Dose volume histogram analysis of lung radiation from chest wall treatment: comparison of electron arc and tangential photon beam techniques.

The technique of electron arc irradiation of the post-mastectomy chest wall was developed to improve dose uniformity and to reduce lung irradiation in comparison to that seen with standard chest wall tangent photon beam methods. Because of the cephalocaudal variation in chest wall shape and thickness, electron arc treatment planning requires anatomical detail provided by multiple axial CT images of the thorax. To compare the fixed beam and rotational techniques, computer simulated beams covering the chest wall and internal mammary lymphatics were retrospectively applied to the CT-derived contours obtained during treatment planning for 12 consecutive patients receiving adjuvant chest wall treatment by electron arc. The lung dose distribution for each technique was calculated using heterogeneity corrections. The multiplanar 2-dimensional isodose distributions were summed to provide estimated 3-dimensional dose distributions of integral histograms. These reveal that for most of these patients a modest to large improvement in volume-dose relationship occurs with the electron arc technique.