Marieke van Son

66 CHAPTER 4 Assessing and targeting intra-prostatic disease After exclusion of metastatic disease, assessment of intra-prostatic disease is nec- essary to adequately target the recurrent lesion. In the past, salvage treatments had to be aimed at the whole prostate gland since localization of the recurrent nodule was inadequate. Nowadays, this has become possible with the use of multi-parametric MRI (mp-MRI), offering both morphological and functional information with T2-weighted, dynamic contrast-enhanced (DCE) and diffusion-weighted imaging (DWI). In the pri- mary setting, diagnostic accuracy of mp-MRI for the detection of clinically significant intra-prostatic disease seems adequate with a sensitivity of 93% [49,50]. Although smaller (secondary) tumor foci are still occasionally being missed (even when harbor- ing higher grade cancers), mp-MRI is often able to detect the larger index tumor [51]. Because of the relatively high contrast of fibrotic prostatic tissue with viable tumor tissue in a previously irradiated prostate, DCE and DWI-MRI are especially capable of adequately detecting radiorecurrent lesions [52-54]. However, in the setting of treatment failure evaluation, the interpretation of mp-MRI is often complicated by treatment-related anatomic and functional changes. Radiologists should be familiar with the findings that are associated with the type of treatment the patient previously received. For instance, T2 hypo-intense intraprostatic lesions can be difficult to distinguish within a diffusely hypo-intense prostate caused by previous irradiation. Although there are no established guidelines for characterizing possible local tumor relapses on mp-MRI, there is an increasing amount of literature discussing the differences between normal post-treatment patterns and suspicious recurrence findings [55-59]. The combination of (68)Ga-PSMA-PET/CT with mp-MRI could provide an even higher accuracy in detecting and delineating intra-prostatic disease [60] (see Figure 1 for image example). A retrospective analysis on the diagnostic value of (68)Ga-PSMA- PET/CT in the recurrent setting revealed a negative predictive value (NPV) and positive predictive value (PPV) of 91.4% and 100%, detecting recurrent prostate cancer in a high number of patients [61]. In line with these promising results, the impact of using (68) Ga-PSMA-PET/CT in patients with recurrent prostate cancer is large, altering the ther- apeutic management in approximately half of all patients. Specifically, the use of dose escalation to boost the target volume and the proportion of focal salvage treatments seems to increase, while systemic treatment decreases [62].