Treatment planning for the breast patient: with or without lung correction?

Patients who have undergone a surgical lumpectomy for breast cancer are routinely referred to our clinic for a course of radiation therapy. This treatment consists of a pair of obliquely opposing radiation fields that encompass the breast and underlying structures. In order to treat the chest wall, a portion of lung must be included in this target volume, as breast tumours can invade the underlying chest wall due to direct invasion, blood invasion, or lymphatic permeation. At our clinic, we have always included this volume as normal tissue (with normal tissue density); thus, an inhomogeneity correction has never been generated on any breast plan. The aim of our study was to accurately measure the volume of lung within the treatment fields at time of simulation, using the simulator films and patient parameters, and to determine the point at which this volume of lung significantly altered the computer plan. This would necessitate the generation of a new distribution in order to produce a homogeneous dose to within +/- 5%. A table was constructed to identify those patients who had excessive lung volumes. At time of simulation it would be possible, using the table, to determine the type of treatment plan required for each patient. Those patients demonstrating an excessive lung volume would require a more extensive planning procedure utilizing CT (computerized tomography). An inhomogeneity correction could then be applied to the computerized distribution.