BIOLOGIC EFFECTS STUDIES ON MICROWAVE RADIATION Time and Power Thresholds for the Production of Lens Opacities by 12.3 Cm. Microwaves

To establish the time and power requirements for experimental opacity production by single doses of 12.3 cm. radiation from the Raytheon Microtherm - director "C" diathermy apparatus.

Opacities were produced in the eyes of anesthetized rabbits by single exposures to 12.3 cm. microwaves from the Raytheon Microtherm - director "C" diathermy apparatus. The cornea of the eye was irradiated at a distance of 2 inches from the director. Power output was varied from 35 to 100 percent, and the duration of the exposure ranged from 3.5 to 90 minutes. Power measurements at 2 inches and at other distances from the director revealed the power density responsible for opacity production in this as well as in otherwise comparable studies of an earlier date. Time and power threshold for experimental opacity production ranged between 5 minutes at 0.59 watt/cm² to 90 minutes at 0.29 watt/cm². These power levels correspond to a thermal flux of 8.4 to 4.1 cal./cm²/min. The trend of the threshold beyond 90 minutes was bracketed between 0.22 and 0.12 watt/cm² for 4.5 hr. of sustained irradiation. The failure of the latter density to cause opacities suggests the proximity of a power level below which opacity production by this method is not practical. The quantity of irradiation reflected and transmitted by the eye is unknown, but sufficient energy is absorbed to produce temperatures of 49° to 53° C. at the site of subsequent lesion formation in the lens. As nearly as could be determined by ophthalmoscopic examination, all opacities were formed in the posterior lens segment and in some respects resemble lens opacities produced by ionizing irradiation. A latent period of 1 to 14 days was required for the onset of discernible opacities. The opacities, in order of increasing severity, were classified according to appearance as minimal, circumscribed, and diffuse. Minimal lesions may not be detrimental to sight, but the latter two types interfere with vision. Other ocular effects were observed, but for the most part, seemed less significant than opacity production. Secondary reactions could not be satisfactorily correlated with the subsequent occurrence of lens opacities, except for some cases of the diffuse type opacity where severe and persisting periorbital edema was associated with subsequent cataract formation. Although only the head was irradiated, increases in body temperature were characteristic of exposures causing opacities. Despite the lower power levels involved, higher body temperatures were associated with the longer periods of exposure. The indications are that the opacity threshold is partially a function of changes in body temperature and/or environmental factors which would affect the capability of the body for dissipating heat. This latter consideration would be of particular significance in the case of whole-body irradiation by microwaves. In humans, lens opacities produced by exposure to microwaves has not been established. Moreover, the equivalent of the experimental threshold has not been established in terms of human analogy. Consideration is being given the development of an opacity threshold in the monkey as an approach to the problem of extrapolation to man. It is suggested in the interim, however, that the present threshold should be regarded as dangerous for human eyes until more definitive information is available. Limited evidence of opacity production as a cumulative effect from multiple subthreshold exposures emphasizes the need to further explore this aspect of ocular irradiation.