DOSIMETRIC COMPARISON OF TREATMENT PLANNING SYSTEMS IN IRRADIATION OFBREAST WITH TANGENTIAL FIELDS

Purpose: The objectives of this study are: (1) to investigate the dosi metric differences of the different treatment planning systems (TPS) i n breast irradiation with tangential fields, and (2) to study the effe ct of beam characteristics on dose distributions in tangential breast irradiation dth 6 MV linear accelerators from different manufacturers. Methods and Materials: Nine commercial and two university-based TPS a re evaluated in this study, The computed tomographic scan of three rep resentative patients, labeled as ''small'', ''medium'' and ''large'' b ased on their respective chest wall separations in the central axis pl ane (CAX) were used, For each patient, the tangential fields were set up in each TPS, The CAX distribution was optimized separately with lun g correction, for each TPS based on the same set of optimization condi tions, The isodose distributions in two other off-axis planes, one 6 c m cephalic and the other 6 cm caudal to the CAX plane were also comput ed, To investigate the effect of beam characteristics on dose distribu tions, a three-dimensional TPS was used to calculate the isodose distr ibutions for three different linear accelerators, the Varian Clinac 6/ 100, the Siemens MD2 and the Philips SL/7 for the three patients, In a ddition, dose distributions obtained with 6 MV X-rays from two differe nt accelerators, the Varian Clinac 6/100 and the Varian 2100C, were co mpared. Results: For all TPS, the dose distributions in all three plan es agreed qualitatively to within +/- 5% for the ''small'' and the ''m edium'' patients, For the ''large'' patient, all TPS agreed to within +/- 4% on the CAX plane, The isodose distributions in the caudal plane differed by +/- 5% among all TPS,In the cephalic plane in which the p atient separation is much larger than that in the CAX plane, six TPS c orrectly calculated the dose distribution showing a cold spot in the c enter of the breast contour, The other five TPS showed that the center of the breast received adequate dose, Isodose distributions for 6 MV X-rays from three different accelerators differed by about +/- 3% for the ''small'' patient and more than +/- 5% for the ''large'' patient, For two different 6 MV machines of the same manufacturer, the isodose distribution agreed to within +/- 2% for all three planes for the ''la rge'' patient. Conclusion: The differences observed among the various TPS in this study were within +/- 5% for both the ''small'' and the '' medium'' patients while doses at the hot spot exhibit a larger variati on, The large discrepancy observed in the off-axis plane for the ''lar ge'' patient is largely due to the inability of most TPS to incorporat e the collimator angles in the dose calculation, Only six systems invo lved agreed to within +/- 5% for all three patients in all calculation planes, The difference in dose distributions obtained with three acce lerators from different manufacturers is probably due to the differenc e in beam profiles, On the other hand, the 6 MV X-rays from two differ ent models of linear accelerators from the same manufacturer have simi lar beam characteristics and the dose distributions are within +/- 2% of each other throughout the breast volume, In general, multi-institut ional breast treatment data can be compared within a +/- 5% accuracy, (C) 1997 Elsevier Science Inc.