Clinical Implications of Occult Systemic Micrometastatic Breast Cancer

Introduction

Overall, around 15% to 20% of breast cancers recur as distant metastatic disease, and recurrence rates are higher if local recurrences or second breast cancers are also included. Clinically apparent metastatic breast cancer is not curable with current treatment modalities. Current clinical guidelines discourage routine testing of asymptomatic patients during follow-up who have completed therapy with curative intent, because two randomized clinical trials conducted 30 years ago failed to show improved survival with more intense follow-up and early intervention with therapies that were available in the late 1980s. In one of these trials, patients were randomly assigned to physician visits and bone scan, liver echography, chest x-ray, and laboratory tests at predefined intervals (n = 655), or to a control arm (n = 665) in which patients were seen by their physicians at the same frequency but no tests were performed. At a median follow-up of 71 months, there was no difference in overall survival between the intensive follow-up and regular follow-up arms. There was also no significant difference in time to detection of recurrence between the two arms, implying that the intense follow-up did not detect cancer earlier. In a second trial (n = 1243) patients were randomized to a standard of care including physical examination and mammography or to intense follow-up with chest x-ray and bone scan every 6 months. Increased detection of isolated intrathoracic and bone metastases was evident in the intense follow-up group, and the 5-year relapse-free survival was lower in this arm due to earlier detection of recurrences; however, 5-year overall survival was the same in both arms. These trials provide the basis for the current guideline recommendations against screening for metastatic recurrence. We continue to perform annual mammograms as screening for new primary breast cancers.

Several important advances have happened since the early 1990s when these trials were conducted. Many highly effective novel therapies were introduced to treat metastatic breast cancer, including CDK4/6 inhibitors, PIK3CA inhibitors, PARP inhibitors, antibody drug conjugates, new HER2-targeted therapies, and immune checkpoint inhibitor therapy. New technologies emerged that can detect impending cancer recurrence in the blood before it becomes clinically apparent including measurements of circulating tumor DNA (ctDNA) and circulating tumor cell (CTC) in the blood. These advances require reevaluation of the old clinical paradigm that screening for early metastatic recurrence is clinically futile (i.e., no impact on survival) and potentially harmful (i.e., deterioration of quality of life). The remarkable success of systemic adjuvant therapies in early-stage breast cancer demonstrates that modern therapies can eradicate clinically invisible micrometastatic disease and improve cure rates in stage I to III breast cancer. The new, more sensitive, and highly specific blood tests can identify signs of impending metastatic relapse while the disease is still micrometastatic. Detection of molecular relapse coupled with early intervention with therapies that are used successfully to treat large volume metastatic disease raises the possibility of preventing clinically apparent, and invariably lethal, metastatic recurrence. This therapeutic strategy has been used successfully in leukemias where diagnostic tests to detect molecular relapse, before any clinical signs of recurrence, have long been available. More recently, two large randomized trials in prostate cancer also demonstrated that early intervention for rising PSA levels, in the absence of any clinically detectable disease, can improve metastasis-free survival. In the PROSPER trial, enzalutamide treatment reduced the risk of metastases or death compared with placebo, and similar results were obtained with apalutamide in the SPARTAN trial.

This chapter will review the rapidly evolving field of molecular detection of micrometastasis using blood-based assays, as well as the ongoing clinical trials whose results might change our follow-up procedures in early-stage breast cancer in the not too distant future ( Table 58.1 ).

Liquid Biopsy

Liquid biopsy refers to blood tests that detect molecular abnormalities that characterize a patient’s cancer. Through the liquid biopsy one can gain insights into the cancer that previously only a direct biopsy of the cancer tissue could yield, including establishing the diagnosis of recurrence or detecting genomic alterations that predict response to targeted therapies. Cancers secrete or passively release a variety of biological molecules (proteins, exosomes, RNA, DNA) into the blood, and intact CTCs can also be directly recovered and quantified from a blood sample. Protein-based tumor markers, including CA15-3/CA27-29, CEA, CA125, CA19-9, were discovered in the 1980s and typically measure aberrant glycosylation of cell surface proteins that are shed into the blood. Despite initial enthusiasm, their clinical utility is limited by low sensitivity and low specificity. Occasionally, a rising CA27-29 level in an asymptomatic patient can herald metastatic recurrence, but several benign conditions and nonbreast cancer malignancies can also lead to mild elevation of this marker (i.e., modest specificity), and in many instances, patients experience recurrence in the absence of elevation of CA27-29 (i.e., low sensitivity). However, in patients with metastatic breast cancer who have elevated CA27-29, this marker may be useful as adjunct to assess response. Protein tumor markers may inform about tumor volume but do not provide any specific information about tumor biology and, therefore, are not considered to be liquid biopsy.

Quantification of CTCs in the blood is a more recent technological breakthrough that continues to evolve in search of a useful clinical niche. CTC positivity is defined as five or more tumor cells (i.e., nucleated cytokeratin positive cells) per 7.5 mL of blood using a cell separating instrument and image analysis software. This threshold was selected because quantification below this number is unstable and error prone. Early-stage breast cancers rarely have CTCs >5/7.5 mL blood which makes the original technology of limited use in stage I to III breast cancers. In metastatic disease, CTC numbers track closely with tumor volume and have strong prognostic value. Changes in CTC count 3 to 4 weeks after starting therapy can also accurately predict subsequent imaging response. However, a prospective randomized trial, S0500, failed to demonstrate benefit from using CTC response as trigger for early treatment switch. Therefore the current clinical utility of CTCs is limited. The CTC analysis technologies are rapidly improving and advances in phenotyping and genotyping of CTC might soon provide insights into sensitivity or resistance to therapies, and these could serve as pharmacodynamic monitoring tools; therefore CTCs are considered liquid biopsies.

Proof of concept studies have also shown that tumor-derived RNA and miRNA can also be detected in a patient’s blood and could, in theory, inform about tumor aggressiveness and drug resistance. Tumor-derived exosomes are secreted in extracellular vesicles ranging from 40 to 160 nm that carry nucleic acids or proteins that reflect the genetics of the tumor cells, and therefore could also function as novel biomarkers in the blood. However, none of these technologies are sufficiently mature enough to be certain about the technical robustness of the assays.

The most broadly used liquid biopsy technology is based on detecting circulating free tumor DNA (ctDNA) in the blood. This technology was originally developed as noninvasive prenatal testing to detect fetal DNA in pregnant women’s blood as a screening tool for congenital defects. ctDNA testing was rapidly adopted in cancer research and was shown to be more sensitive than CTC due to the high efficiency of DNA amplification techniques.

Liquid biopsies are explored in three clinical contexts: (1) molecular target profiling to rapidly detect mutations that predict response to targeted therapies (i.e., PIK3CA mutations), (2) monitor disease recurrence and progression on therapy, and (3) cancer screening in asymptomatic individuals. Of these three uses, only molecular target profiling has proven clinical utility and can be considered standard of care use of ctDNA assays. In the rest of this chapter, we will focus on the emerging use of this technology as a tool for disease monitoring and detection of occult micrometastatic cancer.