In the multidisciplinary management of early-stage extranodal NK/T-cell lymphoma, nasal type (ENKTCL) with curative intent, radiotherapy is the most efficacious modality and an essential component of a combined-modality regimen. In the past decade, utilization of upfront radiotherapy and non-anthracycline-based chemotherapy has improved treatment and the prognosis. This guideline mainly addresses the heterogeneity of clinical features, principles of risk-adapted therapy and the role and appropriate design of radiotherapy. Radiotherapy methods (including target volume definition, dose and delivery methods) are crucial for optimizing cure for patients with early-stage ENKTCL. The application of the principles of involved site radiation therapy (ISRT) in this lymphoma entity often leads to a more extended clinical target volume (CTV) than in other lymphoma types because it usually presents with primary tumor invasion, multifocal lesions, or extensive submucosal infiltration beyond the macroscopic disease. The CTV varies across different primary sites, and is classified mainly into nasal, non-nasal upper aerodigestive tract (UADT), and extra-UADT entities. This review is a consensus of the International Lymphoma Radiation Oncology Group regarding the approach to radiotherapy, target-volume definition, optimal dose, and dose constraints in ENKTCL treatment.