Radiation therapy of breast carcinoma: confirmation of prescription dose using diodes

Abstract

Purpose : To quantitate the dose delivered during tangential breast radiation therapy and measure the scatter dose to the contralateral breast for three different breast setup techniques. Methods and Materials : A commercial semiconductor diode system is used for dose measurements. The diode characteristics were studied by comparing the diode response against a standard ionization chamber response in a reference configuration. In vivo dose measurements on 11 patients undergoing tangential breast radiation therapy with 6 MV photons were performed. Medial and lateral field entrance and exit doses were measured and compared with the expected values from the treatment planning system. Scatter doses to the contralateral breast for three breast setup techniques were measured and documented as a function of distance from the field edge and various beam modifiers commonly used in breast radiation therapy. Results : The diodes used in this study exhibited excellent linearity, dose reproducibility, and minimal anisotropy. The in-phantom measurements resulted in dose accuracy within ± 1.5%. Dose measurements on patients resulted in standard deviations of 1.2 and 2.3% for the medial entrance and exit doses and 1.7 and 2.2% for the lateral entrance and exit doses, respectively. In patients, the scatter doses to the opposite breast at a 5 cm perpendicular distance from the medial field edge resulted in cumulative scatter doses of 2.47 to 5.30 Gy from the tangential fields and an additional 0.50 Gy from the supraclavicular or axillary field, if included. Conclusion : Quantitative verification of the prescribed daily dose is important in breast radiation therapy to ensure precision in patient setup and accuracy in dose delivery. Diodes provide a convenient way of realtime patient dose verification and are easy to use by the therapists.