<h3>Purpose/Objective(s)</h3> It is commonly accepted that reduced volumes and decreased doses has the same efficacy as large volumes and high doses in HL and majority of NHLs. However, there are no relative studies in nasal NK/T cell lymphoma to date. We conducted a cohort study of the local control rates and safety of this therapy to evaluate the dosimetric and clinical outcomes of reduced dose intensity-modulated radiotherapy for patients with nasal NK/T cell lymphoma. <h3>Materials/Methods</h3> Twenty-nine patients with nasal NK/T cell lymphoma were prospectively collected and reviewed. Seventeen patients had Stage I disease, 9 patients had Stage II disease and 3 patients had stage IV with oligometastasis. Except that four patients present with lesions in the Waldeyer ring, the others originate from the nasal cavity and the majority (18/25) with primary tumor invasion. All patients received combination chemotherapy and radiotherapy except for 5 patients treated with radiotherapy alone. New chemotherapy regimens were used mainly gemcitabine-based or L-asparaginase-based regimens. Radiotherapy included extended involved-field (PTV: planning target volume) encompassing the primary tumor and adjacent regions and cervical lymph nodes were included when Waldeyer ring or cervical lymph nodes were involved. Dose was reduced to 45 Gy and the prescribed radiation dose to primary tumor was 50Gy which is different from our routine prescription (median dose was 50Gy for PTV). <h3>Results</h3> For patients with only nasal cavity involved, doses to both lens and optic nerves have reduced compared with 50 Gy to PTV. The median maximum doses to the lens and optic nerves were 617 cGy and 45.1 Gy, respectively. For patients with Waldeyer ring or cervical lymph nodes involved, the mean doses to parotid were 24.3Gy. After a median follow up of 19.0 months, the local control rate for the entire cohort was 100%. The 2-year overall survival and local control rates were 90.7% and 100%, respectively. No Grade 4 or 5 acute toxicities were reported. <h3>Conclusion</h3> Reduced dose to 45 Gy for extended involved-field encompassing the primary tumor and adjacent regions with or without cervical lymph nodes has excellent local control. Longer follow-up time is required.