The effects of therapeutic ultrasound on the clonogenic survival of cells in liquid and semi-solid media
The medical uses of ultrasound for both diagnosis and therapy require a detailed knowledge of the interaction of ultrasound with the tissues. This will allow safety levels to be set and indicate any possible source of hazard. The interactions of ultrasound with tissue are usually divided into three catagories: thermal, cavitation phenomena, and direct action. Much work has been done to show the levels of intensity and the irradiation times which give rise to damage due to thermal and cavitational effects, but the interactions grouped under the term direct action are less well understood. To investigate the direct effects of ultrasound an irradiation system is required in which both the source of the ultrasound and the living system being irradiated can be monitored accurately. The simplest system with which this is possible is the irradiation of some form of cell suspension in a continuous wave ultrasound field. Cell cultures are simpler to use than whole animals and therefore allow better controlled experiments to be performed which result in more accurate quantitative results. Dosimetry of ultrasound is simplest in a continuous wave field. It is possible to make accurate, reproducible measurements of the intensity distribution at the positions where the irradiations are performed. This is important for relating effects to the intensities to which the cells are exposed. Initial work in this thesis was performed on cells suspended in a liquid medium. Clonogenic survival was used to check for any lethal effects of the ultrasound. In this situation the cells are free to move during irradiation which results in some degree of averaging of the intensities present over the whole sample of cells. Also, as it is only possible to measure survival in terms of the effect on the contents of each chamber as a whole, changes resulting from only the highest intensities in the ultrasound beam would not show as a significant alteration in the general level of survival. The results of these experiments showed no effect of ultrasound on clonogenic survival. The second area of research was with suspensions of cells in medium stiffened with agar. In these experiments the cells were mixed with the agar medium and placed in large chambers in which they would be able to grow for a period of at least two weeks. The principle of the experiments was to irradiate the cells in the agar medium and then monitor the growth of colonies in situ after a period of 10-14 days. As the cells are unable to move during or after irradiation, survival at each point across the chamber can be related directly to the intensity at that point. This is probably the only system likely to produce survival curves against actual intensity with presently available ultrasound sources. Also, results from this system may be easier to confirm as they should be reasonably independent of the intensity distribution of the ultrasound source used. Irradiations were performed at a range of intensities in an attempt to obtain a significant effect of ultrasound. No measurable reduction in clonogenic survival was found in any of these experiments. To try and enhance any effect that might be present hyperthermia was applied at the same time as the ultrasound, in a further series of experiments. These showed a significant effect due to the ultrasound, but the measured drop in survival was approximately that which would be expected from heating due to absorbtion of ultrasound alone. This further confirms the absence of any lethal effect of the direct action of ultrasound.