A low modulation factor (MF) maintaining a good dose distribution contributes to the shortening of the delivery time and efficiency of the treatment plan in helical tomotherapy. The purpose of this study was to reduce the delivery time using initial values and the upper limit values of MF. First, patients with head and neck cancer (293 cases) or prostate cancer (181 cases) treated between June 2011 and July 2015 were included in the analysis of MF values. The initial MF value (MFinitial) was defined as the average MFactual value, and the upper limit of the MF value (MFUL) was defined according the following equation: MFUL = 2 × standard deviation of MFactual value + the average MFactual
Next, a treatment plan was designed for patients with head and neck cancer (62 cases) and prostate cancer (13 cases) treated between December 2015 and June 2016. The average MFactual value for the nasopharynx, oropharynx, hypopharynx, and prostate cases decreased from 2.1 to 1.9 (p = 0.0006), 1.9 to 1.6 (p < 0.0001), 2.0 to 1.7 (p < 0.0001), and 1.8 to 1.6 (p = 0.0004) by adapting the MFinitial and the MFUL values, respectively. The average delivery time for the nasopharynx, oropharynx, hypopharynx, and prostate cases also decreased from 19.9 s cm−1 to 16.7 s cm−1 (p < 0.0001), 15.0 s cm−1 to 13.9 s cm−1 (p = 0.025), 15.1 s cm−1 to 13.8 s cm−1 (p = 0.015), and 23.6 s cm−1 to 16.9 s cm−1 (p = 0.008) respectively. The delivery time was shortened by the adaptation of MFinitial and MFUL values with a reduction in the average MFactual for head and neck cancer and prostate cancer cases.