LET-Dependent Survival of Irradiated Normal Human Fibroblasts and Their Descendents

Abstract

Evidence has accumulated showing that ionizing radiations persistently perturb genomic stability and induce delayed reproductive death in the progeny of survivors; however, the linear energy transfer (LET) dependence of these inductions has not been fully characterized. We have investigated the cell killing effectiveness of gamma rays (0.2 keV/microm) and six different beams of heavy-ion particles with LETs ranging from 16.2 to 1610 keV/microm in normal human fibroblasts. First, irradiated confluent density-inhibited cultures were plated for primary colony formation, revealing that the relative biological effectiveness (RBE) based on the primary 10% survival dose peaked at 108 keV/microm and that the inactivation cross section increased proportionally up to 437 keV/microm. Second, cells harvested from primary colonies were plated for secondary colony formation, showing that delayed reproductive death occurred in a dose-dependent fashion. While the RBE based on the secondary 80% survival dose peaked at 108 keV/microm, very little difference in LET was observed in the RBE based on secondary survival at the primary 10% survival dose. Our present results indicate that delayed reproductive death arising only during secondary colony formation is independent of LET and is more likely to be dependent on initial damages having been fixed during primary colony formation.

Killing of Feline T-Lymphocytes by Gamma-Rays and Energetic Carbon Ions

“…Irradiation with carbon ions used in this study is expected to be most effective for cell killing, as it has been shown for human and rodent cells that RBE peaks at 100-200 keV/µm [1,3,8,22]. Carbon-ion irradiation significantly decreased the clonogenicity of FeT-J cells compared to γ-rays, as shown in Fig.…”

“…Irradiation with energetic carbon ions (114 keV/µm), of which LET has been known to maximize biological effects in human and rodent cells [1,3,8,22], exacerbated the killing of feline Tlymphocyte FeT-J cells with RBE of 2.98 relative to the D 10 dose of g-rays (0.2 keV/µm). Inactivation cross-section in cells exposed to carbon ions was 1691-fold higher than that in γ-irradiated cells (Fig.…”

“…For irradiation experiments, 65 µl cell suspension was added to Slide Seal for in situ PCR (1.5 × 1.6 cm, Takara, Tokyo, Japan) and covered with 8 µm thick Kapton polyimide film (TorayDuPont, Tokyo, Japan) to prevent air drying in non-coated 60 mm non-treated plastic dishes (Asahi Techno Glass, Chiba, Japan). The cells were exposed at room temperature to either 0-8 Gy of γ-rays (0.2 keV/µm) from a 60 Co source at a dose rate of 2 Gy/min, or to 0-2 Gy of energetic carbon ions (220 MeV 12 C, 114 keV/µm) delivered from the azimuthally-varying-field (AVF) cyclotron installed at TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) facilities at the Japan Atomic Energy Agency (JAEA) [8]. We calculated the LET value based on the kinetic energy loss assuming water equivalence, and the absorbed dose (Gy) was calculated according to the formula Dose (Gy) = fluence (number of ion particles/cm 2 ) × LET (keV/µm) × 1.6 × 10 -9 [8,16].…”

“…The cells were exposed at room temperature to either 0-8 Gy of γ-rays (0.2 keV/µm) from a 60 Co source at a dose rate of 2 Gy/min, or to 0-2 Gy of energetic carbon ions (220 MeV 12 C, 114 keV/µm) delivered from the azimuthally-varying-field (AVF) cyclotron installed at TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) facilities at the Japan Atomic Energy Agency (JAEA) [8]. We calculated the LET value based on the kinetic energy loss assuming water equivalence, and the absorbed dose (Gy) was calculated according to the formula Dose (Gy) = fluence (number of ion particles/cm 2 ) × LET (keV/µm) × 1.6 × 10 -9 [8,16]. Sham-irradiated control cells were manipulated in parallel with the test cells.…”

“…High-LET radiations, including energetic heavy-ion particles, are more effective for cell killing [6,18,27] and apoptosis [20,24] than low-LET photons such as X-rays and γ-rays. Relative biological effectiveness (RBE) values reach a maximum at a LET of 100-200 keV/µm [1,3,8,22]. High-LET heavy ions have a more selective dose distribution as well as a higher RBE, and have been applied to radiotherapy in human patients for the treatment of malignant tumors in the lung [11], brain [12], liver [15], and prostate tissues [26]; this treatment has been shown to be more effective than therapy using conventional low-LET photons [25].…”