Currently, neoadjuvant therapy has been shown to be effective in treating HER2-positive breast cancer; the development and assessment of novel medications and therapy regimens are being continued. A tumor response to the treatment is the most important factor in planning adjuvant therapy. The use of different systems to evaluate a therapeutic effect gives rise to significant differences in estimating the rate of complete morphological regression according to the results of performed therapy. The paper describes a procedure to estimate cancer burden using the RCB system. This procedure is highly reproducible and recommended by the 2015 European Society for Medical Oncology Clinical Practice Guidelines for the management of primary breast cancer.