Page 40 - Towards personalized therapy for metastatic prostate cancer: technical validation of [18F]fluoromethylcholine

Chapter 2
technique) at thresholds of 8-10 mm is only 7%. In an attempt to reduce the remaining uncertainty, a standard of reference method was used. This approach, as extensively described in the “Materials and Methods” section, consisted of the combination of histopathological examination (whenever available) and the results obtained by clinical or radiological follow-up. This is a commonly used procedure [32-37] to account for the limitations of retrospective studies. In difficult cases, biopsy of the proper radioactive choline avid lymph nodes was improved and verified by using a dedicated gamma-probe [38]. Confirmation seemed feasible in 65% of these pelvic LN (24/37). In 7 patients treated with ADT and/or chemotherapy for coexisting bone metastases, decreases of nodal diameter could not be interpreted since such changes are not necessarily compatible with a ‘malignant tissue’ response to treatment.
Note that in our present context ‘sensitivity’ and ‘specificity’ should not be confused with ‘the accuracy of [18F]FCH PET/CT to diagnose metastatic lymph nodes in prostate cancer’. The results pertain to the ability of tracer uptake time-trends to classify lymph nodes with enhanced [18F]FCH uptake.
The relevance of uptake time-trends to characterize [18F]FCH foci has been demonstrated in malignant bone metastases, in recurrent PC, and in malignant zones of the prostate in preoperative setting [8, 15, 17]. Our findings corroborate and extend those of Beheshti et al. [8] who reported on 18 malignant lymph nodes showing stable or increasing uptake over time. The imaging protocol consisted of a dynamic PET/CT scan of the pelvic region for 8 min, starting 1 min p.i., followed by whole body (WB) images 10 min after [18F]FCH injection and optional supplementary delayed WB acquisitions, 90-120 min p.i., when abnormalities were detected. However, since that study comprised only 4 [18F]FCH positive reactive lymph nodes (with decreasing uptake over time) they urged for validation of these patterns in a larger study.
In our study, all but one inguinal nodes showed decreasing [18F]FCH uptake over time (Figure 3), versus 95% (35/37) of the pelvic category demonstrating stable or increasing uptake (Figure 4).
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