Complex sentence: The role of radiotherapy after potentially curative surgery (so as adjuvant therapy) has been controversial since the 1980s, and the European Society for Medical Oncology recommends that this should only be used for people in clinical trials.
Split into simpler sentences: The role of radiotherapy as an auxiliary (adjuvant) treatment after potentially curative surgery has been controversial since the 1980s. The European Society for Medical Oncology recommends that adjuvant radiotherapy should only be used for people enrolled in clinical trials.