Monte Carlo study on fetal dose assessment for carbon beam craniospinal irradiation during pregnancy

Radiotherapy may be suggested for pregnant cancer patients when treatment cannot be delayed, with careful targeting to minimize fetal dose. Considering that a successful case for a pregnant patient treated with proton beam craniospinal irradiation (CSI) was reported, carbon therapy can be also considered applicable for the CSI treatment to pregnant patients.

We investigated fetal organ doses from carbon beam CSI during pregnancy by performing Monte Carlo dose calculations.

We employed the high-quality pregnant female phantom series developed by University of Florida (UF) for eight gestational ages (8, 10, 15, 20, 25, 30, 35, and 38 weeks). The phantoms were converted into DICOM-RT CT images and implemented in a treatment planning system (TPS) with the prescribed relative biological effectiveness (RBE)-weighted dose of 36 Gy. A carbon CSI plan created from TPS was used to perform TOPAS MC dose calculations after commissioned to the carbon beam measurement data. Fetal organ absorbed doses for 28 organs considered radiosensitive were calculated and compared with those estimated for proton therapy in a previous study.

The organ/tissue doses for 35 weeks showed the largest variation, ranging from 13.2 mGy (esophagus) to 88.3 mGy (lens), whereas those for 8 weeks showed the smallest variation, ranging from 21.0 mGy (gall bladder) to 30.3 mGy (brain). The fetal whole-body doses decreased from early to mid-gestational ages, but increased again during the later stage as the fetus grew predominantly in the superior direction, reducing the overall distance to the beam field.

Considering the significant attention to carbon therapy, as a first dedicated effort, the result would be clinically informative to estimate the fetal dose from carbon therapy during pregnancy.