THE EFFECT OF THE NUMBER OF COMPUTED TOMOGRAPHIC SLICES ON DOSE DISTRIBUTIONS AND EVALUATION OF TREATMENT PLANNING SYSTEMS FOR RADIATION-THERAPY OF INTACT BREAST

Purpose: This study was undertaken to answer the following questions i n breast irradiation: (a) How many calculation planes are sufficient f or three-dimensional (3-D) treatment planning? (b) Is pseudo-3-D plann ing system sufficiently accurate for 3-D treatment planning of a breas t? Methods and Materials: We carried out dose calculations and differe ntial dose-volume analysis on three representative patients covering t he range of breast size encountered in a clinic. The breast volumes we re reconstructed from computed tomography (CT) scans using three slice s, five slices and the full CT scan respectively. An established 3-D d ose algorithm and two pseudo-3-D commercial systems were used in the c alculations. Comparison of isodose distributions were made between the central axis plane, a cephalic and a caudal plane 6 cm above or below the central axis respectively. Results: When comparing isodose distri butions generated with conventional two-dimensional treatment planning with 3-D dose calculations, the former underestimated the size and ma gnitude of the hot spots in the medial and the lateral subcutaneous (S C) regions. When comparing the three-slice with the full CT model, whi le the three-slice model was found to be adequate for the ''small'' an d the ''medium'' size patients, the full CT model provided a more accu rate representation of dose distributions for the ''large'' patient. C omparison of a true 3-D algorithm with pseudo-3-D algorithms showed th at while the tatter systems were adequate for the ''small'' and the '' medium'' patients, significant differences were noted between the true 3-D and the pseudo-3-D algorithms for the ''large'' patient. Conclusi on: For patients whose breast contours vary slowly within the tangenti al fields, a three-slice CT scan as well as a pseudo-3-D approach appe ars to be adequate for clinical decision. However, for patients with l arge variation of contours within the tangential fields, a full scale CT scan with a true 3-D dose algorithm is more accurate than either th e three-slice or the five-slice model.