read the story first then the question at the end please thank you

Dalila was doing a monthly self-exam by palpation when she felt an unusual lump in her breast. Following a mammogram, she was diagnosed with cancer on her left breast. Base of the results of that test ,her  physician suggested  a radical mastectomy instead of a lumpectomy or a mastectomy. Post surgery , a biopsy was done and her follow up treatment included chemotherapy. Soon after she lost her hair, and became more and more tired.  Her arm became edematous and remained uncomfortable for months. Eventually her physician included treatment with erythropoitin.

Why was chemotherapy done for Dalila in spite of her surgery?