Phenomenological Study of Intra-Spill Break Spots in Dose-Driven Continuous Scanning Proton Therapy

Dose Driven Continuous Scanning proton therapy offers reduced beam delivery time compared to discrete spot scanning but requires accurate characterization of temporal beam parameters for precise dose calculation. This study quantified rise time ( $t_{\mathrm{rise}}$ ) and plateau beam current ( $I_{0,\mathrm{measured}}$ ) variations across a complete spill cycle and assessed their impact on average beam current ( $I_{\mathrm{avg}}$ ) per spot.

Beam current measurements were performed using a CROSSmini 2D detector array at 50 μs temporal resolution across 3 proton energies (70.2, 150.2, and 228.7 MeV) and 3 nominal beam currents (8, 14, and 20 MU/s) with break points implemented at every spot. Piecewise function fitting was applied to extract $t_{\mathrm{rise}}$ and $I_{0,\mathrm{measured}}$ parameters for each spot across complete spill durations. Sensitivity analysis was conducted using partial derivatives and experimental validation to quantify the relative impact of the temporal parameter variations on $I_{\mathrm{avg}}$ . Moving averages over 20-spot windows were calculated to characterize intra-spill parameter changes.

$I_{0,\mathrm{measured}}$ observed a decreasing trend throughout spills before a steep terminal decline. Mean $t_{\mathrm{rise}}$ was 1.43 ± 0.71 ms across all conditions. Sensitivity analysis revealed $I_{\mathrm{avg}}$ exhibited substantially higher sensitivity to $t_{\mathrm{rise}}$ variations than $I_{0,\mathrm{measured}}$ changes, with $t_{\mathrm{rise}}$ variations producing up to 80% changes in $I_{\mathrm{avg}}$ compared to <2% for $I_{0,\mathrm{measured}}$ variations across spill.

We have characterized $t_{\mathrm{rise}}$ and $I_{0,\mathrm{measured}}$ within a spill and the impact of the variation of these parameters on $I_{\mathrm{avg}}$ . These findings provide an essential basis for improving dose calculation accuracy in DDCS delivery.