Part III | Chapter 11 222 Identification of recurrent prostate cancer In our clinic, recurrent disease is detected using a combination of mp-MRI and (68-Ga) PSMA-PET/CT, without requiring further confirmation by histological biopsies. Given the morbidity of current focal salvage options, guidelines currently still advice histological proof before salvage treatment.75,77 However, pathological assessment of biopsies is problematic given the risk of false negatives due to sampling errors and radiation effects hindering accurate classification.78,79 In a study by Rasing et al.80 the positive predictive value of combined mp-MRI and PSMA-PET/CT for detecting recurrent prostate cancer was 97.6% in 41 patients treated with FS-HDR-BT, indicating the limited added value of targeted biopsies in these patients. However, these results are only valid for those in whom mpMRI and PSMA-PET/CT are concordant. Histological confirmation by targeted biopsies is therefore encouraged in case of discordance between mp-MRI and PSMA-PET/CT and in case of low or ambiguous suspicion.80 The debate on the exact value of targeted biopsies in the recurrent setting, and specifically its role in selecting patients who are eligible for salvage treatment, is still ongoing. Another important debate is how to best define of the Gross Tumour Volume (GTV). While mp-MRI and PSMA-PET/CT have improved identification of isolated local recurrences (and improved metastatic staging), it is still unclear how to best define and delineate the GTV and therefore a wide variation exists in clinical practice. As reported by Draulans et al.81, the selected PSMA tracer and window-level settings have a major influence on GTV contour. Research on histopathological validation of imaging-based contouring allows for recommendations on what imaging and settings to use to achieve accurate, reliable GTV contours.81–83 It is important that consensus guidelines are established, as studies investigating focal boosting in the primary setting or focal salvage treatment are majorly influenced by accurate definition of the GTV. This becomes even more important with very small error margins of 1 or 2 mm. To improve identification of the DIL and/or local recurrence, PET/CT systems could potentially be replaced by hybrid PET/MRI systems.84 Improving outcomes after salvage treatment: is MRI-guided SBRT the solution? Although the exact role of focal radiotherapy for recurrent prostate cancer is still unclear, the promising outcomes do favour treatment of selected patients within prospective clinical studies. Nevertheless, there seems to be room for improvement with respect to the oncological outcomes. There are several aspects that may influence the observed bDFS rates after focal salvage FS-HDR-BT. First, as also mentioned in chapter 8, there might be a selection problem. Some patients (or better stated: tumours) will inevitably be less suitable for salvage treatment than others. Improved selection strategies might lead to improved bDFS rates. On the other hand, the treatment itself could be improved. For example, it is questionable whether a single radiotherapy fraction is really suitable for the treatment of (recurrent) prostate cancer. Evidence is emerging on improved oncological outcomes after fractionated treatment with two fractions of 13.5 Gy, in both the primary and salvage treatment setting.71,85 Although evidence is too scarce to draw definite conclusions as of yet, this warrants further research into fractionated salvage treatment. In this view, fractionated brachytherapy treatment might not be that attractive, because of the invasive character (need for spinal anaesthesia, catheter insertion into the prostate) and logistical aspects (need for anaesthesiology team, procedure takes several hours) of the treatment. Currently, only small and
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