Fetal Dose Assessment In a Pregnant Patient with Brain Metastasis Using Gamma Knife Esprit
Abstract
Purpose
To demonstrate that Gamma Knife stereotactic radiotherapy with on-board imaging can be safely utilized for pregnant patients using techniques and resources accessible to medical physicists in non-academic clinical settings.
Methods
This study presents a fetal dose assessment and measurement for a patient at 18 weeks gestation undergoing mask-based SRT for two sites (24 Gy at 61% isodose and 20 Gy at 50% isodose) in two fractions. A critical component of this assessment involved estimating the dose from Gamma Knife cone-beam CT (CBCT) imaging. The imaging for treatment guidance included one high-dose scan (CTDIvol = 6.3 mGy) and three low-dose scans (CTDIvol = 2.5 mGy), totaling a cummulative head CTDIvol of 13.8 mGy. TLDs were ordered from a radiation monitoring by mail service and placed inferior to the sternum, equivalent to the fundus height at 36 weeks, and at the level of the umbilicus during imaging and treatment of each fraction. These locations correspond to the fundus height at 36 weeks, thus providing a worst-case dose measurement, and 20 weeks, respectively.
Results
Given a 32 cm separation from the imaging field edge to the umbilicus, the fetal dose was calculated at approximately 0.028 mGy. The TLDs were read by a remote service to validate the contribution from the radiotherapy was also within acceptable limits. ICRP and IAEA guidance indicate deterministic fetal effects are not expected at doses below 100 mGy.
Conclusion
This work indicates the feasibility of safely delivering stereotactic radiotherapy during pregnancy. Although careful management is required due to potential risks to the fetus, this framework demonstrates that a rigorous fetal dose assessment and treatment can be successfully executed in non-academic settings by leveraging diagnostic physics expertise and external dosimetry service.