A novel energy sequence optimization algorithm for efficient spot-scanning proton arc (SPArc) treatment delivery

Spot-scanning proton arc therapy (SPArc) has been proposed to improve dosimetric outcome and to simplify treatment workflow. To efficiently deliver a SPArc plan, it’s crucial to minimize the number of energy layer switches (ELS) a sending because of the magnetic hysteresis effect. In this study, we introduced a new SPArc energy sequence optimization algorithm (SPArc_seq) to reduce ascended ELS and to investigate its impact on the beam delivery time (BDT). An iterative energy layer sorting and re-distribution mechanism following the direction of the gantry rotation was implemented in the original SPArc algorithm (SPArc_orig). Five disease sites, including prostate, lung, brain, head neck cancer (HNC) and breast cancer were selected to evaluate this new algorithm. Dose-volume histogram (DVH) and plan robustness were used to assess the plan quality for both SPArc_seq and SPArc_orig plans. The BDT evaluations were analyzed through two methods: 1. fixed gantry angle delivery (BDT_fixed) and 2. An in-house dynamic arc scanning controller simulation which considered of gantry rotation speed, acceleration and deceleration (BDT_arc). With a similar total number of energy layers, SPArc_seq plans provided a similar nominal plan quality and plan robustness compared to SPArc_orig plans. SPArc_seq significantly reduced the number of ascended ELS by 83% (19 <i>vs.</i>115), 70% (16 <i>vs.</i> 64), 82% (19 <i>vs.</i> 104), 80% (19 <i>vs.</i> 94) and 70% (9 <i>vs.</i> 30), which effectively shortened the BDT_fixed by 65% (386 <i>vs.</i> 1091 s), 61% (235 <i>vs.</i> 609 s), 64% (336 <i>vs.</i> 928 s), 48% (787 <i>vs.</i>1521 s) and 25% (384 <i>vs.</i> 511 s) and shortened BDT_arc by 54% (522 <i>vs.</i>1128 s), 52% (310 <i>vs.</i>645 s), 53% (443 <i>vs.</i> 951 s), 49% (803 <i>vs.</i>1583 s) and 26% (398 <i>vs.</i> 534 s) in prostate, lung, brain, HNC and breast cancer, respectively. The SPArc_seq optimization algorithm could effectively reduce the BDT compared to the original SPArc algorithm. The improved efficiency of the SPArc_seq algorithm has the potential to increase patient throughput, thereby reducing the operation cost of proton therapy.