Purpose: To design a practical breast shield and to investigate its efficacy in reducing scattered radiation to the contralateral breast of patients undergoing radiation therapy for breast cancer.
Methods and materials: We constructed a mobile shield consisting of (a) a mobile base and a counterweight; (b) a vertical column adjustable in height and a diagonal arm adjustable in angle; (c) a curved, 2.5-cm thick lead sheet with a 1-cm thick polystyrene liner for blocking scattered radiation; and (d) diode detectors to verify that the edge of the lead sheet is not in the useful beam in addition to the use of the field light. Measurements were performed with thermoluminescent dosimeters on 10 patients without the shield and on an anthropomorphic phantom with a pair of wax breasts with and without the shield. All of the patients were treated with 6-MV photons (Varian 6/100). The scattered radiation from the medial and lateral fields was measured separately.
Results: The contribution of the medial field to the total scattered dose was 70% to 75%, whether a medial wedge was used or not. However, without a medial wedge, the scattered dose was reduced by nearly 33% at 3 to 9 cm away from the medial border. In the anthropomorphic phantom study with wax breast, the mobile shield reduced the medial field contribution to the total scatter dose to less than the contribution from the lateral field without a shield. With a prescribed dose of 50 Gy and a medial wedge, the median scatter dose to the contralateral breast from 6 patients was 5.3 Gy; without a medial wedge, it was 3.8 Gy from 4 patients at 6 cm from the medial border. In the phantom study, with the shield the total dose to the contralateral breast was 1.0 Gy at 6 cm from the medial border with a same prescribed dose.
Conclusion: The mobile shield reduced the scatter dose to the contralateral breast from the linear accelerator (Varian 6/100, 6-MV photons) by a factor of 3 to 4. The shield greatly reduced the scattered dose in the wax phantom. Equivalent reductions in patients may be clinically significant by reducing the risk of radiation-induced breast cancer in the contralateral breast of woman undergoing radiation therapy for breast cancer. The shield is safe and easy to adjust to each patient.