Comparisons of Tritiated Thymidine, Tritiated Water, and Colbalt-60 Gamma Rays in Inducing Chromosomal Aberrations

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Abstract
Hamster cells growing in vitro were chronically irradiated with either H3TdR incorporated into the DNA, H2 3O contained in the medium, or gamma-rays delivered from a Co60 teletherapy source. After either 10 or 23 hours of continuous irradiation, the cells were fixed, and the number of breaks and exchanges were scored in metaphase cells. In the H3TdR experiments the cells were pulse-labeled for 1 hour with H3TdR. To quantitate the number of tritium disintegrations that occurred in the cells during the 10-hour period, autoradiographic stripping film was applied to the cells, and the number of grains was counted in each metaphase cell. The efficiency of the film (11%) was determined by comparing the average grain count per cell with the disintegration rate per cell determined in the liquid scintillation counter. For all 3 radiations, 20 to 40% of the aberrations were attributed to exchanges, all of which were of the chromatid type. For both H2 3O and Co60 the aberration frequency was related to the 1.8 to 1.9 power of the dose; whereas for H3TdR the relationship approached linearity. To produce 2 visible aberrations per cell after irradiation over a 10-hour period, the following doses were required: 3500 disintegrations or 820 rads to the nucleus from H3TdR, 1780 disintegrations in the nucleus or 490 rads from H2 3O, and 520 rads from Co60 gamma-rays. The amount of chromosomal damage for a given dose of irradiation was less if the radiation was administered over 2 cell cycles (23 hours) instead of over one cell cycle. The average nuclear radiation dose from H3TdR was calculated to be 0.23 rad per disintegration by assuming that the H3TdR was uniformly distributed throughout the nucleuc with a radius of 4.0 [mu]. The dose to the DNA if it was located in a 0.5 [mu] shell at the periphery of the nucleus was calculated to be 0.30 rad per disintegration. The following conclusions were reached: (1) On the basis of energy absorbed in the nucleus or chromosomes, the [beta]-particle from H3TdR was less effective for producing chromosomal aberrations than the [beta]-particle emitted either from H2 3O distributed in the nucleus or from gamma-rays absorbed in the cell. (2) Energy absorption in or near the chromosomes is primarily responsible for producing aberrations, and the transmutation process in H3TdR is relatively ineffective for inducing aberrations. (3) In the cells chronically irradiated by H2 3O or Co60 gamma-rays during the S and G2 periods, many of the breaks remained open for an hour or more.