Marieke van Son

184 CHAPTER 10 therefore deliver high-precision radiotherapy. This enables irradiation of the tumor while sparing the surrounding healthy tissues, potentially allowing for an increased dose to the tumor with smaller margins, a reduction of toxicity and/or a reduction of the number of fractions(60). Regarding the improvement of focal HDR-brachytherapy, the recent literature has shown increasing evidence favoring a fractionated regimen over single-dose treatment. In the primary whole-gland treatment setting, a prospective randomized controlled trial comparing two fractions of 13.5 Gy to a single dose of 19 Gy HDR-brachytherapy revealed a clear 5-year biochemical disease-free survival advantage in the two-fraction arm (95% versus 73.5%)(61). In the focal salvage treatment setting, the evidence is lim- ited to individual cohort studies which show higher estimated 3-year biochemical dis- ease-free survival among multi-fraction studies (±60% versus ±44%), with comparable toxicity rates(62-66). Although these results support the use of fractionated treatment, selection bias may affect this interpretation. Furthermore, there are also disadvantages to fractionated brachytherapy, particularly the logistical aspects of two (or more) im- plant sessions. Here, highly conformal external beam radiotherapy would undoubtedly have procedural advantages over brachytherapy. If adequate tumor tracking becomes possible on the MR-Linac(67), there even is a potential for ultrafocal external beam radiotherapy. The trade-off, however, would likely be less target dose coverage and less conformal dose distributions, with therefore higher doses to the rectum and urethra(68).