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The Steering Committee on Clinical Practice Guidelines for the Care and Treatment of Breast Cancer states that pregnancy is an absolute contraindication for breast irradiation (Whelan et al., 2003) due to the developing foetus being highly radiosensitive. Particularly, the effect of scattered radiation in conventional x-ray treatments is of concern. We investigate proton therapy as a treatment option for pregnant breast cancer patients. A phantom was used to estimate doses to the foetus from intensity-modulated proton therapy (IMPT) during the first trimester of pregnancy.

We use a virtual anthropomorphic phantom (Xu et al., 2007) in the early gestational period of pregnancy, segmented into major organs and tissues. The phantom includes a 3-month-old foetus, segmented into foetal soft tissue and brain. For all segments in the phantom, the relative stopping powers are calculated voxel-wise using corresponding elemental compositions and densities from Chen et al., 2004 (maternal/foetal tissues) and ICRU 46 (remaining tissues), with mean excitation energies (I-values) from ICRP 44 and Bär et al., 2018. Treatment planning was performed in Eclipse, with planning target volumes created by a therapeutic radiographer for the right and left breast. Four IMPT plans were generated (40 Gy (RBE), 15 fractions) using spot scanning and single field optimisation, each with a Lexan range shifter (5 cm water equivalent thickness) to degrade the proton energy for shallow range: 1) Single field (315°) and 2) two fields (0°, 315°) to the right breast; 3) single field (45°) and 4) two fields (0°, 45°) to the left breast. Following parametrisation of the beam properties, treatment plans were imported into TOPAS (Perl et al., 2012), simulating total absorbed and equivalent neutron doses, scored for the foetal brain and soft tissue.

Total absorbed doses and equivalent neutron doses to the foetus with and without range shifter are displayed in Table 1. The average foetal dose (std) across all four plans was 0.044 (0.008) mGy and 0.39 (0.09) mSv for the total absorbed dose and equivalent neutron dose (with range shifter), respectively. We observe an increase in foetal neutron dose caused by the addition of the range shifter, up to 19 % higher in the two-field plan compared to the single field setup. 

Our phantom study suggests that breast IMPT remains well within recommended dose constraints of 100 mGy (ICRP 84). The use of a range shifter considerably increased the foetal dose in all plans; therefore, clinics may consider a material with lower neutron yield to follow the ALARA principle. Compared to conventional x-ray therapy, IMPT of the breast exposes the foetus to 5 orders of magnitude lower stray dose, roughly 120 mGy (Fenig et al., 2000). Future work will investigate later stages of gestation with larger foetal volumes closer to the treatment volume.