No, tumors do not usually shrink by themselves. However, there are certain types of tumors, such as some benign tumors, that may not require treatment and can eventually shrink on their own. However, most tumors, especially malignant tumors, should not be left untreated, as they are likely to grow and spread if left untreated. Treatment options for tumors include surgery, chemotherapy, radiation therapy, targeted therapy, and immunotherapy. Each type of treatment has its own set of risks and benefits, so it is important to discuss these options with your doctor in order to find the best option for you.