The field of microdosimetry continues to demonstrate remarkable vitality and expansion.1 Building upon the momentum established by the Second International Workshop on Microdosimetry held in November 2024,2 the Third International Workshop convened on June 3, 2025, as a satellite event of the Sixty-third Annual Conference of Particle Therapy Co-Operative Group (PTCOG 63) in Buenos Aires, Argentina (https://ptcog63.org/advancing-microdosimetry-in-particle-therapy/). This gathering marked a significant milestone: the first time this workshop series was held in Latin America, reflecting the truly global nature of the microdosimetry community and its growing relevance to particle therapy worldwide. The workshop, themed “Bridging Physics and Biology,” brought together researchers from institutions globally, representing the full spectrum of microdosimetry research—from fundamental detector development to clinical implementation. The organizing committee itself embodied this international spirit, with members from Austria, Italy, the United States, Australia, Argentina, Japan, and the United Kingdom. The workshop program included thirteen accepted abstracts and one invited presentation on microdosimetry in Boron Neutron Capture Therapy (BNCT). Twelve talks were delivered during the session, with all accepted contributions included in the collection of abstracts. The presentations were organized around several interconnected themes: relative biological effectiveness (RBE) modeling and standardization, detector technology advances, computational tools, and expanding clinical applications. The workshop opened with two presentations by Hartzell et al. addressing fundamental questions in carbon ion therapy RBE calculation. The first study examined how different microdosimetric formalisms, including the Analytical Microdosimetric Function and Kiefer-Chatterjee radial dose distributions, influence RBE predictions, finding variations of up to 10% depending on the calculation method and domain geometry. The second presentation supported microdosimetry as a practical quality assurance tool for carbon ion therapy, demonstrating that tissue-equivalent proportional counter (TEPC) measurements can estimate RBE across major models (MKM, LEM, RMF) within approximately 5% accuracy. New radiobiological models featured prominently in the program. Valeriano et al. introduced the Continuous Microdosimetric Photon Isoeffective Dose Model (COMPHID), which parametrizes survival model coefficients using the first three lineal energy moments from microdosimetric spectra. Combined measurements with a Mini-TEPC and lung cancer H460 cell survival experiments at the Trento Proton Therapy Center validated this approach. Battestini et al. presented the MS-GSM2 model, a mechanistic framework that extends microdosimetry to clinically relevant endpoints including tumor control probability and normal tissue complication probability, with validation across protons, helium, and carbon ions under both conventional and ultra-high dose rate irradiation conditions. Computational efficiency for clinical implementation was addressed by Cartechini et al., who presented a systematic benchmark of fast microdosimetry algorithms including Super Track, MONAS, the Analytical Microdosimetric Function, and specific energy look-up tables. This work provides practical recommendations for selecting appropriate computational methods based on accuracy requirements and available resources. The fundamental question of radiation quality characterization was presented by Hamad et al., who compared lineal energy measurements from a silicon-on-insulator microdosimeter with linear energy transfer (LET) measurements from a Timepix3 detector in helium-ion beams. This study, conducted in collaboration between Heidelberg and MedAustron, contributes to ongoing efforts to standardize radiation quality measurements for clinical integration. Detector technology advances spanned multiple presentations. Tran et al. presented 25 years of silicon microdosimeter development at the Centre for Medical Radiation Physics, University of Wollongong, culminating in the new MicroPlusTM2 probe designed for routine clinical LET and RBE-weighted dose quality assurance. Mercado et al. presented their development of novel PIN diode-based microdosimeters in both silicon and gallium arsenide (GaAs), with GaAs offering advantages in radiation hardness and performance under high-dose-rate environments. Romano et al. described a general-purpose Geant4-based Monte Carlo simulation platform for microdosimetric spectra evaluation, integrating models of multiple detector types to provide an open-source, validated tool for the medical physics community. The role of secondary fragments in proton therapy was highlighted by Parisi et al., who demonstrated that while fragments account for less than 0.5% of total events, they contribute over 60% of the dose-mean lineal energy, underscoring their importance for accurate radiation quality assessment despite their rarity. Applications beyond conventional particle therapy were explored through presentations on BNCT. Santa Cruz discussed microdosimetry principles and applications in BNCT, while Selva et al. described developments in miniaturized TEPCs with boron-doped cathode walls for clinical BNCT applications, including work within the EU-funded ANTHEM project. Bianchi et al. introduced the AtoMiQA project, developing a dual-mode TEPC system capable of both single-event and multi-event operation to extend microdosimetric measurements into high-flux clinical environments. The workshop concluded with forward-looking discussions on standardization and clinical quality assurance. Magrin proposed a structured approach to community standardization, using the stochastic nature of radiation interactions as a first “test case” for building consensus through open discussion. The Third Workshop reinforced several themes that have emerged as priorities for the microdosimetry community. First, the field is actively moving toward clinical translation, with multiple presentations addressing quality assurance tools, treatment planning integration, and practical measurement solutions for clinical environments. Second, computational advances are enabling faster and more accurate microdosimetric calculations, essential for routine clinical implementation. Third, the development of novel detector technologies, including solid-state devices in silicon, diamond, and gallium arsenide, continues to expand the capabilities and practicality of microdosimetric measurements. Fourth, the growing international engagement with microdosimetry, reflected in the diverse institutional representation among both organizers and contributors, strengthens the field by fostering collaboration and bringing the diverse perspectives essential for addressing the complex challenges of clinical implementation. The call for standardization, first articulated at the Second Workshop,2 gained further momentum and culminated in a concrete action plan. A key outcome of the Third Workshop discussions was the decision to develop a white paper establishing best practice guidelines for experimental microdosimetry studies. While the recent ICRU Report 981 provides a solid foundation for the theoretical and conceptual aspects of microdosimetry, there remains a critical gap in practical guidance on reporting experimental and Monte Carlo data in a standardized and reproducible manner. The proposed guidelines will present a structured framework for minimum information reporting across detector specifications, radiation field characterization, acquisition settings, data processing, calibration, and uncertainty analysis. By promoting transparency, comparability, and confidence in microdosimetric data, these recommendations will support reproducibility, facilitate inter-laboratory comparisons, and accelerate the translation of microdosimetry into radiobiological modeling and clinical treatment planning. The workshop theme of “Bridging Physics and Biology” aptly captured the current state of the field. Presentations ranged from fundamental physics of energy deposition to mechanistic biological models predicting clinical outcomes, illustrating how microdosimetry serves as the crucial link connecting physical measurements to biological effects and ultimately to improved patient outcomes in particle therapy. The continued growth and vitality of microdosimetry, now evident across three successful international workshops, confirms the field's transition from a specialized research discipline to an essential component of modern particle therapy. As treatment planning systems evolve beyond constant RBE assumptions and as new particle species and delivery modalities emerge, microdosimetry will play an increasingly central role in ensuring safe and effective treatments. The organizing committee thanks all speakers and participants for their contributions to the workshop. We are grateful to the PTCOG 63 organizing committee for hosting this satellite event in Buenos Aires, and we acknowledge the support of the institutions represented in making this international collaboration possible. The authors declare no conflict of interest.
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