Treatment Planning Systems (TPS) for proton therapy are beginning to evolve beyond the conventional use of a constant Relative Biological Effectiveness (RBE) value of 1.1. Inspired by the approach adopted in carbon ion therapy, variable RBE models are being increasingly integrated. This paradigm shift acknowledges the influence of the complex, mixed radiation fields within the patient—comprising different particle types and energies—on both RBE and the resulting biological dose. As a consequence, accurate modeling of radiation quality should be incorporated into TPS, and experimental verification, where feasible, should be included in routine Quality Assurance (QA) protocols. To date, no detector systems are available that can routinely and reliably verify biological dose-related parameters with high precision. Similar to physical dosimetry, there is a growing need for dedicated detectors to assess biological effects. Microdosimetry offers a compelling solution by providing access to single-event energy deposition fluctuations on subcellular scales, which are directly related to RBE and radiation quality. Despite significant progress in R&D, existing microdosimeters still fall short of the demands of clinical application. These include high sensitivity, simulated volumes that are sufficiently small for better biological relevance, and the ability to function under the high particle fluxes typical of hadron therapy. Solid-state detectors can tolerate high fluxes but tend to have lower sensitivity and simulate larger volumes. Conversely, tissue-equivalent proportional counters (TEPCs) offer high sensitivity and small simulated volumes, but in their conventional single-event mode, they are not suited for high-rate environments. At the Legnaro National Laboratories (INFN-LNL), state-of-the-art miniaturized TEPCs have been designed, engineered, and extensively tested. These mini-TEPCs exhibit excellent reproducibility of their response. In particular, the shape of the microdosimetric spectra remains consistent up to particle fluxes of 106 particles per second per cm2, and average values such as dose-mean lineal energy remain reliable within uncertainties up to 108 particles per second per cm2. While these performance levels are significant, they are still insufficient for quality assurance across the entire depth-dose profile of a clinical beam. Currently, their application is mainly limited to lower-flux regions such as the distal fall-off or out-of-field areas. To overcome these flux limitations and enable TEPCs to be used throughout the entire treatment field, a transition to multi-event operation is required. In this mode, rather than resolving individual events, the device integrates the signal over defined time intervals. When coupled with the variance-covariance technique, this approach allows for the extraction of meaningful single-event microdosimetric quantities from the integrated signal. This method provides a pathway to extend the capabilities of TEPCs into high-flux clinical environments while maintaining sensitivity to radiation quality. A new INFN Young Researcher Grant supports the development of a dual-mode TEPC system capable of both single-event and multi-event operation. This novel instrument represents a versatile and promising solution for clinical measurements of radiation quality. The AtoMiQA project, centered on this objective, aims to deliver a fully integrated device including a TEPC sensor, custom front-end electronics, and a digital data acquisition system designed specifically for QA in hadron therapy. The goals, design strategy, and expected impact of the AtoMiQA system will be presented and discussed in this talk. This presentation will explore the advantages, current limitations, and potential future developments of TEPCs in the context of clinical quality assurance.
Bianchi et al. (Thu,) studied this question.