Terminology Proposal for Adaptive Particle Therapy Classification and Strategies

Adaptive radiotherapy (ART) is an increasingly prominent concept in modern radiation oncology, offering the ability to adjust treatment in response to patient-specific anatomical and physiological changes. The concept was first introduced in 1995 as a "closed-loop radiation treatment process where the treatment plan can be modified using a systematic feedback of measurements.^"1 In this manuscript, we use ART to denote a treatment paradigm in which the treatment plan is intentionally modified during the course of radiotherapy in response to observed or anticipated changes in patient geometry, anatomy, or biology, with the aim of maintaining or improving the therapeutic ratio. More recently, ART has gained momentum due to advances in imaging, computing, and automation, which enable such feedback to be implemented more systematically and at different timescales.2, 3, 4, 5 Therefore, ART could eventually become a standard of care for some traditionally difficult-to-treat indications. These developments are particularly significant for particle therapy (PT), which is highly sensitive to (anatomical) variations. While PT offers dosimetric advantages over conventional photon therapy, its precision also means that even small deviations from the planned density and/or geometry can translate into clinically relevant changes in water-equivalent path length and thus proton range, with substantial impact on the dose distribution.