DESIGN OF COMPENSATORS FOR BREAST RADIOTHERAPY USING ELECTRONIC PORTAL IMAGING

A novel method of designing intensity modulated beams (IMBs) to achiev e compensation in external beam radiotherapy of the breast, without th e need for CT scans, is presented. The design method comprises three p arts: (1) an electronic portal image is used to generate a map of radi ological thickness; (2) this map is then used to obtain an estimate of the breast and lung outline; (3) a TMR-based dose calculation algorit hm is then used to determine the optimum beam profile to achieve the b est dose distribution. The dose distributions calculated for IMBs were compared with those calculated for the use of simple wedges. The resu lts for two patients studied indicate that the dose inhomogeneity for IMBs is +/- 5%, compared with a value of +/- 10% for a wedged plan. Th e uncertainty in radiological thickness measurement corresponds to a d osimetric error of +/- 2%, Other errors associated with outline estima tion are typically less than 2%, with a largest value of +5% for one o f the patients who had a large and highly asymmetrical breast, The res ults for the two patients studied suggest that the uncertainties in th e method are significantly smaller than the improvement in dose unifor mity produced.