Introduction

The presence of circulating tumor cells (CTCs) is a predictor of minimal residual disease (MRD) and treatment outcomes. Abscopal effects of targeted intraoperative radiotherapy have been observed clinically, and the tumor microenvironment may influence these outcomes. Recent reports have shown activation and transit of CTCs following simulated radiotherapy from irradiated regions in vitro. Thus, the cellular extravasation and invasion phenotype cascade of CTCs in irradiated tumor regions could be highly concerning and raises several clinical questions. For the first time, we retrospectively analyzed patients who underwent radiotherapy to observe a clinical correlation between the presence of CTCs and overexpression of the PD-L1 protein in blood circulation as an indicator of minimal residual disease and a potential radiotherapy-associated effect.

Methods

A cohort of 26 pan-cancer patients (female = 10, male = 16) was analyzed, including cases of colorectal cancer (n = 4), lung cancer (n = 4), endometrial cancer (n = 4), head and neck cancer (n = 4), pancreatic cancer (n = 1), ovarian cancer (n = 1), renal cell carcinoma (n = 4), breast cancer (n = 2), and bone cancer (n = 2). Blood samples were analyzed retrospectively based on clinical and treatment history. Enumeration of CTCs was performed using the immunomagnetic multi-component OncoDiscover platform approved by CDSCO India, mediated by anti-EpCAM antibodies. CTCs were identified based on the presence of CK18+, PD-L1+, DAPI+, and CD45− staining using an automated Zeiss microscope in 1.5 ml of blood samples.

Results

In the retrospective analysis, a total of 88% (23/26) of patients showed the presence of CTCs in 1.5 ml of blood. Among these patients, 46% (n = 12/26) had undergone radiotherapy at some point during their treatment history. Three patients had received focused radiotherapy for brain metastasis with primary cancers of head and neck (1 case) and endometrium (2 cases). Among patients who underwent radiotherapy, 88% (8/9) were observed to have at least one CTC along with PD-L1 overexpression in at least one CTC. In addition, one patient who underwent radiotherapy showed the presence of a CTC cluster.

Conclusions

For the first time, we demonstrate the association of CTCs following radiotherapy in irradiated tumor regions. The disseminated CTCs may enhance tumor cell reinvasion through active transit in circulation, supported by a favorable microenvironmental milieu. Further validation with larger clinical cohorts across multiple cancer types is required.