Lumps in the armpit can arise from a variety of conditions. In lactating women, the breast can enlarge to the extent that the tail of the breast, which lies in the armpit region, can become enlarged as well ("axillary breast tissue"). When this is present, it is an entirely benign condition. Breast masses can occasionally arise in the same area of the breast (axillary tail of the breast), and when they are present, they should be evaluated just like any other breast mass, in order to determine whether they are benign or malignant. Finally, there are numerous lymph nodes in the armpit area. Most of the time, enlarged lymph nodes in the armpit are due to benign inflamamtory or infectious causes. Occasionally, however, axillary lymh nodes can become enlarged due to malignant causes, including primary malignancy of the lymph nodes (lymphoma) or secondary malignancy of the lymph nodes (metastatic cancer).
A visit with a qualified general surgeon would be a good place for your daughter to start, in order to begin to sort out these various possibilities in her case.
Sincerely/ Robert A. Wascher, MD, FACS