The investigators propose to conduct a randomised study on cT2, cT3a-b tumours less than 5 cm using two different techniques of radiotherapy boost following neoadjuvant chemoradiotherapy (nCRT) (CAP45): EBRT (9 Gy/5 fractions) or CXB (90 Gy/3 fractions). The endpoint will be organ preservation at 3 years without non-salvageable local pelvic recurrence. The proof of this concept will be of most benefit for all patients but especially for the elderly who usually are not fit for or keen to undergo major surgery.