Clinical utility of genetic signatures in selecting adjuvant treatment: Risk stratification for early vs. late recurrences.

Adjuvant endocrine therapy (ET) reduces the odds of distant recurrence and mortality by nearly one-half in women with hormone receptor (HR) positive early stage breast cancer. While the risk of recurrence is lower for HR positive than negative patients during the first 5-7 years, HR positive patients suffer ongoing recurrences between 0.5 and 2% year over subsequent years. Extended adjuvant ET further reduces recurrence during this late phase of follow-up. ET is associated with post-menopausal side effects (hot flashes, sexual dysfunction, mood changes, and weight gain), and occasional major toxicities (thrombosis and endometrial cancer with tamoxifen; bone mineral loss and possibly heart disease with AIs) persist throughout therapy. Accurate and reliable estimates of the risk of recurrence after five years of ET for women with prior HR positive breast cancer would permit appropriate extended ET decisions. The risk of long-term relapse is related to lymph node status and size of tumor, but these are relatively crude. Several groups have investigated whether multi-parameter tumor biomarker tests might identify those patients whose risk of recurrence is so low that extended ET is not justified. These assays include IHC4, the 21-gene "OncotypeDX", the 12-gene "Endopredict," the PAM50, and the 2-gene "Breast Cancer Index (BCI)" assays. The clinical validity of all these tests for this use context have been established, with at least one paper for each that shows a statistically significant difference in risk of distant recurrence during the 5-10 years after the initial five years of adjuvant endocrine therapy. However, the stakes are high, and although each of these represents a "prospective retrospective" study, they require further validation in subsequent datasets before they should be considered to have "clinical utility" and are used to withhold potentially life-saving treatment. Perhaps more importantly, the clinical breast cancer community, and especially the patient, need to determine how low the risk of late recurrence needs to be to forego the toxicities and side effects of extended adjuvant ET.