There is insufficient evidence that intentional weight loss will affect cancer risk for any cancer. A very limited number of observational studies have examined the effect of weight loss, and a few found some decreased risk for breast cancer among women who have lost weight. However, most of these studies have not been able to evaluate whether the weight loss was intentional or related to other health problems.
One recent study that examined the effect of intentional weight loss found that women who experienced intentional weight loss of 20 or more pounds and were not currently overweight had cancer rates at the level of healthy women who never lost weight. However, unintentional weight loss episodes were not associated with decreased cancer risk.
This answer is based on source information from the National Cancer Institute.