This study presents a validated approach for accurate retrospective dose reconstruction in carbon-ion radiotherapy (CIRT) using Monte Carlo (MC) simulation. The primary objective is to enable precise estimation of both in-field and out-of-field dose distributions from historical patient data, particularly where conventional treatment planning systems (TPS) lack detailed information on secondary particle transport. A key challenge lies in converting CT-number values into physically realistic materials that accurately reflect elemental composition and mass density.
We propose a two-step material assignment method grounded in the Hounsfield unit–stopping power ratio (HU-SPR) curve implemented within the clinical TPS. First, standard human tissues—air, lung, fat, adipose, soft tissue, muscle, bone-scapula, bone-mineral, and tooth—are mapped to predefined CT-number intervals based on ICRP Publication 110 reference phantoms. Second, the mass density of each material is derived directly from the TPS’s calibrated HU-SPR curve, ensuring that the stopping power of incident carbon ions matches the original treatment plan exactly.Hexamidine diisethionate Formula This eliminates uncertainties caused by scanner-specific calibration differences, beam energy variations, or image reconstruction protocols, allowing faithful reproduction of past clinical treatments.
The method was evaluated using PHITS 3.02 simulations on homogeneous and heterogeneous phantoms irradiated with a 400 MeV/u carbon beam featuring an 8 cm spread-out Bragg peak (SOBP). Depth-dose distributions were computed and compared against those generated by the TPS (XiO-N, Elekta). Results showed excellent agreement: peak position deviations were within ±1 mm across all tested materials, and d50 positions differed by less than 1.6 mm. Gamma index analysis under stringent criteria (1 mm distance-to-agreement, 3% dose difference) revealed pass rates of 97.5% for the proposed method and 99.8% for the varying-density-water approach—both exceeding the typical clinical acceptance threshold of 95%.
While in-field dose agreement was strong, significant discrepancies emerged in secondary particle fluences. The varying-density-water method underestimated nuclear interaction probabilities in high-CT-number materials such as bone-mineral, resulting in up to 2% lower C-12 fluence and reduced yields of protons, helium, and neutrons. In contrast, the proposed method, which uses real elemental compositions from ICRP Publication 110, correctly captures these interactions.EDNRA ProteinSynonyms For example, neutron fluence behind a bone slab was overestimated by more than 10% when using water-equivalent models due to incorrect backscattering behavior.PMID:34985677 Similarly, the spatial distribution of secondary particles showed marked differences in regions with abrupt density changes.
These results demonstrate that while water-equivalent models may suffice for basic in-field verification, they are inadequate for detailed assessments involving radiation-induced biological effects, especially secondary cancer risk. The proposed method enables reliable prediction of out-of-field doses, including contributions from neutrons and light fragments, which are critical for long-term risk modeling.
This validated framework will be integrated into a new MC-based dosimetry system at the National Institute of Radiological Sciences (NIRS), supporting large-scale retrospective analysis of historical CIRT patient data. It provides a robust tool for independent treatment verification, algorithm benchmarking, and epidemiological research. Future work will focus on enhancing artifact handling for metallic implants, improving noise resilience, and enabling automated integration with clinical databases to streamline the re-evaluation process.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com