The purpose of this study was to investigate methods used to modulate dose distributions in radiotherapy planning, to determine the fundamental features of these and to establish the attainable dose uniformity. Published modulation methods were categorized, and a simple physical model devised to predict the weight of the wedged beam and the relative dose distribution for each category. Each technique was applied to patient data with planning target volume sizes ranging from below 500 cm(3) to 2200 cm(3). The spatial distribution of high-dose regions in the breast, and maximum dose for the heart and lung, were determined for each plan. The dose uniformity was analysed by evaluating the volume of the breast (V(I)) receiving <95% and <105% of the prescribed dose. The difference between V(105%) and V(95%) for each method for each patient data set was also calculated. The simple model predicted the trend in percentage weight of the wedge beam and the form of the dose distribution in the transverse plane with the modulation method. Improvements in the dose uniformity were seen for the majority of modulation methods. The magnitude of the change was between 5.6% and 11.1% (p<0.05) of the breast volume for breast sizes above 500 cm(3). Some modulation methods introduced high dose at the chest wall. In conclusion, the majority of the methods improved dose uniformity for breast sizes of 500 cm(3) or greater. No method showed a clear advantage over the others. The use of modulation methods should be governed by consideration of its effects relative to a simple wedge plan.