FETAL DOSE ASSESSMENT FOR BREAST CANCER RADIATION THERAPY WITH COBALT-60 QUADRATE TECHNIQUE

Abstract

Objective There is no current information about the estimated fetal dose from an extensive breast cancer radiation treatment which include internal mammary chain (IMC), supraclavicular (SPC) and tangential chest wall. The aim of this work was to determine an appropriate irradiation technique and to build a fetal dose data set for the management of pregnant women needing breast irradiation.

Methods Measurements with thermoluminescent (TLD-100) dosimeters were performed in an anthromorphic phantom which was modified to simulate a pregnant patient at first month to sixth month of pregnancy: i.e., 4, 12 and 24 weeks of gestation. Two similar treatment plans, quadrate technique with the open and wedge tangential field, were delivered with a total dose of 50 Gy using Cobalt-60 gamma-ray. Abdominal shielding was constructed and its efficacy was verified. Results of the measured doses were analyzed and plotted as a function of depth and distance from the tangential field edge.

Results Minimum fetal doses in all three gestational periods were detected by the open tangential field quadrate technique with the shielding. With the total prescription dose of 50 Gy, the corresponding average measured doses at 4, 12 and 24 weeks gestation were found to be 5.4±1.19, 11.0±5.18 and 19.6±17.3 cGy or 0.11%, 0.22% and 0.39 % of the total dose, respectively. The modification device, a wedge filter, was found out to yield more external scattered radiation dose to the fetus, about 17-27%, in comparison with the open tangential field technique. The measured dose in the shielding technique, both the open and wedge tangential field technique, was lower than the non-shielding technique approximately 50-60%. For all three periods in a simulated pregnant phantom, the fetal doses showed a small change with depth. But the fetal doses were likely to decrease exponentially with the distance from the primary beam edge. This observation was seen both in the second and third trimesters with correlation coefficients, R? = 0.93 and 0.94 respectively.

Conclusion _A reliable and accurate data set to assess the doses to fetus for breast cancer pregnant patient receiving Cobalt-60 gamma ray with quadrate technique irradiation was obtained. Fetal doses presented in a graph, plotted as the function of depth and distances were found to be useful in the risk management for any individual pregnant patient requiring cobalt-60 quadrate technique radiation therapy at our institution.