Radiobiological Characterization of 60-keV Quasimonoenergetic Convergent Photons

To characterize the biological effects of low-energy photons generated by a ∼60-keV quasimonoenergetic focused convergent-beam system (patented x-ray converging lens) compared with orthovoltage and 6-MV clinical photon beams.

Human (NCI-H460 and NCI-H2172) and murine (KLN-205 and LKR-13) lung cancer cell lines and a normal human bronchial epithelial cell line (Beas-2B) were irradiated with photons from a 6-MV linear accelerator, an 180-kVp orthovoltage unit (average energy ∼60 keV), or a system that generates 60-keV quasimonoenergetic convergent photons. Clonogenic cell survival was analyzed to assess dose-response metrics and relative biological effectiveness (RBE) in all cell lines. DNA damage and cell cycle analyses were assessed in human-derived lung cancer cell lines. A human lung cancer (NCI-H460) xenograft model was used to test tumor growth delay in vivo.

Relative to 6-MV photons, the 60-keV quasimonoenergetic convergent and orthovoltage photons induced comparable cell kill effect and RBE for the LKR-13, KLN-205, and Beas-2B cell lines and higher cell kill effect and RBE for the NCI-H460 (significant) and NCI-H2172 (nonsignificant) cell lines. The enhanced biological response to orthovoltage and 60-keV quasimonoenergetic convergent photons in NCI-H460 cells was also evidenced by increased DNA damage signaling and cell cycle checkpoint activation. Radiosensitivity differed considerably among cell lines, even for the same radiation type. The 60-keV quasimonoenergetic convergent photons significantly delayed tumor growth, achieving effects comparable to 6-MV photons, in the NCI-H460 xenograft model.

In vitro and in vivo evaluations confirmed that the biological effectiveness of 60-keV quasimonoenergetic convergent photons is similar to that of photons from an orthovoltage unit (average energy ∼60 keV) and either comparable (3 cell lines) or higher (2 cell lines, but 1 was not significant) than photons from a 6-MV linear accelerator.