In1 J Radialr on Oncology Bf ol Phs., Vol. 26, pp. 883-890 0360.3016193 $6.00 + .I% Pnnted in the U.S.A. All rights reserved. Copyright 0 1993 Pergamon Press Ltd. ?? Technical Innovations and Notes TOLERANCES IN SETUP AND DOSIMETRIC ERRORS IN THE RADIATION TREATMENT OF BREAST CANCER INDRA J. DAS, PH.D.,’ CHEE-W. CHENG, PH.D.,* HELEN FOSMIRE, M.D.,* KENNETH R. KASE, PH.D.’ AND THOMAS J. FITZGERALD, M.D.’ ‘Department of Radiation Oncology, University of Massachusetts Medical Center, Worcester, MA 0 1655: and ‘University of Arizona Health Sciences Center, Tucson, AZ 85724 Purpose: Treatment failure in radiation therapy, as well as unexpected complications, can be associated with set up changes or variations that can cause deviations from the prescribed radiation dose distribution both inside and outside the target volume. The effect of various deviations from the planned setup on the delivery of the prescribed radiation dose to the desired treatment volume was studied. Methods and Materials: Adding a second simulation was investigated as means of minimizing setup changes on treatment. The first simulation was used for planning the treatment and the second simulation was essentially a mock treatment. Dosimetric evaluations based on dose volume histograms were analyzed for each deviation in the setup. Results: In 95% of the patients, the frequency of the changes in the setup parameters between the second simulation and the treatment setup were reduced significantly from the changes that occurred between the first simulation and the second simulation. The changes in isocenter coordinates up to +l.O cm have minimal effects (f2%) on the dose distributions. Gantry angle variations up to +4” produce a change of less than fS% in the dose distribution within the target volume. However, this angular variation resulted in additional tissue irradiation outside of the desired treatment field (about 10 cm3 for a large patient). A gantry angle variation of +6” can change the volume of tissue that receives the prescribed dose by at least tlO%. In addition, such a change can increase the volume of tissue outside the desired treatment field that is irradiated. Conclusion: It is concluded that individually, deviations in one of the parameters from the planned setup of +l.O cm in isocenter position and +4” in gantry angle do not produce significant deviations from the planned dose distribution. However, a significant change in dose distribution is observed if the setup parameters are concurrently changed. A second simulation may minimize the deviations of the treatment setup from the planned setup and maximize the precision in dose delivery to the target volume. Breast cancer, Radiation treatment, Setup error, Dose volume histogram, Simulation. INTRODUCTION Radiation therapy has become an integral part of the management of breast cancer. It has been demonstrated that conservative surgery combined with radiation therapy has a comparable outcome when compared with total mastectomy (3, 18). A successful breast cancer treatment should provide a long disease-free survival and good breast cosmesis. Conservative surgery and radiation treatment have added to both of these goals in the management of breast cancer. Several late sequelae can be associated with the improper radiation treatment of breast cancer, in- cluding asymmetry, breast edema, skin thickening, fibro- sis, retraction, dimpling, telangiectasia, and radiation pneumonitis (4, 7, 11, 12, 17, 19). Because of the prox- imity of lung to breast tissue, a portion of lung is often in treatment field. The irradiated lung volume can be re- duced by choosing proper beam angles when parallel op- posed beams are used for the tangential field treatment of the breast. Numerous tangential field techniques used for breast cancer treatment have been described in the literature (6, 13, 16). It has become an accepted clinical practice to treat breast cancer using tangential fields, that properly cover the breast tissue, with nondivergent posterior beam edge to minimize the irradiated lung volume. If a third field is added its inferior border must be matched to the superior borders of the tangential fields such that the three beams form a match plane to avoid hot and cold spots (Fig. 1). This geometry can be achieved by using inde- Presented at the Breast Cancer Symposium in San Antonio, Texas, 5-7 December 1991. Reprint request to: Indra J. Das, Ph.D. Acknowledgements-We are grateful to our radiation thera- pists Barbara DeGaspe, Marsha O’Neil, and Stefania Zak for their help and patience in this study. Accepted for publication 14 January 1993. 883