A mobile shield to reduce scatter radiation to the contralateral breast during radiotherapy for breast cancer: Preclinical results

Purpose: To design a practical breast shield and to investigate its efficac y in reducing scattered radiation to the contralateral breast of patients u ndergoing radiation therapy for breast cancer. Methods and Materials: We constructed a mobile shield consisting of (a) a m obile 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 she et with a l-cm thick polystyrene liner for blocking scattered radiation; an d (d) diode detectors to verify that the edge of the lead sheet is not in t he useful beam in addition to the use of the field light. Measurements were performed with thermoluminescent dosimeters on 10 patients without the shi eld and on an anthropomorphic phantom with a pair of wax breasts with and w ithout the shield. All of the patients were treated with 6-MV photons (Vari an 6/100). The scattered radiation from the medial and lateral fields was m easured separately. Results: The contribution of the medial field to the total scattered dose w as 70% to 75%, whether a medial wedge was used or not. However, without a m edial 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 brea st, the mobile shield reduced the medial field contribution to the total sc atter dose to less than the contribution from the lateral field without a s hield. With a prescribed dose of 50 Gy and a medial wedge, the median scatt er dose to the contralateral breast from 6 patients was 5.3 Gy; without a m edial 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 b reast 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 facto r of 3 to 4. The shield greatly reduced the scattered dose in the wax phant om. Equivalent reductions in patients may be clinically significant by redu cing the risk of radiation-induced breast cancer in the contralateral breas t of woman undergoing radiation therapy for breast cancer. The shield is sa fe and easy to adjust to each patient. (C) 1999 Elsevier Science Inc.