Recurrent tumors refer to the reemergence or regrowth of cancer cells after initial treatment and remission. These tumors can occur in the same location as the original cancer or in different parts of the body. Recurrent tumors are a significant challenge in cancer treatment, as they indicate that the cancer has become resistant to previous treatments and may require a different approach for management. Understanding the mechanisms behind recurrent tumors and developing targeted therapies to prevent their occurrence are ongoing areas of research in the field of oncology.