Objective The aim of this study was to investigate tumor volume changes with kilovoltage cone-beam computed tomography(k V-CBCT) and their dosimetric consequences for non-operative lung cancer during intensity-modulated radiotherapy(IMRT) or fractionated stereotactic radiotherapy.Methods Eighteen patients with non-operative lung cancer who received IMRT consisting of 1.8–2.2 Gy/fraction and five fractions per week or stereotactic radiotherapy with 5–8 Gy/fraction and three fractions a week were studied. k V-CBCT was performed once per week during IMRT and at every fraction during stereotactic radiotherapy. The gross tumor volume(GTV) was contoured on the k V-CBCT images,and adaptive treatment plans were created using merged k V-CBCT and primary planning computed tomography image sets. Tumor volume changes and dosimetric parameters,including the minimum dose to 95%(D95) or 1%(D1) of the planning target volume(PTV),mean lung dose(MLD),and volume of lung tissue that received more than 5(V5),10(V10),20(V20),and 30(V30) Gy were retrospectively analyzed.Results The average maximum change in GTV observed during IMRT or fractionated stereotactic radiotherapy was –25.85%(range,–13.09% ––56.76%). The D95 and D1 of PTV for the adaptive treatment plans in all patients were not significantly different from those for the initial or former adaptive treatment plans. In patients with tumor volume changes of >20% in the third or fourth week of treatment during IMRT,adaptive treatment plans offered clinically meaningful decreases in MLD and V5,V10,V20,and V30; however,in patients with tumor volume changes of < 20% in the third or fourth week of treatment as well as in patients with stereotactic radiotherapy,there were no significant or clinically meaningful decreases in the dosimetric parameters.Conclusion Adaptive treatment planning for decreasing tumor volume during IMRT may be beneficial for patients who experience tumor volume changes of >20% in the third or fourth week of treatment. Objective The aim of this study was to investigate tumor volume changes with kilovoltage cone-beam computed tomography (k V-CBCT) and their dosimetric consequences for non-operative lung cancer during intensity-modulated radiotherapy (IMRT) or fractionated stereotactic radiotherapy. Methods Eighteen patients with non-operative lung cancer who received IMRT consisting of 1.8-2.2 Gy / fraction and five fractions per week or stereotactic radiotherapy with 5-8 Gy / fraction and three fractions a week were studied. k V-CBCT was performed once per week during IMRT and at every fraction during stereotactic radiotherapy. The gross tumor volume (GTV) was contoured on the k V-CBCT images, and adaptive treatment plans were created using merged k V-CBCT and primary planning computed tomography image sets. Tumor volume changes and dosimetric parameters, including the minimum dose to 95% (D95) or 1% (D1) of the planning target volume (PTV), mean lung dose (MLD), and volume of lung tissue that received more than Results The average maximum change in GTV observed during IMRT or fractionated stereotactic radiotherapy was -25.85% (range, -13.09% - 5 (V5), 10 (V10), 20 -56.76%). The D95 and D1 of PTV for the adaptive treatment plans in all patients were not significantly different from those for the initial or former adaptive treatment plans. In patients with tumor volume changes of> 20% in the third or fourth week of treatment during IMRT, adaptive treatment plans offered clinically meaningful decreases in MLD and V5, V10, V20, and V30; however, in patients with tumor volume changes of <20% in the third or fourth week of treatment as well as in patients with stereotactic radiotherapy, there were no significant or clinically meaningful decreases in the dosimetric parameters. Contact adaptive treatment planning for decreased tumor volume during IMRT may be beneficial for patients who experience tumor volume changes of> 20% in the third or fourth wee k of treatment.