ESTRO 2026 Congress Report I physics track

Cancer during pregnancy, although relatively rare (about 1 in 1,000 pregnancies), is becoming increasingly relevant due to delayed childbearing and rising cancer incidence in younger populations. Management remains clinically challenging, as radiotherapy (RT), despite being a standard treatment for many malignancies, is often avoided or postponed during pregnancy due to concerns regarding fetal radiation exposure. However, treatment delays or premature delivery may compromise maternal prognosis, particularly in aggressive or locally advanced disease.

From a radiobiological perspective, fetal radiation exposure is associated with both deterministic and stochastic effects. Current evidence indicates a threshold for malformations of around 100 mGy (ICRP recommendations), while higher doses (>300 mGy) during sensitive gestational periods (weeks 8–25) have been linked to severe neurodevelopmental impairment. Conversely, epidemiological data suggest an increased—albeit small—risk of childhood cancer at considerably lower dose levels (~10–30 mGy), motivating the need for dose minimisation.

Proton therapy (PT), particularly pencil beam scanning (PBS), offers substantial advantages over photon RT due to its superior dose conformity and significant reduction in out-of-field (OOF) dose, even when the secondary neutron contribution is considered. Reported reductions of 1–2 orders of magnitude in OOF dose make PT a promising modality for treating pregnant patients, potentially expanding treatment eligibility while maintaining fetal safety, even in the third trimester.

To evaluate current European practice, a joint initiative between the European Particle Therapy Network (EPTN) and the European Radiation Dosimetry Group (EURADOS) conducted a comprehensive survey across PT centres, complemented by dedicated workshops. The survey included 18 centres, of which 9 had treated a total of 26 pregnant patients. Treatments were primarily delivered at later gestational stages (median week 23), most frequently for head and neck or intracranial indications, although mediastinal and lung cases were also treated, with typical prescriptions around 60 Gy (RBE). PBS was the dominant technique, except in intraocular cases, where passive scattering was used.

 

 

EPTN–EURADOS working group on cancer during pregnancy established during the first workshop held at Maastro Clinic (11 March 2025).

A key outcome of the survey was the identification of substantial heterogeneity in clinical practice, particularly regarding fetal dose estimation methodologies. Approaches ranged from Monte Carlo (MC) simulations to phantom measurements and literature-based extrapolations, with different dose quantities reported (absorbed dose, equivalent dose, and operational quantities such as ambient dose equivalent and personal dose equivalent). This variability highlights the urgent need for harmonisation and standardisation.

Workshop discussions further emphasised critical challenges, including accurate neutron dosimetry, appropriate dose metrics (absorbed dose vs dose equivalent), and limitations of current measurement techniques. On the one hand, area monitors are the most commonly available systems in proton centres, but they provide operational quantities that are not representative of fetal exposure. Therefore, further investigation is needed to define a methodology that could correlate these measurements with fetal dose. On the other hand, the use of anthropomorphic pregnant phantoms, passive detectors, and/or MC simulations has been suggested as a better approach, although clinical implementation remains complex. Importantly, a paradigm shift towards treating cancer during pregnancy, rather than delaying therapy, was supported by clinical stakeholders.

To address existing knowledge gaps, several task groups have been established focusing on dosimetry protocols, detector evaluation, MC methodologies, and the development of clinical guidelines. The initiative also underscores the necessity of international registries to improve understanding of long-term fetal outcomes.

Overall, the use of PT in the management of cancer during pregnancy is a highly promising option, although coordinated efforts between physicists, physicians and technologists are required to standardise practices and strengthen the evidence base.

Maite Romero-Expósito

Medical Radiation Physics, Stockholm University.

maite.romero@fysik.su.se

 

 

Go Deeper: Watch our on-site congress video featuring Maite Romero-Expósito alongside Dominik Wawrzuta, as they discuss the critical geographic accessibility of proton therapy in Europe and its specific application for pregnant patients.

Watch the video