External photon beam radiotherapy of pregnant patients with brain tumors: Reducing fetal dose through treatment parameter adjustment

This is the authors' abstract. We don't add key points for this paper.

PURPOSE: This study aims to evaluate the potential of the parameters of the treatment plan (collimator angle, beam filtering and modulation) to reduce fetal dose for a pregnant woman with a brain tumor undergoing external photon radiotherapy. METHODS: Dose measurements were performed with a TrueBeam STx 6 MV using a Farmer ionization chamber, water-equivalent slabs (out-of-field dose profiles), and the ATOM phantom with additional homemade bellies affixed to simulate the three pregnancy trimesters. Eight treatment plans were evaluated, including 3D conformal radiotherapy (3D-CRT) and Volumetric Modulated Arc Therapy (VMAT) plans with varying collimator angles, beam filtering, and beam modulations. RESULTS: Adjusting the beam collimator angle can reduce fetal exposure by a factor of 2. The collimator angle minimizing dose in the out-of-field region (in inline direction) is not always achieved by placing the distal jaw in the cranio-caudal axis of the patient, as suggested in the literature. Fetal doses for FFF plans are systematically about 40% lower than for FF plans. CONCLUSIONS: Although the lowest fetal dose was obtained with 3D-CRT, adjusting beam collimator angle and beam modulation in VMAT FFF plans with a Truebeam machine resulted in a comparable fetal dose and better dose distribution to the primary tumor. This study provides indications on how to improve the benefit-risk ratio between minimal fetal dose and optimal target dose delivery through treatment plan parameter adjustment and highlights the need to characterize the out-of-field dose of the machine used.