The purpose of this study was to introduce a new parameter which enhances breast cancer detection using X-ray mammography. We used the database of X-ray mammograms generated by the Japan Society of Radiological Technology. The new parameter called 'quasi-fractal dimension (Q-FD)' was calculated from the relationship between the cutoff values for the maximum image intensity in the lesion set at 21 levels from 20% to 100% at equal intervals and the number of pixels with an intensity exceeding the cutoff value. In addition to Q-FD, the image features such as curvature (C) and eccentricity (E) were extracted. The conventional fractal dimension (C-FD) was also calculated using the box-counting method. We used artificial neural networks (ANNs) as a classification method. When using C, E, C-FD and age as inputs in 208 ANNs and taking the number of neurons in the hidden layer as 50, we found the area under the receiver operating characteristic curve (A(z)) was 0.87+/-0.07 in the task differentiating between benign and malignant masses. When Q-FD was added to inputs in addition to the above parameters, the A(z) value was significantly improved to become 0.93+/-0.09. These results suggested that Q-FD is effective for discriminating between benign and malignant masses.