Room: Track 5
Purpose: Treatment planning and fetal dose estimation for pregnant patients can be an urgent and time-consuming process. The purpose of this work is to provide a framework for minimizing fetal dose while maintaining high plan quality.
Methods: A patient in her first trimester of pregnancy was treated for a brain tumor to 2500 cGy in 5 fractions on a TrueBeam with HDMLC. A series of 30 treatment plans were created to explore the trade-offs between fetal dose contributions and plan quality for the following parameters: beam energy (6X, 10XFFF), technique (step-and-shoot IMRT, sliding window IMRT, VMAT), and couch angle (up to 15° superior and inferior). Collimator and gantry angles were chosen according to our previous work indicating minimal doses around collimator 45° and posterior gantry angles with the use of custom lead shielding placed under the patient. MU objectives were used to minimize the total MU of each plan. 13 plans were selected for measurement performed using an anthropomorphic phantom, solid water, and ion chamber at distances from isocenter corresponding to the expected nearest position of the fundus throughout the course of pregnancy. The plan for treatment was chosen based on minimizing fetal dose and then optimizing conformity index, PTV coverage, brainstem dose, and V12Gy.
Results: The fetal dose measured from the 10XFFF plans were slightly lower than 6XFFF, however, 10XFFF was not used due to increased neutron production. Couch kicks increased dose by 14% for a 50 couch angle up to 60% for 150. The final dose for treatment was estimated to be 0.12cGy and was achieved by IMRT and VMAT. A VMAT plan was chosen that provided the best conformity index (1.2), target coverage (98%), and V12Gy.
Conclusion: Low fetal dose and high plan quality are achievable during intracranial SRS with careful selection of plan parameters.