S1640
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Results: As seen in Figure 2, the Advanced Markus chamber exhibited kB,M,Q values consistent with previous measurements for monoenergetic beams [2]. Notably, for SOBPs mimicking clinical radiation fields, kB,M,Q approached unity with no statistically significant deviation, suggesting reduced MF perturbation at low proton energies as found in modulated beam configurations. In contrast, the microDiamond detector showed no significant MF-induced response changes across all beam energies (both mono- energetic and SOBP) and MF strengths, demonstrating its MF strength independence and superior suitability for dosimetry and PSQA in MRiPT.
comprehensive correction factors for MR-integrated proton therapy (MRiPT) remain limited, particularly for spread-out Bragg peak (SOBP) configurations relevant to patient-specific quality assurance (PSQA). Plane- parallel ionization chambers (e.g., the Advanced Markus chamber) are standard in proton dosimetry but can exhibit significant MF-induced response changes. The microDiamond detector, a solid-state detector, offers the possibility of being unaffected by MFs, reducing dosimetry workflow complexity. This study compares MF correction factors for Advanced Markus chamber and microDiamond detector in monoenergetic and SOBP proton radiation fields to assess microDiamond detector suitability for MRiPT dosimetry. Material/Methods: Both detectors were irradiated simultaneously in a water phantom placed inside a dedicated in-beam electromagnet at proton beam energies of 102.5 MeV and 223.4 MeV, and in an SOBP configuration (10×10 cm²) under MF strengths of 0 T, 0.32 T, and 0.5 T. Detectors were positioned at 2 cm water depth (Figure 1). The Advanced Markus chamber served as reference for cross-calibration of the microDiamond detector. Based on previous work [2], the dose conversion factor (cB = DB/D) was assumed to be unity. Two key parameters were evaluated: (1) change in detector response, calculated as the ratio of signal with and without MF (kB,M = M/MB) and (2) MF correction factor, (kB,M,Q = (cB)·(M/MB)) which corrects for MF- induced detector response changes to enable accurate dosimetry in MRiPT.
Conclusion: This study presents the first direct comparison of Advanced Markus chamber and microDiamond detector for MRiPT dosimetry using monoenergetic radiation fields and clinically realistic SOBPs. The microDiamond's MF independence and stable response across MF strengths up to 0.5 T demonstrate its suitability for PSQA. The measured correction factors can be applied in future MRiPT dosimetry performed at different centers. References: [1] Fuchs H, et al. MR-guided proton therapy: Impact of magnetic fields on the detector response. Med Phys. 2021;48(5):2572-2579.[2] Gebauer B, et al. Proton dosimetry in a magnetic field: Measurement and
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