3D range modulators for fast, conformal carbon ion therapy: anthropomorphic phantom validation and robustness analysis

<i>Objective</i>. Fast and precise delivery of ion-beam therapy is essential for improving clinical throughput and intrafractional motion management, yet synchrotron-based systems require multiple energy layers for depth dose coverage, resulting in delays on the order of minutes. To eliminate energy layer switching times, a fast Monte Carlo (MC)-based workflow for patient-specific 3D range modulators was developed to enable monoenergetic, conformal carbon irradiation at clinically viable speeds. To mirror realistic clinical use, the dosimetric impact of setup and RM geometry deviations from simulated models were assessed.<i>Approach</i>. The workflow begins with spot extraction from clinical intensity modulated particle therapy (IMPT) plans, followed by RM geometry optimization, fast MC dose verification using MonteRay, and 3D printing final geometries. Experimental validations were performed for spread-out Bragg peaks (SOBPs) in water, and two targets in an anthropomorphic head phantom: (1) in a homogeneous brain region and (2) across a heterogeneous bone-soft tissue interface. Robustness against realistic setup and printing errors were assessed in the heterogeneous case.<i>Main results</i>. Each RM geometry was optimized in under one minute and the RM-based plans achieved dose distributions comparable to IMPT with similar target coverage and homogeneity. Simulated and measured depth dose profiles for SOBP plans agreed within 1.2% local deviation in the target. In the head phantom, measured 2D dose maps achieved local gamma pass rates >99% (2%/2 mm, 10% threshold) in both uniform and anatomically complex settings. Plans were robust to setup deviations up to 1 mm, and manufacturing deviations up to 100<i>µ</i>m.<i>Significance</i>. This rapid, clinically feasible workflow enables conformal, monoenergetic carbon ion delivery with dosimetric quality comparable to IMPT even in heterogenous scenarios. The substantially reduced treatment delivery time facilitates motion mitigation and higher patient throughput, and may also provide a technical basis for exploring FLASH regimes in synchrotron-based ion beam facilities.