SUMMARY Glioblastoma is one of the most common primary brain tumors in adults and the prognosis is very poor. The standard treatment is gross total resection and postoperative radiotherapy with concurrent and adjuvant chemotherapy with temozolomide. Given its propensity to spread to areas of brain parenchyma surrounding the gross tumor volume (GTV), a generous margin is typically created around the GTV. Peritumoral edema is frequently included in the GTV, especially in the USA. The margin of expansion from GTV to clinical target volume also varies widely among different institutions. There is a lack of consensus as to what constitutes the most appropriate clinical target volume for glioblastoma.