There was an early discussion about the meaning of

There was an early discussion about the meaning of CTC counts. I am attaching some articles regarding the subject, however, make no mistake, zero count CTC means a lot as you can see from some of the papers below. Metastasis is the real killer in cancer. Zero CTC's means that this has been stopped or slowed down big time. I copy-and-paste some small portions of the articles for illustration purposes only.

CTCs are an important tool for cancer detection and quantification:

https://www.sciencedirect.com/science/article...via%3Dihub

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Conclusions
We confirm the identification of two subgroups of MBC, Stage IVindolent and Stage IVaggressive, independent of clinical and molecular variables. Thus, CTC count should be considered an important tool for staging of advanced disease and for disease stratification in prospective clinical trials.

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Role of Circulating Tumor Cell (CTC) Monitoring in Evaluating Prognosis of Triple-Negative Breast Cancer Patients in China

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5493060/

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Results"
Compared with preoperative levels, the average CTC content in peripheral blood of breast cancer patients was significantly increased at 3 days after surgery, and then decreased to the preoperative baseline level by 7 days after surgery. The 3-year overall survival rate and progression-free survival rate in patients with CTC >5/7.5 mL peripheral blood were significantly lower than in patients with CTC <5/7.5 mL peripheral blood detected preoperatively and at 3 and 7 days postoperatively.
Conclusions:
Dynamic monitoring of preoperative and postoperative CTC levels can accurately predict recurrence and progression of disease, and is important in postoperative monitoring and prognosis evaluation.

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https://www.ncbi.nlm.nih.gov/pubmed/30244877

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RESULTS:
Forty-eight consecutive patients (T1a [73%], T1b [21%], and T2a [6%]) were enrolled. Median follow-up was 14.2 months. Twenty patients (42%) had a positive CTC level pre-RT, with a median CTC count of 4.2 CTCs per mL (interquartile range [IQR], 2.2-18.7). Of these 20 patients, 17 had evaluable post-RT CTC evaluations showing reduced CTC counts at 1 month (median, 0.2; IQR, 0.1-0. and 3 months (median, 0.6; IQR, 0-1.1). Three of these 17 patients experienced disease progression at a median of 19.9 months; all 3 experienced ≥1 positive post-RT CTC test predating clinical progression by a median of 16 months (range, 2-17 months). In contrast, among patients presenting with CTC-detectable disease and for whom all post-RT CTC tests were negative, none experienced recurrence or progression.
CONCLUSIONS:
CTC monitoring after SBRT for presumed early stage NSCLC may give lead-time notice of disease recurrence or progression. Conversely, negative CTC counts after treatment may provide reassurance of disease control. CTC analysis is thus potentially useful in enhancing clinical diagnosis and follow-up in this population.

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https://www.targetedonc.com/news/study-of-sm8...tic-cancer

Pancreatic Cancer

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Additional analyses were conducted to show the association between CTCs and survival. The median decrease of CTCs was 63%, and 56.3% of patients with CTC decreases had an OS of >180 days compared with 37.5% of those with CTC increases. Patients who had ≥80% CTC reduction (n = 24) trended toward improved OS (HR, 0.4; 95% CI, 0.11-1.5;P= 1. , and those with ≤50 cells/4 mL at nadir also had favorable OS outcomes (HR, 0.64; 95% CI, 0.2-2.1; P= .45). Patients with the longest survival at 261+, 262+, and 343+ days had both an >80% reduction and <50 cells/4 mL.

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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6505632/

About difficulties of measuring CTC and meaning:

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We have recently asked whether the detection of CTCs in breast cancer patients could be influenced by clinico-pathological features [8]. One of the main motivations for addressing this question was the observed fluctuations in CTC numbers in patients with comparable disease status (e.g. progressive disease), overall burden and metastatic profile, suggesting that CTC shedding rates are unequal across patients and even within individual patients that are sampled longitudinally, and that possibly, certain clinical features could “accidentally” influence cancer spread. On the other hand, clear correlatives between CTC abundance and patient prognosis have been demonstrated in large clinical studies [2, 3], suggesting a logic connection between CTC release and tumor aggressiveness.

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