ESTRO 2026 Congress Report I Radiobiology track

By the ESTRO Biology committee

 

A packed room, four brilliant speakers, and a motion deliberately designed to provoke discussion:

“This House believes that re-irradiation outcomes can only be improved with radiobiological insights.”

The ESTRO debate on re-irradiation brought together members of the Re-irradiation Focus Group for an engaging and thought-provoking exchange between biology, physics, technology, and clinical practice.

As tradition dictates, the audience was asked to vote before the debate began. The starting position seemed clear: 72% voted for the motion and 28% against. But would the speakers manage to change our minds?

 

Opening the Debate

Opening for the motion, Laure Marignol made the case that, although modern technology has made re-irradiation increasingly feasible, our ability to predict who will benefit—and who will experience toxicity—remains limited by biological uncertainty. Two patients may have the same tumour site, previous dose, and calculated EQD2, yet experience dramatically different outcomes. Tissue tolerance, she argued, is not simply a number but a biological state shaped by recovery, fractionation, hypoxia, stem-cell depletion, chronic inflammation, and individual radiosensitivity. Re-irradiation means treating not the same normal tissue again, but a biologically transformed ecosystem.

“Physics made re-irradiation possible, but biology will help determine its future.”

— Laure Marignol

Taking the opposite side, Ane Appelt immediately challenged one crucial word in the motion: “only”. Re-irradiation is already being delivered on a substantial scale, and this expansion was not triggered by a breakthrough in our understanding of tissue recovery. Rather, advances in imaging, treatment planning, image registration, dose accumulation, and highly conformal techniques such as SBRT have enabled clinicians to safely push previous boundaries. She illustrated how differences of up to 20 Gy in estimated cumulative OAR dose between institutions could be dramatically reduced by implementing advanced imaging, registration, and 3D dose accumulation workflows. Her argument was equally clear: radiobiology is essential, but physics and technology are also independently driving progress—and over the next five to ten years, major gains may still come from better imaging, planning, and treatment delivery.

 

Deepening the Arguments

The second round made the debate even more animated. Bartłomiej Tomasik, firmly on “Team Biology”, introduced his imaginary biology detector, which, he joked, had been “beeping” throughout the physicist’s presentation. Recovery, tolerance, treatment interval, and even EQD2 all contain biological assumptions. The  ratio is itself a biological parameter, while tissue recovery remains largely inferred rather than directly measured. Importantly, re-irradiation is not only delivered to previously altered normal tissues; the tumour itself has changed through clonal selection and treatment pressure. Emerging tools such as ctDNA dynamics, molecular radiosensitivity signatures, hypoxia profiling, and immune characterization could therefore move us from estimating biology to actually measuring it, ultimately enabling truly individualized re-irradiation.

Finally, Piotr Wojcieszek brought the discussion firmly back to the clinic with a distinctly pragmatic perspective. His question was simple: can we use radiation after radiation failure? Clinical experience with SBRT and brachytherapy tells us that we can, even when the underlying biology is incompletely understood. For him, careful patient selection, experienced multidisciplinary teams, prospective clinical evidence, technical precision, and safe delivery remain fundamental.

“Quality over quantity, precision is progression, and conformity is above biology.”

— Piotr Wojcieszek

 

Bridging Technology and Biology

The discussion that followed was equally lively and highlighted how much the debate depends on the way we define radiobiology. The conversation naturally broadened from classical radiobiology to a wider concept of biology, encompassing tissue recovery, tumour evolution, genomics, biomarkers, immune response, and patient-specific vulnerability. This broader perspective also reflects the evolution within ESTRO from a Radiobiology Committee to a Biology Committee.

A strong theme emerging from the discussion was the need to bridge technological and biological advances more effectively. While imaging, treatment planning, dose accumulation, and delivery techniques continue to evolve rapidly, biological technologies have also made remarkable progress. Translating these advances into re-irradiation, however, requires access to biological samples, prospective data collection, and integration of translational research into clinical studies. Combining high-quality dosimetric and clinical data with biological information may ultimately allow us to move beyond population-based estimates and empirical assumptions towards more individualized predictions of tumour response and normal-tissue toxicity.

 

The Final Verdict

And then came the final vote.

After an hour of passionate arguments, provocative questions, and plenty of laughter, the room had clearly been challenged: 42% voted for the motion and 58% against.

Voting Stage	FOR the Motion	AGAINST the Motion
Initial Vote	72%	28%
Final Vote	42%	58%

Yet the change in vote did not signal a rejection of biology. If anything, the debate helped clarify the meaning—and the deliberate ambiguity—of the motion. To say that re-irradiation outcomes can only be improved with biological insight is not to say that they can be improved by biology alone.

Perhaps the strongest message emerging from this packed and enthusiastic session was therefore one of integration rather than competition. Physics, imaging, and technology allow us to know where the tissue is, how much dose it has received, and how precisely we can treat it; biology can help us understand how previously irradiated tissues recover, how tumours evolve, and why individual patients may respond differently. Bringing these perspectives together will be essential to make re-irradiation increasingly personalized, predictable, and safe.

 

Monica Mangoni

Radiation Oncologist

University of Florence

Florence, Italy
Member of the ESTRO Biology Committee