CHICAGO — Most women with the most common form of early-stage breast cancer can safely skip chemotherapy without hurting their chances of beating the disease, doctors are reporting from a landmark study that used genetic testing to gauge each patient’s risk. The study is the largest ever done of breast cancer treatment, and the results are expected to spare up to 70,000 patients a year in the United States and many more elsewhere the ordeal and expense of these drugs. “The impact is tremendous,” said the study leader, Dr.