Background

The role of circulating tumor cells (CTCs) has been well established in predicting survival in metastatic settings, particularly in breast, colorectal, and prostate cancers. However, their clinical utility has been limited due to high costs, variability in sensitivity and accuracy, and the use of cutoff-based interpretations. The biological significance of CTCs—from extravasation and invasion to their contribution to tumor microenvironment dynamics and tumor burden—suggests greater clinical relevance than is currently applied in practice.

Their role in monitoring minimal cellular residual disease (MCRD), especially in early-stage cancers post-surgery, remains underexplored, including decisions regarding therapy duration in diseases such as colorectal cancer and longitudinal monitoring for recurrence. Dynamic PD-L1 expression on CTCs may indicate incomplete tumor resection or treatment response and may also reflect cellular dormancy in circulation, potentially enabling immune evasion. In this study, we report the expression of PD-L1 on CTCs and CTC clusters in colorectal cancer patients.

Methods

We retrospectively analyzed 666 colorectal cancer patients (63.06% male and 36.94% female), spanning early- to late-stage disease, for the presence of CTCs with and without PD-L1 expression, as well as CTC clusters. CTCs were detected using the CDSCO-approved OncoDiscover platform in 1.5 mL of peripheral blood. Cells were classified as CTCs if they were EpCAM⁺, CK18⁺, DAPI⁺, and CD45⁻, and were identified using an automated Zeiss microscope system.

Results

At baseline analysis, 74.25% (n = 591) of patients had ≥1 CTC per 1.5 mL of blood. CTC counts ranged from 1 to 20 cells. Among patients with detectable CTCs, 74.62% (n = 441) exhibited PD-L1 expression. The highest proportion of CTCs (~25.86%, n = 352) was observed in the 61–70 years age group. CTC clusters were detected in 13.00% (n = 156) of patients, and notably, more clusters were observed during follow-up compared with baseline. The mean CTC count (including clusters) was 1.71, while the mean PD-L1–positive CTC count was 1.02.

Conclusions

PD-L1 expression on CTCs may contribute to their ability to persist in circulation through immune evasion, potentially enabling dormancy via surface protein overexpression that helps them avoid elimination by immune T cells. The CTC–PD-L1 assay shows strong potential for patient surveillance both before and after treatment in assessing minimal cellular residual disease. Further clinical studies in this direction are strongly warranted.