Proton therapy is of great interest to pediatric
cancer patients because of its optimal depth dose distribution. In view of healthy tissue damage and the increased risk of secondary
cancers, we investigated DNA damage induction and repair of radiosensitive hematopoietic stem and progenitor cells (HSPCs) exposed to therapeutic
proton and photon irradiation due to their role in
radiation-induced leukemia. Human CD34+ HSPCs were exposed to 6 MV X-rays, mid- and distal spread-out Bragg peak (SOBP)
protons at doses ranging from 0.5 to 2 Gy. Persistent chromosomal damage was assessed with the micronucleus assay, while DNA damage induction and repair were analyzed with the γ-H2AX foci assay. No differences were found in induction and disappearance of γ-H2AX foci between 6 MV X-rays, mid- and distal SOBP
protons at 1 Gy. A significantly higher number of micronuclei was found for distal SOBP
protons compared to 6 MV X-rays and mid- SOBP
protons at 0.5 and 1 Gy, while no significant differences in micronuclei were found at 2 Gy. In HSPCs, mid-SOBP
protons are as damaging as conventional X-rays. Distal SOBP
protons showed a higher number of micronuclei in HSPCs depending on the radiation dose, indicating possible changes of the in vivo
biological response.