Fetal dose from proton pencil beam scanning craniospinal irradiation during pregnancy: a Monte Carlo study

Abstract

Objective: We conducted a Monte Carlo study to comprehensively investigate the fetal dose resulting from proton pencil beam scanning (PBS) craniospinal irradiation (CSI) during pregnancy. Approach: The gestational-age dependent pregnant phantom series developed at the University of Florida (UF) were converted into DICOM-RT format (CT images and structures) and imported into a treatment planning system (TPS) (Eclipse v15.6) commissioned to a IBA PBS nozzle. A proton PBS CSI plan (prescribed dose: 36 Gy) was created on the phantoms. The TOPAS MC code was used to simulate the proton PBS CSI on the phantoms, for which MC beam properties at the nozzle exit (spot size, spot divergence, mean energy, and energy spread) were matched to IBA PBS nozzle beam measurement data. We calculated mean absorbed doses for 28 organs and tissues and whole body of the fetus at eight gestational ages (8, 10, 15, 20, 25, 30, 35, and 38 weeks). For contextual purposes, the fetal organ/tissue doses from the treatment planning CT scan of the mother’s head and torso were estimated using the National Cancer Institute dosimetry system for CT (NCICT, Version 3) considering a low-dose CT protocol (CTDIvol: 8.97 mGy). Main Results: The majority of the fetal organ/tissue doses from the proton PBS CSI treatment fell within a range of 3 to 6 mGy. The fetal organ/tissue doses for the 38-week phantom showed the largest variation with the doses ranging from 2.9 mGy (adrenals) to 8.2 mGy (eye lenses) while the smallest variation ranging from 3.2 mGy (oesophagus) to 4.4 mGy (brain) was observed for the doses for the 20-week phantom. The fetal whole-body dose ranged from 3.7 mGy (25 weeks) to 5.8 mGy (8 weeks). Most of the fetal doses from the planning CT scan fell within a range of 7 to 13 mGy, approximately 2-to-9 times lower than the fetal dose equivalents of the proton PBS CSI treatment (assuming a quality factor of 7). Significance: The fetal organ/tissue doses observed in the present work will be useful for one of the first clinically informative predictions on the magnitude of fetal dose during proton PBS CSI during pregnancy.

Challenges and opportunities for proton therapy during pregnancy

“…Notwithstanding the dosimetric uncertainties of those fetal dose estimations, this clear reduction might already be enough to apply PBS proton‐RT for these patients. On the other hand, proton‐RT might also allow for the treatment of more challenging cases such as craniospinal irradiation 27,28 . However, in such cases it is important to account for dosimetric uncertainties as well as uncertainties on the fetal position, in order to ensure fetal doses remain below acceptable threshold levels.…”

“…Moreover, these computational and physical models were never validated against each other. For instance, whereas Kalbasi et al estimated the fetal dose during 36 Gy craniospinal irradiation to be 72 mSv based on the literature, MC simulations by Yeom et al 27 estimated this dose to only be 26–31 mSv, despite using the same Q‐factor. Often, these discrepancies are acceptable, as fetal doses typically remain well below 100 mSv and considering more than the 10‐fold dose reduction typically observed compared with photon‐RT.…”

“…On the other hand, proton‐RT might also allow for the treatment of more challenging cases such as craniospinal irradiation. 27 , 28 However, in such cases it is important to account for dosimetric uncertainties as well as uncertainties on the fetal position, in order to ensure fetal doses remain below acceptable threshold levels.…”

“…This shows that the estimated fetal dose is highly dependent on the individual geometry of patient and fetus. Finally, Yeom and colleagues 27 used MC simulations to estimate the fetal dose across different stages of pregnancy and for different fetal organs during craniospinal irradiation with PBS proton‐RT for a 36‐Gy prescribed dose, based on a clinical case by Kalbasi et al 28 (see below). The total fetal absorbed dose (ie not accounting for RBE) varied between 3.66 and 5.80 mGy, amounting to 26–41 mSv equivalent fetal dose when using a fixed quality factor of Q = 7, with lower doses occurring later in pregnancy as the fetus moves anteriorly.…”

TET2DICOM-GUI: Graphical User Interface Based TET2DICOM Program to Convert Tetrahedral-Mesh-Phantom to DICOM-RT Dataset

“…(6) Define the density of each VoxelPhantom by calculating the weight average of the points accumulated on all VoxelPhantoms in the BoxPhantom. (7) The user converts the density of VoxelPhantom to the HU value using the predefined Density to HU table in the program.…”