Background: Liquid biopsy-based biomarkers provide valuable insights into tumor biology, dynamics, burden, relapse prediction and therapeutic responsiveness. The stress-inducible heat shock protein 70 (Hsp70), which is frequently overexpressed in highly aggressive solid tumors and is presented on the cell membrane of tumors but not normal cells, is found in the circulation either as a free protein originating from dying cells or in the context of extracellular vesicles (EVs) that are actively released by viable tumor cells. This study demonstrates the potential value of circulating Hsp70 (eHsp70) levels across multiple solid tumor entities as an entity- and stage-dependent diagnostic biomarker reflecting tumor burden and disease stage. Methods: Circulating eHsp70 levels, as determined using the Hsp70-exo ELISA which detects free and EV-associated Hsp70, in plasma samples collected from patients with different tumor entities (n = 389) prior to the initiation of any oncological therapy and healthy controls (n = 108) between 2021 and 2025, were analyzed retrospectively. Tumor stages were categorized as early, locally advanced, or metastatic. The Kruskal–Wallis test was used for group comparisons and the Receiver Operating Characteristic (ROC) curve was used to evaluate the diagnostic performance of eHsp70 levels. DeLong’s test was used to calculate differences between AUC values. Results: In tumor patients (n = 389), circulating eHsp70 levels were significantly higher than those in healthy controls (n = 108) (Kruskal–Wallis, p < 0.001). eHsp70 levels progressively increased from early-stage to locally advanced and metastatic disease in a stage-dependent manner. Although ROC analysis demonstrated the limited discriminatory performance of eHsp70 levels in early-stage disease (AUC 0.569), increased discrimination was apparent in locally advanced disease (AUC 0.751), metastatic tumors (AUC 0.784) and combined advanced tumor diseases (AUC 0.765; significant by DeLong’s Test comparing early-stage to locally advanced and metastatic tumors), irrespective of the tumor entity with the highest AUC values in metastatic breast cancer (AUC 0.872), sarcoma (AUC 0.861) and non-small cell lung cancer (NSCLC) (AUC 0.835). Apart from minor entity-specific differences, the correlation of eHsp70 levels with the tumor stage remained consistent across all measured tumor entities. Conclusions: Circulating eHsp70 levels are markedly elevated in patients with highly malignant solid tumors and show a consistent, stage-dependent increase across multiple tumor types. These findings suggest that circulating eHsp70, as an indicator of tumor-associated cellular stress and overall tumor burden, represents a valuable biomarker for assessing disease stage, monitoring disease progression, and evaluating therapeutic responses.

Objectives/Background: Heat shock protein 70 (Hsp70) is frequently overexpressed and released from malignant tumors and is associated with tumor aggressiveness and resistance to treatment. While elevated levels of extracellular Hsp70 (eHsp70) in the blood have been associated with unfavorable tumor characteristics, its prognostic significance in the context of long-term follow-up has not yet been sufficiently analyzed. The primary objective of this study was to investigate the prognostic association between preoperative circulating eHsp70 levels and subsequent event-free survival (EFS) in patients with NSCLC undergoing thoracic surgery. Patients with pulmonary metastases from other primary tumors were analyzed as an exploratory subgroup. Methods: The association between eHsp70 levels, clinical outcome, and event-free survival in patients with malignant thoracic diseases was investigated in a retrospective study. Results: A total of 76 patients undergoing thoracic surgery for non-small cell lung cancer (NSCLC) or pulmonary metastases from other primary tumors were included and subsequently followed for disease progression, metastasis, or death. Preoperative eHsp70 levels were significantly higher in patients who experienced an adverse clinical event than in event-free patients (median 194.3 ng/mL [IQR 19.9–493.0] vs. 27.8 ng/mL [IQR 14.3–53.4], p = 0.0064). ROC analysis showed moderate discriminatory performance in the overall cohort (AUC 0.68, 95% CI 0.55–0.81), with an AUC of 0.72 (95% CI 0.56–0.88) in patients with NSCLC and 0.59 (95% CI 0.34–0.83) in patients with pulmonary metastases. In patients with NSCLC, elevated preoperative eHsp70 levels were associated with significantly shorter event-free survival (log-rank p < 0.0001). In multivariable Cox regression analysis restricted to patients with NSCLC, eHsp70 remained independently associated with shorter event-free survival after adjustment for tumor stage, age, and sex (HR 1.33, 95% CI 1.07–1.65, p = 0.010). Conclusions: These findings suggest that circulating eHsp70 may have prognostic relevance in patients with NSCLC. However, prospective validation in larger and independent cohorts is required before its clinical utility can be established.