BACKGROUND: The efficacy of focal continuous low dose-rate brachytherapy (CLDR-BT) for prostate cancer requires that appropriate margins are applied to ensure robust target coverage. In this study we propose a method to establish such margins by emulating a focal treatment in patients treated with CLDR-BT to the entire gland. MATERIAL AND METHODS: In 15 patients with localized prostate cancer, prostate volumes and dominant intra-prostatic lesions were delineated on pre-treatment magnetic resonance imaging (MRI). Delineations and MRI were registered to trans-rectal ultrasound images in the operating theater. The patients received CLDR-BT treatment to the total prostate volume. The implantation consisted of two parts: an experimental focal plan covering the dominant intra-prostatic lesion (F-GTV), followed by a plan containing additional seeds to achieve entire prostate coverage. Isodose surfaces were reconstructed using follow-up computed tomography (CT). The focal dose was emulated by reconstructing seeds from the focal plan only. The distance to agreement between planned and delivered isodose surfaces and F-GTV coverage was determined to calculate the margin required for robust treatment. RESULTS: If patients had been treated only focally, the target volume would have been reduced from an average of 40.9 cm3 for the entire prostate to 5.8 cm3 for the focal plan. The D90 for the F-GTV in the focal plan was 195±60 Gy, the V100 was 94% [range 71-100%]. The maximum distance (cd95) between the planned and delivered isodose contours was 0.48 cm. CONCLUSIONS: This study provides an estimate of 0.5 cm for the margin required for robust coverage of a focal target volume prior to actually implementing a focal treatment protocol.