Marieke van Son

163 RE-SALVAGE FOCAL HDR-BT INTRODUCTION Despite improvements in primary prostate cancer care, 15%-55% of patients undergoing radiotherapy develop biochemical recurrence after a 10 years’ follow-up [1]. Even with external beam radiation therapy (EBRT) dose escalation, the risk of disease progres- sion remains significant, especially in higher-risk groups [2]. In the management of recurrent disease, physicians are faced with difficult considerations regarding sal- vage treatment options. Although early recurrences are often confined to the prostate without lymph-node or distant metastases, around 98% of patients are still treated with (delayed) androgen deprivation therapy (ADT). This treatment is associated with significant side-effects, such as erectile dysfunction, osteoporosis, increased risk of diabetes, gynaecomastia, hot flashes, and depression. Moreover, hormonal treatment is palliative and castration resistance usually occurs within one to three years [3]. In contrast, curative whole-gland salvage treatments, such as prostatectomy, brachyther- apy, high-intensity focused ultrasound (HIFU), and cryosurgery remain unpopular due to high toxicity rates and, in earlier series, a significant risk of failure [4]. In an effort to reduce toxicity in the salvage setting, research has shifted towards organ preserving approaches. Although prostate cancer is usually multifocal, the “index lesion” hypothesis states there is one clinically important tumor focus in the prostate (the index lesion), which harbors the metastatic precursor cell [5]. After whole-gland irradiation, the disease often recurs unifocally [6], indicating that smaller secondary lesions have been treated while the index lesion remains. On that premise, salvage focal therapy aimed at this lesion should achieve the same oncological control as whole- gland treatments. The success of focal salvage treatment depends on the degree of tumor visualization and reliable exclusion of metastases. This has significantly im- proved with advancements in multiparametric (mp)-MRI and prostate-specific mem- brane antigen (PSMA) positron emission tomography (PET) imaging. The Radiation Oncology department of the University Medical Center Utrecht (UMCU) is equipped with a 1.5T MRI treatment facility, enabling brachytherapy treatment under MRI guidance. The convergence of these technologies allows for an optimal implanta- tion procedure, supporting focal treatment. In 2013, the UMCU introduced MRI-guided, single-fraction (19Gy) focal high-dose-rate brachytherapy (HDR-BT) with iridium-192 as the salvage treatment for local radio-recurrent prostate cancer. With this treatment, the radiation dose to the tumor is escalated while exposure to the surrounding organs at risk (OAR) is limited. Results with regard to toxicity are promising and, therefore, the question arises whether re-treatment with focal HDR-BT is possible for future post-sal- vage recurrences. This could prevent or further delay the initiation of ADT, thereby avoiding hormone-induced toxicity. Furthermore, postponing castration resistance could potentially increase prostate cancer-specific survival. We present a novel case of second MRI-guided focal salvage HDR-BT. 9