SBRT ADT or the relative benefits of SBRT to only PET positive lesion(s) versus larger volume irradiation (e.g. the involved lymph node chain) remains to be investigated. Further research is also warranted to characterize the time-course of activity in treated lesions on post-SBRT [18F]- fluoromethylcholine PET/CT scans, and to avoid over-diagnosing local failure (Figure 8.1). In conclusion, the number of detected oligometastases seems prognostic. Additional data, including from prospective randomised controlled trials (e.g. NTC01558427 and NTC02680587), will hopefully determine whether the advantage of SBRT for oligometastatic prostate cancer is limited to deferring systemic therapy and identify additional patient or tumour characteristics predictive for disease progression. 8 Figure 8.1: [18F]-fluoromethylcholine PET/CT images of a 74-year-old patient with persisting, but decreasing, activity in a right para-iliac lymph node after SBRT. PET, low-dose CT, and fused PET/CT images before (A-C) SBRT, 20 months (D-F) and 32 months (G-I) after SBRT, respectively. Oligometastatic prostate cancer 179