Micrometastasis is a form of metastasis

Metastasis

Metastasis, or metastatic disease , is the spread of a disease from one organ or part to another non-adjacent organ or part. It was previously thought that only malignant tumor cells and infections have the capacity to metastasize; however, this is being reconsidered due to new research...

 (the spread of a cancer from its original location to other sites in the body) in which the newly formed tumors are too minuscule to be detected

Adjuvant chemotherapy

Adjuvant chemotherapy

Adjuvant therapy, also called adjuvant care, is treatment that is given in addition to the primary, main or initial treatment. The surgeries and complex treatment regimens used in cancer therapy have led the term to be used mainly to describe adjuvant cancer treatments...

 and adjuvant radiotherapy are directed at killing any micrometastases, thus preventing relapse (regrowth of the cancer) and the (likely) death of the patient.