Since 2006 a clinical phase I/II study is carried out treating prostate cancer patients with a combination of Cion irradiation and IMRT (C-ion treatment at GSI, IMRT in Heidelberg). In parallel to this therapy, a study investigating chromosome aberrations in blood lymphocytes of prostate cancer patients is performed, as described in [1]. Chromosome aberrations in blood lymphocytes are widely used in biological dosimetry as they represent a sensitive marker for ionizing radiation. Since they are unavoidably exposed to radiation during tumor treatment, this study makes it possible to investigate in vivo the cytogenetic effects of C-ion radiation and to compare with conventional tumor irradiation. Patients are irradiated with C-ion boost (6x3GyE) and IMRT (30x2Gy) or solely with IMRT (38x2Gy). Blood samples are drawn from each patient before, during, at the end of and one year after therapy. Lymphocytes are cultured according to standard techniques [2]. Chromosome spreads are prepared and slides are stained using Fluorescence-Plus-Giemsa staining (FPG) and multiplex fluorescence in situ hybridization (mFISH), respectively. To investigate the individual radiosensitivity, a blood sample from each patient before therapy is irradiated in vitro (3Gy X-rays). The results are compared to those of healthy donors (fig. 1). Slight differences in the aberration yield were observed, but no significantly increased radiosensitivity of one patient was found, nor an enhanced sensitivity of all patients compared to healthy donors.