Radiobiology of protons: Preclinical efforts to understand and mitigate toxicity

 

As part of the proton therapy community and within the framework of the European Particle Therapy Network (EPTN), it is becoming increasingly clear that we must strengthen our efforts in preclinical radiobiological research within proton radiation. To refine and safeguard proton therapy moving forward, we need targeted, high-quality preclinical studies that explore the underlying biology, especially focusing on linear energy transfer (LET), fractionation, and comparative photon cohorts.

 

Proton therapy (PT) is an advanced radiotherapy modality offering superior dose distribution compared to photon-based therapies, characterised by low entrance doses and no exit dose beyond the tumour due to the Spread-Out Bragg Peak (SOBP). This unique physical advantage is particularly beneficial in brain tumour therapy, significantly reducing radiation exposure to surrounding healthy brain tissue [1,2]. Currently, clinical PT widely employs a fixed Relative Biological Effectiveness (RBE) value of 1.1 [3,4], based primarily on early in vitro studies [5,6] and formally adopted by the International Commission on Radiation Units (ICRU) in 2007 [7]. RBE defines how much more biologically effective protons are compared to photons at producing a given biological effect. However, it is now well accepted that the RBE is not constant but varies significantly depending on LET, fractionation schemes, tissue type, dose, and biological endpoint measured. LET peaks at the distal edge of proton beams, potentially causing increased toxicity in adjacent normal tissue. This has been shown preclinically in multiple studies [8–14].

 

Clinical studies conducted over the last decade have observed unexpected normal tissue toxicities such as radiation-induced brain injury, correlating strongly with elevated LET regions [15–19]. These findings raise critical questions about the validity of employing a constant RBE in PT, especially for sensitive tissues like the brain.

Three recent preclinical studies provide compelling evidence that these parameters warrant deeper investigation. Our group at Aarhus University Hospital in 2025 [20] demonstrated that fractionation enhances the biological effectiveness of protons in vivo, with a higher RBE observed for late normal tissue damage when treatment is delivered in fractions rather than a single dose (Figure 1A). This challenges the assumption of a uniform RBE across dose schedules and highlights the potential for underestimated toxicity in fractionated clinical protocols. More fractionated studies have been launched by our group, which results we await.

Figure 1.A) Dose-response relationships for mice developing fibrosis after 1 fraction or 4 fractions of 6 MV photons or protons, resulting in a higher RBE for 4 fractions [20]. B) Heatmap of microglial priming response after irradiation with photons, plateau protons or SOBP protons, showing slightly stronger priming response after SOBP protons [21]. C) CT-based dose plan for a C57BL/6 mouse with little dose deposited in the contralateral hemisphere [22].

 

 

 

A study from Groningen in 2024 [21] further showed that LET influences neuroinflammatory responses in the brain. While photon and proton plateau irradiation had comparable effects, proton irradiation with the target located in the proximal end of the proton SOBP resulted in elevated microglial priming (Figure 1B). This spatially localised biological response underscores the importance of considering even slight LET elevations, not only for dose deposition, but also for their direct biological impact on sensitive tissues.

 

Finally, the work by a Dresden group in 2021 [22] introduced a robust preclinical model with high spatial precision, capable of linking proton dose distributions to both MRI-detectable and histological markers of brain damage (Figure 1C). Their results confirmed a dose- and LET-dependent pattern of late brain injury, further validating the relevance of preclinical models for translational insight.

 

Together, these studies point towards the conclusion that current clinical assumptions of an RBE of 1.1 do not adequately capture the biological complexities of PT. To address this gap, further preclinical studies specifically designed to investigate how LET and fractionation influence biological outcomes after PT are needed.

 

 

Cathrine Bang Overgaard Post Doctoral Associate/Fellow Aarhus University Hospital, Denmark		Brita Singers Sørensen Professor Aarhus University, Denmark,

 

 

References:
 

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