Gantry-period-guided modulation factor selection in helical tomotherapy: effects on delivery efficiency, dosimetry and verification accuracy.

PURPOSE: To explore a gantry-period-guided strategy for identifying a lower useful modulation factor (MF) in helical tomotherapy under fixed field width and pitch. METHODS: Twenty previously treated cases from head, head-and-neck, abdomen and pelvis were replanned with a field width of 2.5 cm and a pitch of 0.287. MF was initialized at 5.0 and reduced by 0.2 with unchanged objectives. At each level, plans were reoptimized and clinically reviewed. MFlimit was defined as the last clinically acceptable MF before the step-down sequence entered the mechanical gantry-period plateau (~11.8 s); MFmin was the lowest clinically acceptable MF identified after further reduction. Gantry period, delivery time, leaf-open-time (LOT) metrics, dosimetric indices and pretreatment gamma pass rates were compared. RESULTS: All cases entered the efficiency-limited pathway. MFlimit ranged from 1.9 to 2.8. MF reduction produced a biphasic gantry-period response, with an initial decrease followed by a mechanical plateau. Further reduction from MFlimit to MFmin did not meaningfully reduce delivery time. LOTmax decreased and LOTmean remained relatively stable, whereas Tgap, defined as the temporal gap between the available projection time and LOTmax, increased markedly at MFmin, indicating unused projection time after plateau entry. Compared with MFmin, MFlimit was associated with improved PTV homogeneity and conformity, more favorable exploratory pooled OAR dose trends, and higher 1%/1 mm gamma pass rates, while 3%/2 mm and 2%/2 mm pass rates were similar. CONCLUSIONS: The MF immediately before gantry-period plateau entry may provide a practical reference point for limiting MF reduction. In this pilot cohort, this gantry-period-guided approach helped maintain plan quality and avoid reductions in modulation that offered little efficiency benefit. Further studies across additional planning configurations and disease sites will be needed to confirm its broader applicability.