Session Item

Proton arc therapy has dosimetrical gains over conventional intensity modulated proton therapy (IMPT) for diverse indications. A relationship exists between normal tissue complication probability (NTCP), and the number of energy layers (ELs) -and beams in a proton arc plan. This work aims to investigate the number of ELs and beams required for optimal plan quality and impact on NTCP for oropharyngeal cancer patients selected for IMPT.

The RaySearch energy layer reduction algorithm iteratively selects ELs from beams equidistantly spaced over a 360 degree arc. The number of ELs and beams were varied, to determine their relationship with NTCP and the estimated delivery time for three patients. The delivery time was estimated for a “step and shoot” type delivery, in which the gantry remains stationary while the fields are delivered. Proton arc plans with optimized EL reduction settings were generated for ten oropharyngeal cancer patients previously treated with IMPT. Proton arc and clinical IMPT plans were compared in terms of integral dose and NTCP for dysphagia and xerostomia, while target coverage was robust.

Figure 1 illustrates the dependence of NTCP and delivery time on number of ELs K and beams B. We found that 360 ELs distributed over 30 beams generated proton arc plans with minimal NTCP. Relative to corresponding IMPT plans, an average reduction of 21±3% in integral dose was observed. The average NTCP for grade≥2 and grade≥3 xerostomia at six months after treatment decreased with 4.7 ± 1.8% and 2.3 ± 0.8%, respectively, while the average NTCP for grade≥2 and grade≥3 dysphagia decreased with 4.3 ± 2.9% and 0.8 ± 0.4%, respectively. Figure 2 shows the expected reduction in NTCP per patient, when proton arc therapy is employed.

Figure [1]: The dependence of average reduction in plan toxicity with respect to clinical IMPT plans (∆ NTCP), (graph A,B) and average estimated "step and shoot" delivery time (graph C,D) on the number of beams B, in proton arc plans with 360 energy layers (graph A,C) and on the number of energy layers K, in proton arc plans with 30 beams (graph B,D) for 3 oro-pharynx patients. The dashed lines in gaph B represent the average ∆ NTCP when all available enegy layers were used. The red line in graphs C and D show the average delivery time for clinical 6 field IMPT.

Figure [2]: NTCP values at six months after treatment for grade≥2 toxicity (top) and grade≥3 toxicity (bottom) for clinical IMPT and proton arc plans employing 30 beams and 360 energy layers of 10 oropharyngeal patients.

Proton arc therapy demonstrates potential to further reduce toxicity relative to clinical IMPT, especially when 360 ELs and 30 beams are employed for oropharyngeal patients.