E17. Modern radiotherapy planning in the treatment of breast cancer

Abstract

Radiotherapy (RT) is an integral part of the management of early stage breast cancer following breast-conserving surgery and after mastectomy in patients at moderate to high risk for loco-regional recurrence. Absolute improvements in breast cancer-specific and overall survival have been demonstrated by the Early Breast Cancer Trialists’ Collaborative Group due to improvements in loco-regional control with RT and are approximately 4−5% at 15 years. 1 The gains in survival would likely be greater were it not for excess mortality observed from causes other than breast cancer, which have been shown to be primarily cardiac in nature. Therefore, treatment planning approaches which minimise treatment of uninvolved tissues are critical to the ultimate success achieved with RT. Radiation treatment planning for breast cancer has changed dramatically through the years with modifications in clinical target volumes, treatment delivery systems, and techniques. Early target volumes reflected a Halstedian philosophy in which extensive loco-regional fields were felt to be necessary for cure but through successive studies, volumes have been refined that maintain high rates of loco-regional control while minimising toxicity. Treatment delivery systems have evolved from orthovoltage units where skin dose and shallow depth dose distributions limited dose delivery to deep seated lesions to sophisticated computer-controlled treatment delivery systems which allow highly conformal therapy by the creation of dose distributions that closely conform to the shape of the target in three dimensions. While improvements in target definition, treatment delivery systems and changes in techniques are all critical in radiation delivery, the remainder of the discussion will be devoted to modern treatment planning techniques. Two-dimensional (2D) planning systems utilising tangential fields to the breast have resulted in high rates of tumour control. However, dose homogeneity across the entire breast is difficult to achieve with 2D planning due to variations in breast contour and tissue density. Use of wedges with lung density correction significantly reduced dosimetric hot spots along the central axis 2 ; however, lack of homogeneity within the entire field was still problematic particularly in large-breasted women,