Treatment options for patients with cancer that is detected at a late stage are severely limited, which usually leads to an unfavorable prognosis for such patients. Indeed, the options available for patients with metastatic solid cancers are scarcely curative. Therefore, early diagnosis of neoplasia remains a fundamental mainstay for improving outcomes for cancer patients.
Histopathology is the current gold standard for cancer diagnosis. Biopsy is an invasive procedure that provides physicians with further samples to test but that furnishes limited information concerning tumor heterogeneity. Biopsy specimens are usually obtained only when there is clinical evidence of neoplasia, which significantly limits their usefulness in early diagnosis.
Around 20 years ago, it was discovered that the presence of circulating tumor cells (CTC) in patients with metastatic breast cancer who were about to begin a new line of treatment was predictive of overall and progression-free survival. The prognostic value of CTC was independent of the line of treatment (first or second) and was greater than that of the site of metastasis, the type of therapy, and the time to metastasis after complete primary resection. These results support the idea that the presence of CTC could be used to modify the system for staging advanced disease.
Since then,
Liquid vs. tissue
Liquid biopsy is a minimally invasive tool that is easy to use. It is employed to detect cancer, to assess treatment response, or to monitor disease progression. Liquid biopsy produces test material from primary and metastatic (or micrometastatic) sites and provides a more heterogeneous picture of the entire tumor cell population, compared with specimens obtained with tissue biopsy.
Metastasis
The notion that metastatic lesions are formed from cancer cells that have disseminated from advanced primary tumors has been substantially revised following the identification of disseminated tumor cells (DTC) in the bone marrow of patients with early-stage disease. These results have led researchers to no longer view cancer metastasis as a linear cascade of events but rather as a series of concurrent, partially overlapping processes, as metastasizing cells assume new phenotypes while abandoning older behaviors.
The initiation of metastasis is not simply a cell-autonomous event but is heavily influenced by complex tissue microenvironments. Although colonization of distant tissues by DTC is an extremely inefficient process, at times, relatively numerous CTC can be detected in the blood of cancer patients (> 1,000 CTC/mL of blood plasma), whereas the number of clinically detectable metastases is disproportionately low, confirming that tumor cell diffusion can happen at an early stage but usually occurs later on.
Early dissemination
Little is currently known about the preference of cancer subtypes for distinct tissues or about the receptiveness of a tissue as a metastatic site. What endures as one of the most confounding clinical phenomena is that patients may undergo tumor resection and remain apparently disease free for months, years, and even decades, only to experience relapse and be diagnosed with late-stage metastatic disease. This course may be a result of cell seeding from minimal residual disease after resection of the primary tumor or of preexisting clinically undetectable micrometastases. It may also arise from early disseminated cells that remain dormant and resistant to therapy until they suddenly reawaken to initiate proliferation into clinically detectable macrometastases.
Dormant DTC could be the main reason for delayed detection of metastases. It is thought that around 40% of patients with prostate cancer who undergo radical prostatectomy present with biochemical recurrence, suggesting that it is likely that hidden DTC or micrometastases are present at the time of the procedure. The finding is consistent with the detection of DTC many years after tumor resection, suggesting they were released before surgical treatment. Nevertheless, research into tumor cell dormancy is limited, owing to the invasive and technically challenging nature of obtaining DTC samples, which are predominantly taken from the bone marrow.
CTC metastases
Cancer cells can undergo epithelial-to-mesenchymal transition to facilitate their detachment from the primary tumor and intravasation into the blood circulation (step 1). Dissemination of cancer cells from the primary tumor into circulation can involve either single cells or cell clusters containing multiple CTC as well as immune cells and platelets, known as microemboli. CTC that can survive in circulation (step 2) can exit the bloodstream (step 3) and establish metastatic tumors (step 4), or they can enter dormancy and reside in distant organs, such as the bone marrow.
Use in practice
CTC were discovered over a century ago, but only in recent years has technology been sufficiently advanced to study CTC and to assess their usefulness as biomarkers. Recent evidence suggests that not only do the number of CTC increase during sleep and rest phases but also that these CTC are better able to metastasize, compared to those generated during periods of wakefulness or activity.
CTC clusters (microemboli) are defined as groups of two or more CTC. They can consist of CTC alone (homotypic) or can include various stromal cells, such as cancer-associated fibroblasts or platelets and immune cells (heterotypic). CTC clusters (with or without leukocytes) seem to have greater metastatic capacity, compared with individual CTC.
A multitude of characteristics can be measured in CTC, including genetics and epigenetics, as well as protein levels, which might help in understanding many processes involved in the formation of metastases.
Quantitative assessment of CTC could indicate tumor burden in patients with aggressive cancers, as has been seen in patients with primary lung cancer.
Early cancer diagnosis
Early research into CTC didn’t explore their usefulness in diagnosing early-stage tumors because it was thought that CTC were characteristic of advanced-stage disease. This hypothesis was later rejected following evidence of local intravascular invasion of very early cancer cells, even over a period of several hours. This feature may allow CTC to be detected before the clinical diagnosis of cancer.
CTC have been detected in various neoplastic conditions: in breast cancer, seen in 20% of patients with stage I disease, in 26.8% with stage II disease, and 26.7% with stage III disease; in nonmetastatic colorectal cancer, including stage I and II disease; and in prostate cancer, seen in over 50% of patients with localized disease.
The presence of CTC has been proven to be an unfavorable prognostic predictor of overall survival among patients with early-stage non–small cell lung cancer. It distinguishes patients with pancreatic ductal adenocarcinoma from those with noncancerous pancreatic diseases with a sensitivity of 75% and a specificity of 96.3%.
CTC positivity scoring (appropriately defined), combined with serum prostate-specific antigen level, was predictive of a biopsy diagnosis of clinically significant prostate cancer.
All these data support the utility of CTC in early cancer diagnosis. Their link with metastases, and thus with aggressive tumors, gives them an advantage over other (noninvasive or minimally invasive) biomarkers in the early identification of invasive tumors for therapeutic intervention with better cure rates.
This article was translated from Univadis Italy. A version appeared on Medscape.com.