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Background: Beam-on time in Total Marrow Irradiation (TMI) delivery with helical tomotherapy is more than 30 minutes. The purpose of this study was to investigate extended time output variation in tomotherapy machine without dose servo system and its impact on the dosimetry of TMI planning. Materials and methods: The calibration procedures with 1800 seconds delivery were conducted. The slab and cylindrical phantoms were used for static and rotational output variation measurements, respectively. All measurements were performed in 0.1 second interval with an Exradin A1SL ionization chamber (Standard Imaging Inc., Madison, WI, USA) connected to the tomoelectrometer supplied by the manufacture. Simulated TMI treatment planning with a slab phantom was delivered and verified with ion chamber and EDR-2 films. Results: The static output variations during 30 min averaged -2.9% ± 0.2%, -3.4% ± 0.3%, and -3.4% ± 0.3% at 10 min, 20 min, and 30 min, respectively. The rotational output variations from start averaged -2.5% ± 0.7%, -3.1% ± 0.7%, and -3.5% ± 0.8% at 10 min, 20 min, and 30 min, respectively. The maximum output variation was up to 4.5%. In a TMI planning model, in which beam-on time was over 30 min, planned dose and dose measured with ion chambers in both cranial and caudal sides agreed within 3%. Film measurements in cranial and caudal sides also showed the pass rates of 97.7% and 92.2% with the criteria of 3 mm/3% in gamma analysis. Conclusion: These results suggest that long TMI delivery by helical tomotherapy, even without dose servo system, does not pose a risk for significant deviations from the original treatment plan regardless of the output variation. However, very long time output variation should be checked before the first treatment. [ABSTRACT FROM AUTHOR]