Circulating tumor cells (CTCs) have been recognized as a major contributor to distant metastasis. Their unique role as metastatic seeds renders them a potential marker in the circulation for early cancer diagnosis and prognosis as well as monitoring of therapeutic response. In the past decade, researchers mainly focused on the development of isolation techniques for improving the recovery rate and purity of CTCs. These developed techniques have significantly increased the detection sensitivity and enumeration accuracy of CTCs. Currently, significant efforts have been made toward comprehensive molecular characterization, ex vivo expansion of CTCs, and understanding the interactions between CTCs and their associated cells (e.g., immune cells and stromal cells) in the circulation. In this review, we briefly summarize existing CTC isolation technologies and specifically focus on advances in downstream analysis of CTCs and their potential applications in precision medicine. We also discuss the current challenges and future opportunities in their clinical utilization.

Purpose: Immunotherapy (IT) and radiotherapy (RT) can act synergistically, enhancing antitumor response beyond what either treatment can achieve separately. Anecdotal reports suggest that these results are in part due to the induction of an abscopal effect on non-irradiated lesions. Systematic data on incidence of the abscopal effect are scarce, while the existence and the identification of predictive signatures or this phenomenon are lacking. The purpose of this pre-clinical investigational work is to shed more light on the subject by identifying several imaging features and blood counts, which can be utilized to build a predictive binary logistic model. Materials and methods: This proof-of-principle study was performed on Lewis Lung Carcinoma in a syngeneic, subcutaneous murine model. Nineteen mice were used: four as control and the rest were subjected to combined RT plus IT regimen. Tumors were implanted on both flanks and after reaching volume of ~200 mm³ the animals were CT and MRI imaged and blood was collected. Quantitative imaging features (radiomics) were extracted for both flanks. Subsequently, the treated animals received radiation (only to the right flank) in three 8 Gy fractions followed by PD-1 inhibitor administrations. Tumor volumes were followed and animals exhibiting identical of better tumor growth delay on the non-irradiated (left) flank as compared to the irradiated flank were identified as experiencing an abscopal effect. Binary logistic regression analysis was performed to create models for CT and MRI radiomics and blood counts, which are predictive of the abscopal effect. Results: Four of the treated animals experienced an abscopal effect. Three CT and two MRI radiomics features together with the pre-treatment neutrophil-to-lymphocyte (NLR) ratio correlated with the abscopal effect. Predictive models were created by combining the radiomics with NLR. ROC analyses indicated that the CT model had AUC of 0.846, while the MRI model had AUC of 0.946. Conclusions: The combination of CT and MRI radiomics with blood counts resulted in models with AUCs of 1 on the modeling dataset. Application of the models to the validation dataset exhibited AUCs above 0.84, indicating very good predictive power of the combination between quantitative imaging and blood counts.

Purpose: To investigate the relationships between circulating tumor cells (CTCs), free circulating DNA (fcDNA) and biochemical response in prostate cancer patients treated primarily versus salvage radiotherapy (RT). Methods and Materials: Blood was collected prospectively from patients, enrolled in two institutional Phase II trials for primary and salvage RT. Three blood samples were collected at: (i) prior to treatment [RT or androgen deprivation therapy (ADT)], (ii) last week of RT, and (iii) three months post-RT. CTCs were quantified in 31 samples from 12 primary patients and 30 samples from 12 salvage patients; fcDNA were analyzed in 11 primary (28 samples) and 5 (9 samples) salvage patients. CTCs were visualized by immunofluorescence after microfilter capture and fcDNA was quantified using real-time Polymerase chain reaction (PCR). CTCs and fcDNA were correlated with early biochemical response by subdividing patients into early favorable and unfavorable response at 3 months after RT. Results: For those treated primarily, there was a direct correlation with CTC counts and prostate specific antigen (PSA) pre-RT that changed to a reciprocal relationship 3 months post-RT. CTCs increased significantly (p=0.03) at 3 months after primary RT in the biochemical favorable patients, while no significant association was observed for fcDNA. Correspondingly, post-RT fcDNA levels were inversely related to CTC counts. In salvage patients, the number of CTCs was related to pre-RT PSA, but it was not correlated to RT response. In post-RT series, a significant direct correlation was observed between CTCs and PSA. Conclusion: Our preliminary studies suggest that RT affects CTC counts, which are thus associated with prostate cancer biochemical response. A larger cohort with longer follow-up will be needed to establish the association with more recognized treatment endpoints.