Purpose: Prior efforts to assess fetal dose during radiotherapy utilize Monte Carlo methods that incorporate limited fetal anatomy and focus on treatment sites distant to the fetus, e.g., intracranial. The purpose of this study was to use newer, sophisticated pregnant computational phantoms to explore the feasibility of treating a site closer to the fetus without exceeding the 100 mSv deterministic fetal threshold while also investigating differences between modern treatment approaches using 3DCRT, IMRT, and proton PBS.
Methods: The UF Family of hybrid pregnant phantoms was used for this study. For an initial assessment, the 25-week gestational age phantom was voxelized and converted to DICOM format. The dataset was imported into RayStation, and target/OAR contours were provided by a physician expert. 3DCRT, IMRT, and PBS plans were created in accordance with institutional protocols for breast radiotherapy using 42.4 Gy in 16 fx. Dose to the fetal sac was assessed using volume-based metrics.
Results: The distance between the inferior border of the target and superior border of the fetus was 11 cm. The max D1cc was 310 mGy, 10 mGy, and 0 cGy for 3DCRT, IMRT, and PBS, respectively. The mean dose was 20 mGy for 3DCRT but negligible for IMRT and PBS. The distance to the 100 mGy isodose line was 1 cm for IMRT and 7 cm for PBS.
Conclusion: Considering patient scatter only, PBS provides clear advantages for treating sites in close proximity to the fetus. Though fetal dose was also low for IMRT, the nearby dose gradient likely precludes the use of IMRT due to the fact that, clinically, the exact distance between the target and fetus would be variable and difficult to accurately quantify. Future work using Monte Carlo methods will quantify neutron and imaging dose to further assess the feasibility of PBS for treating pregnant patients.
TH- External Beam- Particle/high LET therapy: Proton therapy – computational dosimetry-deterministic