S1717
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
Conclusion: For PBS breast irradiation during pregnancy, fetal doses were already well below the threshold of 100 mSv. Further reduction towards public dose limits was achieved by optimizing the use of the RS. The largest reductions were obtained by replacing the RS with a 3D-bolus, or by applying BS with small air gaps, which approximately halved the intrauterine dose. References: [1] Blommaert J, De Saint - Hubert M, Depuydt T, Oldehinkel E, Poortmans P, Amant F, e.a. Challenges and opportunities for proton therapy during pregnancy. Acta Obstet. Gynecol. Scand. 2023;103:767- 74. [2] Maynard MR, Long NS, Moawad NS, Shifrin RY, Geyer AM, Fong G, e.a. The UF Family of hybrid phantoms of the pregnant female for computational radiation dosimetry. Phys. Med. Biol. 2014;59:4325- 43. [3] Colson D, Blommaert J, Poels K, De Saint-Hubert M, Reniers B, Depuydt T. Extended in-field and out-of- field validation of a compact Monte Carlo model of an IBA PROTEUS®ONE proton beam in TOPAS/GEANT4. Phys. Med. Biol. 2023;68:21NT02. Keywords: pregnancy, Monte Carlo, plan optimization Fine-Tuning Plan Quality in Adaptive Proton Therapy Using an Interactive Dose Modification Tool Lisa Stefanie Fankhauser 1,2 , Francesca Albertini 1 , Erik Traneus 3 , Antony John Lomax 1,2 , Matthew Lowe 4 1 Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland. 2 Department of Physics, ETH Zürich, Zürich, Switzerland. 3 Particle Therapy, Raysearch Laboraties AB, Stockholm, Sweden. 4 Christie Medical Physics & Engineering, The Christie NHS Foundation Trust, Manchester, United Kingdom Digital Poster Highlight 1414
However, OOF dose increases when a range shifter (RS) is used, on which many PBS systems rely for superficial targets. This study evaluates planning strategies that optimize RS use in PBS to further minimize intrauterine dose while maintaining target coverage and plan quality, using breast cancer as an example case. Material/Methods: Nine PBS breast treatment plans were created in RayStation 12A to deliver 42.30 Gy(RBE) to the breast and regional nodes with a 52.20 Gy(RBE) boost, using a 20-week computational pregnant phantom from the University of Florida [2]. Superficial target coverage was achieved using three RS configurations: (i) a conventional 3.5 cm Lexan RS with varying air gaps; (ii) beam splitting (BS) with varying air gaps, a technique in which the RS was removed for deeper energy layers (affecting 52% of MU); and (iii) replacement of the RS with a 3D-bolus (3 cm water-equivalent).Plans were exported to TOPAS/GEANT4 for Monte Carlo simulation using a PBS beam model [3], previously validated for OOF dose in pregnant breast irradiation. For each plan simulation, the mean intrauterine dose equivalent was calculated by summing neutron and gamma components and scaling to the full treatment course. Results: Across all treatment plans, the simulated combined neutron and gamma dose equivalent within the uterus remained below 5.0 mSv (Figure 1). Smaller air gaps consistently resulted in lower dose. BS provided a further reduction, with doses of 2.3-2.6 mSv depending on the air gap. Replacing the RS with a 3D- bolus reduced the dose to 2.0 mSv. Overall, the lowest combined doses were obtained with the 3D-bolus and with BS at a 5 cm air gap, reducing the combined dose by 60% and 54%, respectively, relative to the conventional RS configuration with a 25 cm air gap.
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