A unique circulating stromal cell biomarker was identified in the blood of cancer patients with solid tumors. This biomarker is found in all stages of cancer and applicable to all treatments. LifeTracTM assays efficiently collect this biomarker. Clinical data of LifeTracTM assays demonstrated applications of this biomarker for cancer screening, companion diagnostics, prediction of treatment response in 30-60 days after start of therapy including immunotherapy, determination of residual disease at end of therapy, early detection of recurrence, and delivery of high quality tumor DNA.