Purpose: In case of external breast radiotherapy, the usual treatment consists of two tangential beams homogeneously attenuated by a dynamic or physics wedge in order to obtain the most homogeneous dose distribution as possible. Depending of the shape and size of the breast volume, we may observe with this technique dose heterogeneity over 20% from the recommendation of the International Committee on Radiation Units and Measurements (95-107%). We propose to study breast treatment planning by compensating tissues thickness in order to decrease dose heterogeneity observed on the dose distribution for conventional treatment.
Materials and methods: We have segmented the initial tangential beams used for this kind of treatment into several smaller beams. Their shape was adapted to the distribution of the greys level on the DRR image. Therefore, we have compensated the thickness gradient and we have given the right dose to the right thickness group.
Results: Dose distribution performed with this method shows an improvement of the dose homogeneity in the three space dimensions and a decrease of the maximal dose between 5 and 10% over the ICRU recommendation.
Conclusion: This technique allows us to perform breast irradiation on a single photon energy linac even if the treated volume presents important thickness gradient. However, in case of large breast, this method is not able to reduce the overdosage at the entry of the volume due to inappropriate photon energy relative to the breast thickness.