The study found that women who underwent bilateral mastectomy had a significantly lower risk of developing contralateral breast cancer compared to those who had a lumpectomy or unilateral mastectomy. Specifically, the 20-year risk of developing a new cancer in the opposite breast was 6.9% in the lumpectomy and unilateral mastectomy groups, compared to just 0.3% in the bilateral mastectomy group. This suggests that removing both breasts can indeed prevent the occurrence of cancer in the other breast, but this preventive measure does not necessarily translate into a survival advantage.