Focal therapy is an emerging alternative treatment option, which offers great hopes in terms of cancer control and decreased morbidity (i.e. impotence and incontinence) for patients with localized low- and intermediate grade prostate cancer (PCa). The challenge of focal therapy is to treat only the tumor, sparing the rest of the prostate, especially near the neurovascular bundles and the urethral sphincter to minimize potential morbidity1. The focal therapy concept remains controversial because PCa is a multifocal disease. However, 13–33% of patients have one PCa lesion and would be eligible for focal therapy2. Consistent with the “index lesion theory”, indicating that most tumors other than the index cancer may not be of clinical significance, even more patients would be suitable3. A relatively new technique to perform focal therapy is laser ablation. A laser fiber is positioned into the tumor and the targeted tissue is irreversibly damaged and destroyed in only a few minutes. Magnetic resonance imaging (MRI) can be used during laser ablation for correct targeting of the PCa lesion and accurate laser fiber placement. Furthermore, it can monitor the ablation process in real-time with the help of temperature mapping and can verify complete tumor ablation afterwards. Thus far, only a small amount of studies have been published on MR guided focal laser ablation in prostate cancer patients 4-8. The purpose of this study is to evaluate therapy success, complications and technical feasibility and safety of MR-guided focal laser ablation as primary treatment for prostate cancer.This study was approved by the Institutional Review Board and all patients signed informed consent. Five patients with newly diagnosed, histopathologically proven PCa were treated with MR-guided focal laser ablation as primary treatment. Inclusion criteria were a prostate specific antigen (PSA) level ≤20, Gleason Score ≤7, no previous prostate treatment, no evidence for nodal or metastatic disease and a cancer lesion located >1 cm of the neurovascular bundle. Patients who were unable to undergo MR imaging or with contra-indications to MR-guided focal laser ablation (i.e. rectal pathology) were excluded. All MRI-guided focal laser ablation procedures were performed in a wide-bore 3T MR scanner (Skyra, Siemens, Erlangen, Germany) under local anesthesia. A needle guide was inserted in the rectum, directed to the cancer lesion and after correct alignment the laser fiber (Visualase Inc. Houston, Texas, USA) was inserted. The ablation procedure was continuously monitored with real-time proton-resonance frequency shift (PRF) MR thermometry (TMAP: TR 44.8 ms, TE 13.03 ms, flip angle 30°, resolution 1.5x1.5 mm, slice thickness 5 mm), acquired in a single plane through the laser fibre with a temporal resolution of 4.4s (figure 1B). Per patient, multiple ablations were performed, depending on the size of the cancer lesion. Directly after the ablation T1-weighted fat-saturated contrast enhanced images (figure 1C) were acquired (T1 TSE axial: TR 704 ms, TE 13 ms, flip angle 120°, resolution 0.8 x0.8 mm, slice thickness 3 mm). Follow-up consisted of PSA-level measurement every three months and a multi-parametric prostate MR examination after 1 – 3 weeks, 6 and 12 months. After 12 months, MR-guided biopsy of the ablation zone was performed. Next to this, at baseline and every three months after treatment, patients were asked to fill in validated questionnaires (IPSS, IIEF) to monitor post-treatment morbidity, i.e. impotence and incontinence, and health-related quality of life (EQ-5D).Five patients were successfully treated with transrectal MR-guided focal laser ablation. All patients were dismissed within one hour after treatment. In the weeks after treatment the first patient experienced some pneumaturia. Additional cystoscopy and MR imaging were performed and did not show any signs for a rectal fistula. Patient’s symptoms resolved spontaneously after three months. Median follow-up was 6 months (range, 3 – 12 months). The PSA level decreased in all patients and multi-parametric MRI showed no presence of residual or recurrent tumor. The IPSS, IIEF and EQ-5D scores for monitoring incontinence, impotence and quality of life did not significantly change after treatment. Twelve months after treatment, MRI-guided biopsy of the ablated area was performed in one patient and no malignant cells were found.Transrectal MR-guided focal laser ablation of newly diagnosed PCa was technically feasible and safe. PSA level decreased in all patients and follow-up MRI showed no residual or recurrent cancer; indicating local cancer control, without compromising for increased morbidity or decrease in quality of life. Initial results are promising and more patients have to be included with longer follow-up.