A study on contralateral breast surface dose for various tangential field techniques and the impact of set-up error on this dose

The risk of inducing contralateral breast (CLB) cancer in patients undergoing tangential field irradiation for the treatment of breast cancer is a serious concern in radiation oncology. A bilateral breast phantom made of wax attached onto the Alderson Rando phantom was used for studying the CLB dose for techniques using physical wedges, EDWs, IMRT and open fields. The skin dose to the CLB was measured at four different points (3 cm from the medial border of the tangential field (PI), nipple (P3), axilla (P4), midpoint between P3 and P1 (P2)). The highest measured dose occurred at P I with the 60 degrees physical wedges; it was 15.3 % of the dose at isocentre. Similarly, the dose measured at P3 (nipple) with 60 degrees physical wedges was 1.90 times higher than the dose with 60 degrees EDWs. The dose at P I for IMRT (7.8%) was almost the same as that for the open field (8.7%). The skin dose measured at the nipple was 2.1 - 10.9 % of the isocentre dose. The highest CLB doses were contributed by medial wedged fields. The dose to the CLB can be reduced by using IMRT or avoiding wedging the medial tangential fields. A set-up error in the longitudinal direction has little impact on the CLB dose. Set-up errors > 1 cm in the vertical and lateral directions have significant impact on the CLB dose.