KEY POINTS
- Investigators retrospectively replanned 20 patients—five each from head, head-and-neck, abdomen and pelvis sites—using helical tomotherapy with a fixed 2.5-cm field width and pitch 0.287. Modulation factor was initialized at 5.0 and then reduced in steps of 0.2 with identical planning objectives.
- Every plan was reviewed for clinical acceptability, including PTV V95 ≥95%, Dmax ≤110% of prescription and site-specific organ-at-risk constraints. The proposed MFlimit was the last acceptable modulation factor immediately before the gantry reached its mechanical minimum period of approximately 11.8 seconds.
- All 20 cases reached this efficiency-limited pathway. Median modulation factor fell from 5.0 to 2.3 at MFlimit and 1.8 at MFmin, while median gantry period dropped from 26.9 to 12.6 and 11.8 seconds, respectively.
- Most of the useful efficiency gain had therefore already occurred by MFlimit. Median delivery time was 9.6 min at MF 5.0, 5.3 min at MFlimit and 5.0 min at MFmin—only about 18 seconds of additional median saving once the plan was pushed into the mechanical plateau.
- Leaf-open-time analysis explains why. Maximum leaf-open time declined from 449.9 to 247.4 to 194.2 ms, but mean leaf-open time remained near 123–125 ms. Once below MFlimit, the unused interval between the longest leaf opening and the fixed projection window jumped from 0.8 to 37.3 ms, meaning the extra reduction created idle time rather than faster gantry motion.
- Plan quality subtly favored stopping before the plateau. Boost-PTV homogeneity index was 0.04 versus 0.05 (p=0.010) and conformity index 0.79 versus 0.77 (p=0.012) for MFlimit versus MFmin. Exploratory pooled OAR doses were also slightly lower at MFlimit.
- Standard QA did not distinguish the plans: 3%/2-mm gamma pass rates were 99.5% versus 99.1% (p=0.943). Under the much stricter 1%/1-mm criterion, however, pass rates were 85.3% versus 79.4% (p=0.014) in favor of MFlimit. The study used one planning system, one field-width/pitch combination and only 20 cases, so the numerical threshold cannot be generalized to every tomotherapy workflow.
CLINICAL TAKEAWAY
The practical message is simple: once gantry period hits its mechanical floor, further reduction in modulation factor can sacrifice modulation without making treatment meaningfully faster. The pre-plateau point offers an intuitive patient-specific stopping rule, but it needs external validation across different field widths, pitches and planning systems before becoming a general standard.