Circulating soluble fibroblast activation protein (FAP) in patients with intra-thoracic cancer undergoing chest radiation.

INTRODUCTION: Circulating soluble fibroblast activation protein (FAP) is implicated in myocardial infarction, stroke, fibrosis and various cancers. This study investigates changes in FAP concentrations in patients with intra-thoracic malignancies undergoing chest radiation therapy to assess its potential as a biomarker for radiation-induced organ injury. METHODS: Eighteen patients with intra-thoracic cancers (lung, esophagus and metastatic) received chest radiation therapy. Blood samples were taken before and after treatment, and FAP concentrations were measured using an ELISA assay. A control group of 53 healthy volunteers was included for comparison. RESULTS: Baseline FAP concentrations were significantly lower in cancer patients (median 91 ng/mL, 25th-75th percentiles 72-123 ng/mL) compared to healthy controls (median 118 ng/mL, 25th-75th percentiles 104-140 ng/mL, P = 0.0002). No significant difference in FAP concentrations was found between baseline and post-radiation samples (median 91 vs. 108 ng/mL, P = 0.19). FAP concentrations were not influenced by cancer type, radiation dose or chemotherapy and did not predict patient survival. Time between baseline and final blood sampling was 31 days median (range 9-46 days) and median follow-up period was 20 months (range 15-30 months). CONCLUSION: Our findings indicate that FAP concentrations do not reflect radiation-induced inflammation or fibrosis in the early period after radiation therapy. The small patient cohort and short duration between radiation therapy and FAP measurement may have limited our ability to detect changes in FAP related to long-term radiation effects. Further research with larger cohorts and longer follow-up periods is needed to better understand the role of FAP in cancer and response to radiation injury.