Effects of low dose rate (0.003-0.025 Gy/h) chronic X-irradiation on radioresistant and radiosensitive L5178Y mouse lymphoma cells.

Cultures of radioresistant (LY-R) and radiosensitive (LY-S) strains of L5178Y mouse lymphoma cells were exposed continuously to X-rays delivered at dose rates ranging from 0.003 to 0.025 Gy/h for up to 35 days. Populations of both strains proliferated actively during the exposure, but the growth rates were reduced in a dose rate-dependent manner. The reduction of growth rate occurred for strain LY-S earlier during the exposure and at lower dose rates than for strain LY-R. The survival (as measured by colony forming ability) of strain LY-R was affected only slightly at all dose rates applied. For strain LY-S, a decrease in the surviving fraction was observed in the initial part of the exposure. This decrease was followed by a plateau and eventually by an increase, in some cases to values close to the control level. The increase in the surviving fraction indicated that the radioresistance of the exposed LY-S cells had increased. This pattern was particularly clear for dose rates greater than 0.014 Gy/h. The pre-irradiated cells exhibited radioresistance when exposed to acute X-radiation after termination of the chronic exposure. The increase in radiation resistance was stable for at least 70 days after termination of the protracted exposure. These results show that mutagenic and/or selective phenomena leading to an increase in radiation resistance of mammalian cells can be caused by protracted exposures to X-rays at dose rates permitting active proliferation.

A revised estimate of the risk of carcinogenesis from x-rays to scoliosis patients.

A considerable amount of concern has been felt recently for the risk of carcinogenesis from x-rays to scoliosis patients. This paper re-evaluates risk in quantitative terms by using some data measured by us and other data recently published in the literature. The risks are considerably smaller than had been estimated earlier. Compared with the natural incidence of cancer in the general population, the cumulative additional risk for scoliosis patients varies from 0.2% for breast carcinoma to 5% for leukemia.

Quasifree electron attachment to carcinogens in liquid cyclohexane.

The quasifree electron attachment rate constant, kappa e, was measured in liquid cyclohexane at 20 degrees C for 76 chemicals that had been tested for bacterial mutagenicity in at least one of five earlier studies designed to determine the correlation between bacterial mutagenicity and animal carcinogenicity. A millionfold range of kappa e's was observed with the kappa e of 37 of 42 carcinogens being equal to or greater than the diffusion-controlled electron attachment rate constant, kappa d, of 2.9 x 10(12) M-1 s-1, whereas kappa e was less than kappa d for 30 of 34 noncarcinogens. These results yield a sensitivity of 88 percent for kappa e greater than or equal to kappa d indicating a chemical's carcinogenicity and a specificity also of 88% for kappa less than kappa d indicating a noncarcinogenic chemical. The observed quasifree electron attachment sensitivity and specificity compare favorably with previously observed Ames-test mutagenicity/carcinogenicity correlations which suggests that kappa e measurements also may be of value in a carcinogen-screening program.

Computed tomography with microwaves.

Computed tomographic images were generated using collimated microwaves of frequency 10.5 GHz instead of x rays. In the authors' laboratory version of a rotate-translate scanner, the beam is fixed and a phantom is moved between source and detector, the apparatus being suspended in a tank of fluid to provide impedance matching between phantom, source, and receiver. Phantoms consisting of test tubes of water, cylinders of wood, and animal tissues show spatial resolutions of about 2 cm. It is planned to apply the technique first to mammographic examination, which can best utilize the large difference in microwave absorption between fatty and nonfatty tissues.

Correlation of cell cycle parameters with radiation sensitivity in a series of murine L5178Y cells.

The X-ray dose--response parameters (Do, n and Dq) of 10 strains of murine L5178Y cells spanning a wide range of radiation sensitivities were characterized. The proportions of cells residing in G1, S and G2 + M phases of the cell cycle in exponentially growing cell populations were estimated using DNA microfluorometric techniques. The radiosensitive strains contained a lower proportion of cells in G1 phase and a somewhat larger proporation of cells in S phase than did the radioresistant strains, reflecting a direct correlation between radioresistance and the duration of G1 phase (and possibly an inverse correlation with the duration of the S phase) in these closely related cell strains. These results are discussed in terms of the putative role of G1 in governing radiosensitivity of cells and tumours.

Effect of low dose rate radiation on cell growth kinetics.

Experimental determinations were made of cell number as a function of time for two strains of L5178Y mammalian cells maintained continuously in various environments of radiation. One strain possessed a shoulder in its dose response curve whereas the other did not. Neither strain showed any significant difference in growth rate for interdivision doses on the order of the median lethal dose or less delivered continuously at a low dose rate or pulsed every 4 h at a high instantaneous dose rate. It was also shown that large numbers of dead cells have little effect on growth rate and that these dead cells last as discrete entities for many days. A simple theory of growth rate in the presence of radiation is presented, and the agreement with the observations implies that there is no effect of any sublethal low dose rate radiation received in one generation on the growth rate or radiation sensitivity of the succeeding generation. Further analysis of the data also showed that for the no-shoulder cells at 37 degrees C, tritiated water had a relative biological effect close to unity for cell sterilization.

Risks of exposure to X-rays in patients undergoing long-term treatment for scoliosis.

Thirteen healthy teenage girls with idiopathic adolescent scoliosis were studied using multiple thermoluminescent dosimeters while undergoing standard diagnostic roentgenograms. Average organ doses for each anteroposterior and lateral examination were calculated for bone marrow, gastrointestinal tract, lungs, breast tissue, and gonads. Given an average of twenty-two roentgenograms over a three-year Milwaukee brace-treatment program, the increase in organ carcinogenic risk due to x-ray radiation ranged from 3.4 to fifteen per million (1.3 per cent to 7.5 per cent), except for breast tissue which increased from 140 to 290 per million (110 per cent). Using posteroanterior rather than anteroposterior exposures reduced the increased risk to 5.3 per million (3.8 per cent). The genetic risks of scoliosis roentgenographic studies were considered to be negligible, especially with gonadal shielding and infrequent roentgenograms made every three to four months. Good technique and judicious ordering of roentgenograms added significantly to the safety of the patient.

Radiographic quality, tube potential, and patient dose.

Measurements were made to determine if there was an optimal kVp that would maximize radiographic contrast for various materials while minimizing average patient dose. Radiographs were made of a 17-cm-thick plexiglass phantom (to simulate a typical abdomen) with added regions of plexiglass, hydroxyapatite, and iodine with peak kilovoltages from 40 to 150 kVp and 2 mm A1 added filtration. Radiographs were also made of a 4-cm-thick plexiglass phantom to simulate a typical breast, with added regions of plexiglass, hydroxyapatite, and fat at a peak kilovoltage from 35 to 100 kVp and with 1 mm added filtration. For the thicker phantom it was found that the contrast per average absorbed dose for the added plexiglass, hydroxyapatite, and iodine increased by about 3, 2, and 0.4, respectively, when going from 50 to 150 kVp. Contrast was measured relative to a reference region on the phantom. Similar data for the thinner phantom showed an increase of about 35% in contrast per average absorbed dose for plexiglass and fat but a decrease by about 30% for hydroxyapatite as the kVp changes from 35 to 100 kVp.

Subtraction radiography: techniques and limitations.

Subtraction radiography permits the use of 7-10 times less contrast material than is required without subtraction techniques. The first-order, second-order, and composite mask techniques are described and evaluated. While the second-order technique gives more complete cancellation of common structures, the more commonly used first-order technique was found to be adequate for most clinical uses. The smallest change in contrast which can be detected using subtraction radiography is limited by inherent background film noise in the original radiographs and not by statistical fluctuations in the x-ray quanta.

Designing a wax phantom to simulate a flat tumor with diffuse edges.

A suitable was phantom was designed for evaluating various film techniques used in subtraction radiography. Holes are drilled in a slab of wax and filled with various concentrations of iodinated wax, producing disk-shaped regions with diffuse borders. The technical details of construction and a representative application are described.

Radiation risks with diagnostic x-rays.

Calculations are presented which correlate the risk of carcinogenesis with various diagnostic radiological procedures. Even though "worst case" conditions are assumed, such as a linear non-threshold dose-response relationship, most of the radiographic procedures appeared to be quite conservative in terms of carcinogenic risk. This was not true, however, for mammography of younger women.

An analysis of the value of additional diagnostic procedures.

When several diagnostic procedures are performed on a patient, the probability of his having or not having a specific ailment will change as the result of each procedure is known. This paper describes how such a change is readily calculated using Bayes' theorem. Information theory is also used to find the increment in information from each successive test. Numerical estimates are made using data pertinent to lung X-ray examinations. Finally, some implications for diagnostic examinations in general are drawn.