IGF1 and prostate cancer: To our knowledge, however, only three prospective studies of prostate cancer risk have measured circulating free IGF-I concentration using immunoradiometric assay (IRMA) [16] or enzyme-linked immunosorbent assay (ELISA) [17, 18], with sample sizes up to 1076 men; they did not identify associations but may not have had the power to detect small to moderate associations.