Research Article: Gantry-period-guided modulation factor selection in helical tomotherapy: effects on delivery efficiency, dosimetry and verification accuracy
Abstract:
To explore a gantry-period-guided strategy for identifying a lower useful modulation factor (MF) in helical tomotherapy under fixed field width and pitch.
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. MF limit was defined as the last clinically acceptable MF before the step-down sequence entered the mechanical gantry-period plateau (~11.8 s); MF min 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.
All cases entered the efficiency-limited pathway. MF limit 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 MF limit to MF min did not meaningfully reduce delivery time. LOT max decreased and LOT mean remained relatively stable, whereas T gap , defined as the temporal gap between the available projection time and LOT max , increased markedly at MF min , indicating unused projection time after plateau entry. Compared with MF min , MF limit 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.
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.
Introduction:
Helical tomotherapy (HT) is an established modality for intensity-modulated radiotherapy, characterized by continuous gantry rotation, couch translation, and binary multileaf collimator modulation. During delivery, radiation fluence is modulated through leaf open times (LOTs) across multiple projections, enabling highly conformal target coverage and organ-at-risk (OAR) sparing ( 1 ). In HT treatment planning, plan quality and delivery efficiency are strongly influenced by machine-related parameters, mainly field…
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