Picking pitch, modulation factor and field width

A 450-plan study across five head-and-neck patients mapped the trade-off directly. For a 5 cm field width the Pareto-optimal region is pitch 0.23–0.45 with modulation factor above 2.0, which corresponded to treatment times from 4 min 05 s down to 2 min 45 s.

Pitch is not free to choose. The standard rule is pitch = 0.86 / n for integer n, which suppresses the thread effect — the helical ripple you get when the couch advance and the beam width beat against each other. That is why the common vendor setting is 0.430 with MF ≈ 3.0: it is 0.86/2.

Modulation factor behaves differently either side of pitch 0.20.

• At pitch ≤ 0.20 the gantry is already at maximum speed, so MF does not change treatment time at all. Lowering it only costs you plan quality, for nothing.
• At pitch ≥ 0.25 lowering MF shortens the treatment and degrades the plan, until maximum gantry speed is reached.

The minimum useful MF for a given pitch fits MF = 20·pitch² − 21·pitch + 6.8 over pitch 0.25–0.50.

Dynamic jaws are a time feature, not a quality feature. TomoEdge cut treatment time by nearly half against a 2.5 cm field width, but produced no improvement in plan quality over that same 2.5 cm plan, because the jaws only move at the edges of the target.