S1719
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
Proffered Paper 1522 Status and practice of proton therapy for pregnant patients in Europe Gloria Vilches - Freixas 1 , Christan Bäumer 2,3 , Marijke De Saint-Hubert 4 , Maite Romero-Expósito 5 , Liliana Stolarczyk 6 1 Department of Radiation Oncology (Maastro), GROW Research Institute for Oncology and Reproduction, University Medical Centre+, Maastricht, Netherlands. 2 ., West German Proton Therapy Centre Essen, Essen, Germany. 3 Department of Physics , Germany, TU Dortmund University, Dortmund, Germany. 4 Research in Dosimetric Applications, Belgian Nuclear Research Centre (SCK CEN), Mol, Belgium. 5 Oncology Pathology Department, Karolinska Institutet, Stockholm, Sweden. 6 Dansk Centre for Partikelterapi, Universitetshospital, Aarhus, Denmark Purpose/Objective: Recent case reports and dosimetric studies indicate that proton therapy (PT) may result in lower out-of- field (OOF) doses compared to photon therapy, potentially benefiting pregnant cancer patients. With rising maternal age and a potential increase in cancer diagnoses during pregnancy, a European initiative was started to evaluate current practices, standardise intrauterine dose assessment and reporting, and to develop clinical guidelines for PT treatment planning in this population. Material/Methods: A survey was conducted across European PT centres to gather insights on treating pregnant patients. The initiative, led by EURADOS WG9 and the EPTN with ESTRO's endorsement, included an online workshop and an in-person meeting (more than 70 participants including medical physicists, dosimetry experts, and researchers) hosted in Maastro (Maastricht, Netherlands). Expert presentations and subgroup discussions covered epidemiology, imaging, treatment planning, experimental OOF dosimetry, and related Monte Carlo simulations. Knowledge gaps were identified, and short- and mid-term goals for working groups set and refined in a follow-up online meeting. Results: Out of 28 operational PT centres in Europe, 18 responded to the survey (Table 1), reporting 21 treated pregnant patients, confirming that such cases remain rare. The median [average] week of pregnancy at start of treatment was 26 [22], with most patients (52.4%) in the third trimester. The median [average] dose prescription was 60 Gy(RBE1.1) [57.8 Gy(RBE1.1)]. Of 21 treatments, 20 targeted the head, intraocular region or neck, with one additional case located in the mediastinum and one in the lung.
The survey revealed strong interest in evaluating current practices, harmonising fetal dose reporting and developing practical OOF dose estimation methods. Although epidemiological evidence suggests that fetal effects from in-utero doses <100 mGy are considered negligible, accurate dose assessment remains crucial, and neutron dosimetry is the biggest challenge. Accordingly, guidance documents and checklists were developed to support imaging, treatment planning, and dose assessment in such cases. Conclusion: Evidence indicates that intrauterine exposure to PT is acceptable for treatment areas as low as the mediastinum. Given the rarity of such treatments, sharing medical physics expertise is essential, and the workshop marked an important initial step, as no guidelines for PT exist.It was concluded that dose equivalent can be used as a reporting quantity (Figure 1), but further optimisation, harmonisation of OOF dose measurements, and alignment with ICRP guidance remain necessary. Monte Carlo simulations and inter-centre collaborations were encouraged to support individual cases and develop reference dosimetry set-ups and standardised validation protocols.
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