Inadequacy of conventional computerized tomography scans for treatment planning of tangential breast (chest wall)...

The use of transverse tomography treatment-planning scans, which have to be taken in the exact treatment position of the patients, is essential for precision radiotherapy of breast cancer. Transverse tomography scans give all necessary informations for an optimal isodose distribution within the target volume for the individual patient, and maximal sparing of normal tissues (e.g. lung) is achieved. The exact topography of internal mammary nodes may also be incorporated in the planning scan with the help of lymphoscintigraphy. Conventional computerized tomography (CT) scanners have too small gantry openings (50-70 cm) to allow for proper positioning of the patients under treatment conditions. Using such treatment-planning scans for patients in the actual treatment position would cause an underdosage in excess of 5% within the target volume, when 60Co fields are used. Also, a much larger volume of normal tissues (mainly lung) would unnecessarily be irradiated. Treatment-planning scans obtained by TAT (transverse analog tomography) do not have the crucial positioning problems of conventional CT scanners and enable the precise transformation of a valid isodose distribution to each patient. In addition, positioning wedges (with adjustable wedge angles) may be placed under the patient to achieve the optimal treatment position. A useful alternative of TAT scanning for precise treatment planning of patients with breast cancer would be a CT scanner with a wide enough gantry opening to allow for any patient positioning under actual treatment conditions.