Comparison of Virtual Wedge versus Physical Wedge Affecting on Dose Distribution of Treated Breast and Adjacent Normal Tissue for Tangential Breast Irradiation

Kim, Yeon Sil; Kim, Sung Whan; Yoon, Sei Chul; Lee, Jung Seok; Son, Seok Hyun; Choi, Ihl Bong

Abstract

PURPOSE: The ideal breast irradiation method should provide an optimal dose distribution in the treated breast volume and a minimum scatter dose to the nearby normal tissue. Physical wedges have been used to improve the dose distribution in the treated breast, but unfortunately introduce an increased scatter dose outside the treatment field, particularly to the contralateral breast. The typical physical wedge (PW) was compared with the virtual wedge (VW) to determine the difference in the dose distribution affecting on the treated breast and the contralateral breast, lung, heart and surrounding peripheral soft tissue.
METHODS
AND MATERIALS: The data collected consisted of a measurement taken with solid water, a Humanoid Alderson Rando phantom and patients. The radiation doses at the ipsilateral breast and skin, contralateral breast and skin, surrounding peripheral soft tissue, and ipsilateral lung and heart were compared using the physical wedge and virtual wedge and the radiation dose distribution and DVH of the treated breast were compared. The beam-on time of each treatment technique was also compared. Furthermore, the doses at treated breast skin, contralateral breast skin and skin 1.5 cm away from the field margin were also measured using TLD in 7 patients of tangential breast irradiation and compared the results with phantom measurements.
RESULTS
The virtual wedge showed a decreased peripheral dose than those of a typical physical wedge at 15degrees, 30degrees, 45degrees, and 60degrees. According to the TLD measurements with 15degrees and 30degrees virtual wedge, the irradiation dose decreased by 1.35% and 2.55% in the contralateral breast and by 0.87% and 1.9% in the skin of the contralateral breast respectively. Furthermore, the irradiation dose decreased by 2.7% and 6.0% in the ipsilateral lung and by 0.96% and 2.5% in the heart. The VW fields had lower peripheral doses than those of the PW fields by 1.8% and 2.33%. However the skin dose increased by 2.4% and 4.58% in the ipsilateral breast. VW fields, in general, use less monitor units than PW fields and shortened beam-on time about half of PW. The DVH analysis showed that each delivery technique results in comparable dose distribution in treated breast.
CONCLUSIONS
A modest dose reduction to the surrounding normal tissue and uniform target homogeneity were observed using the VW technique compare to the PW beam in tangential breast irradiation The VW field is dosimetrically superior to the PW beam and can be an efficient method for minimizing acute, late radiation morbidity and reduce the linear accelerator loading by decreasing the radiation delivery time.