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Purpose <p>This study evaluated the feasibility of magnetic resonance imaging (MRI)-based transverse relaxation rate (R₂) mapping for three-dimensional (3D) dose verification of N-isopropylacrylamide (NIPAM) polymer gel dosimeters under proton irradiation and sought to identify an optimal gel formulation.</p> Methods <p>Three formulations were prepared with different ratios of gelatin: N,N’-methylene bis-acrylamide: tetrakis (hydroxymethyl) phosphonium chloride, denoted as 5-3-3-10, 5-5-3-5, and 5-5-3-10. Cylindrical gels were irradiated on a Varian ProBeam<sup>®</sup> 360° proton system with doses from 0 to 7.25&#xa0;Gy. The NIPAM polymer gel was scanned using a 3 T MRI system 96&#xa0;h after irradiation to obtain R₂–dose calibration curves and depth–dose profiles, which were subsequently compared with ion chamber measurements and treatment-planning system (TPS) calculations, respectively. The best-performing formulation underwent additional testing under a spread-out Bragg peak (SOBP) at depths of 5, 10, and 15&#xa0;cm and was assessed for temporal stability on days 1, 3, 6, and 8.</p> Results <p>All formulations showed excellent linearity between R₂ and absorbed dose (all R² &gt; 0.98, <i>P</i> &lt; 0.05). The 5-5-3-10 gel exhibited the closest agreement with TPS depth–dose data and maintained stable R₂–dose responses for up to 3 days post-irradiation, after which minor deviations were observed. Depth-dose curves reproduced the Bragg-peak shape, with a mild under-response in the high linear energy transfer distal region.</p> Conclusion <p>MRI-read 5-5-3-10 NIPAM gels demonstrated high sensitivity, reproducibility, and temporal stability, confirming their suitability for quantitative 3D proton-beam dosimetry. These results provide a strong foundation for further refinement and future clinical implementation of MRI-based polymer gel verification.</p>

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Optimization and Evaluation of MRI-Read NIPAM Polymer Gel Dosimeters for 3D Proton Beam Dose Verification

  • 期刊论文

  • 发表日期：

    2026年2月18日

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  • Yi-Fang Zeng,
  • An-Cheng Shiau,
  • Li-Wei Kuo,
  • Shin-Lei Peng,
  • Chun-Hsu Yao

摘要

Purpose

This study evaluated the feasibility of magnetic resonance imaging (MRI)-based transverse relaxation rate (R₂) mapping for three-dimensional (3D) dose verification of N-isopropylacrylamide (NIPAM) polymer gel dosimeters under proton irradiation and sought to identify an optimal gel formulation.

Methods

Three formulations were prepared with different ratios of gelatin: N,N’-methylene bis-acrylamide: tetrakis (hydroxymethyl) phosphonium chloride, denoted as 5-3-3-10, 5-5-3-5, and 5-5-3-10. Cylindrical gels were irradiated on a Varian ProBeam® 360° proton system with doses from 0 to 7.25 Gy. The NIPAM polymer gel was scanned using a 3 T MRI system 96 h after irradiation to obtain R₂–dose calibration curves and depth–dose profiles, which were subsequently compared with ion chamber measurements and treatment-planning system (TPS) calculations, respectively. The best-performing formulation underwent additional testing under a spread-out Bragg peak (SOBP) at depths of 5, 10, and 15 cm and was assessed for temporal stability on days 1, 3, 6, and 8.

Results

All formulations showed excellent linearity between R₂ and absorbed dose (all R² > 0.98, P < 0.05). The 5-5-3-10 gel exhibited the closest agreement with TPS depth–dose data and maintained stable R₂–dose responses for up to 3 days post-irradiation, after which minor deviations were observed. Depth-dose curves reproduced the Bragg-peak shape, with a mild under-response in the high linear energy transfer distal region.

Conclusion

MRI-read 5-5-3-10 NIPAM gels demonstrated high sensitivity, reproducibility, and temporal stability, confirming their suitability for quantitative 3D proton-beam dosimetry. These results provide a strong foundation for further refinement and future clinical implementation of MRI-based polymer gel verification.