Measurement of scattered radiation to the fetus during radiation therapy for Hodgkin's disease of the mediastinum was 0.0026 rads per rad given to the tumor, or 7.9 rads for 14 treatments or 3,500 rads. Measurements of scattered radiation made on a phantom for three different modalities indicate the scattered dose to a fetus would be 3.5 to 14 rads in early pregnancy, 31 to 49 rads in midpregnancy, and 129 to 248 rads in late pregnancy, for 14 treatments or 3,500 rads.
Covington EE, Baker AS. Dosimetry of Scattered Radiation to the Fetus. JAMA. 1969;209(3):414–415. doi:10.1001/jama.1969.03160160050018