Development and dosimetric verification of static SHArc: Step-and-shoot carbon ion arc therapy for LETd escalation in pancreatic tumors.

BACKGROUND: Carbon ion radiotherapy (CIRT) offers higher linear energy transfer (LET) and superior relative biological effectiveness, making it a promising option for treating hypoxic, radioresistant tumors. Spot-scanning Hadron Arc (SHArc) therapy enables dose-averaged LET (LETd) escalation in the tumor but increases planning and setup complexity. Dynamic delivery remains impractical for gantry-based carbon ion arc therapy due to the system's large size and complex control requirements. Step-and-shoot delivery, while less efficient, provides a feasible alternative and represents a key step towards clinical SHArc implementation. PURPOSE: This work establishes the first step-and-shoot planning and delivery technique for carbon ion arc therapy (static SHArc) at the Heidelberg Ion-beam Therapy Center using the gantry system. Static SHArc therapy is evaluated in terms of plan quality and delivery feasibility for pancreatic cancer. METHODS: Static SHArc plans were optimized for seven pancreatic cancer cases, considering 20 gantry angles. Two distinct energy layer (EL) selection techniques were investigated: (1) Central EL, selecting the seven central ELs per beam, and (2) MU-Based EL, prioritizing ELs contributing the highest monitor units (MU). LETd optimization was performed to escalate the minimum LETd to ∼50-80 keV/µm within the gross tumor volume. Static SHArc plans were compared against conventional IMPT using two single-field optimized posterior oblique beams (2-SFO), in terms of dose conformality, LETd, and robustness against setup (5 mm) and range (1.5%) uncertainties. Inter-fractional robustness was assessed via forward dose calculation on daily control CT scans. Dosimetric validation and delivery time verification for static SHArc using the heavy ion gantry system were conducted via end-to-end testing with ion chamber and film measurements in a cylindrical PMMA phantom. RESULTS: Static SHArc plans improved LETd distributions in the tumor without compromising target coverage and clinical OAR constraints. The MU-based EL method increased minimum target dose, whereas Central EL enabled higher LETd concentration in the tumor center. Both energy selection methods for static SHArc exhibited reduced inter-fractional robustness compared to the two-SFO. Dosimetric verification showed deviations < 3% and total delivery time was ∼27 min. CONCLUSIONS: This study investigates static SHArc, a step-and-shoot approach for delivering carbon ion arc therapy. While static SHArc can provide dosimetric advantages, particularly in terms of LETd distribution, EL selection plays a key role. Improving inter-fraction robustness remains crucial for clinical implementation.