Patient characteristics
This study enrolled 72 patients with sarcoma and 20 long-term survivors. The STARD reporting diagram is given in Fig. 1A. The patients’ clinical characteristics and sarcoma types are summarized in Table 1. The median age of the patients was 14 years; 18 (20%) patients were 10 years old or younger. We found no differences in CTC detection based on age, sex, or type of tumor (Table 1).
Manual and automated capture of CSV + CTCs yields similar results
Our previous research demonstrated the utility of a CSV-targeted antibody in the isolation and identification of CTCs, in that CSV is a universal marker of CTCs.13,20−23 We initially used the manual isolation method described previously to capture and image CTCs.13 To ensure that our technology could be used in CLIA-certified labs, we developed an automated technique (Fig. 1B) to ensure reproducibility of results. In a similar manner to the manual CSV+ CTC process, the automated method can capture and isolate CTCs in the blood of patients with any type of sarcoma (Fig. 1D). To ensure the consistency of these 2 sets of data, we performed statistical comparisons. As shown in Fig. 1C, the two processes did not significantly differ in their ability to isolate and enumerate CTCs.
CSV + Cells are CTCs
We previously showed that the captured CSV+ CTCs are tumor cells via FISH analysis, tumor cell spike assays, and some sequencing analyses.13,24,25 To further validate these observations for this study, we previously stained the CTCs of an angiosarcoma patient with CD31, an angiosarcoma biomarker, and reinforced this argument by staining the CTCs of a patient with embryonal rhabdomyosarcoma with smooth muscle actin (Fig. 1D).23 Expression of CD31 and smooth muscle actin, respectively, confirmed that the CSV+ cells observed were indeed angiosarcoma and embryonal rhabdomyosarcoma CTCs. These data, together with the oncogene amplification we reported in our previous publications, confirmed that the CSV+ cells captured by our technology were indeed tumor cells.13,22
CSV + CTCs are more abundant in the blood of patients with active sarcomas compared to long-term survivors
With this information, we were able to enumerate CSV+ CTCs in the blood of pediatric and AYA patients with sarcoma. Sarcoma patients who were in remission for at least 5 years (long-term survivors) formed a control group. We found that patients with active sarcoma had significantly more (p < 0.0001) CSV+ CTCs per 6 mL of blood than did long-term survivors (Fig. 2A). A range of 0 to 26 CSV+ CTCs per 6 mL of blood were found in these patients. Of the 72 patients with active sarcoma, 17 had no detectable CSV+ CTCs. Of the 20 control samples, 5 contained CSV+ CTCs, though the number of CTCs was low, ranging from 0 to 3 CTCs per 6 mL of blood.
Sensitivity and specificity of CSV + CTCs for detecting sarcoma
We next constructed a receiver operating characteristic (ROC) curve using this information (Fig. 2B). The area under the curve (AUC) of the ROC curve was 0.831. Because the Youden J value was 1 or more CTCs per 6 mL of blood, this level was used as the cutoff for CTC positivity. Using this cutoff value, the sensitivity and specificity of the test were 75% and 85.3%, respectively.
To further improve the test’s sensitivity and specificity, we added the results of genetic biomarker testing obtained in the course of standard care. The rationale for adopting this strategy was that our recent report found that including the genetic biomarker MYCN to CSV + CTC positivity boosted the accuracy of predictions of non-relapse from 95–100% in neuroblastoma patients who were in remission and receiving maintenance therapy.20 Because it is difficult to pinpoint a single mutation as a marker of sarcoma, we used the entire panel of genetic analysis results obtained as part of the standard of care to detect its impact on the ROC of CSV+ CTCs. Indeed, when the ROC curve took also took the genetic mutations listed in Table 2 into account, the AUC increased to 0.875 (Fig. 2C), with maximum combined sensitivity and specificity of 75% and 91.2%, respectively.
Because metastatic sarcoma with genetic mutations indicates a high disease burden, we constructed the ROC curve to compare patients with metastatic sarcoma to those for long-term survivors using the combination of genetic mutations and CSV + CTC. The AUC increased to 0.902 (Fig. 2D).
CSV + CTC positivity is associated with poorer overall survival
Because CTCs have been associated with poor overall survival, we hypothesized that the presence of CSV+ CTCs is also associated with poor survival. To this end, we constructed survival curves. Figure 3A shows that sarcoma patients who were CSV+ CTC negative had longer overall survival durations than did patients who were CSV+ CTC positive. The median overall survival time for patients who were CSV+ CTC negative was undefined, as 76% of CSV+ CTC-negative patients survived. Of the CSV+CTC− patients who did not survive, the median overall survival time was 1597 days. The median survival time for patients who were CSV+ CTC positive and did not survive was 773 days. The median overall survival time for patients who were CSV+ CTC positive was 1987 days.
Again, we incorporated clinical molecular testing results into the survival analysis, as this could reveal subsets of patients who require less-aggressive therapies (Fig. 3B). We excluded long-term survivors from this analysis. When accounting for genetic mutations, we again found that patients with no genetic mutations and patients with no CSV+ CTCs and no sarcoma-associated genetic mutations had the longest survival times; all patients in this category survived. We additionally found that patients who were CSV+ CTC negative and mutation positive had the lowest median survival time, 1096 days. CSV+ CTC-positive patients without any sarcoma-associated genetic mutations had a median survival time of 1372 days, while CSV+ CTC-positive patients bearing sarcoma-associated gene variants had a median survival time of 1509 days.
We additionally examined differences in CSV+CTCs per 6 mL of blood in patients in an attempt to determine whether we could use our test to distinguish between patients with non-metastatic vs metastatic sarcomas (Fig. 4A). However, we were unable to find any differences between the two groups in total CSV+CTCs per 6 mL of blood.