Absorbed dose from X rays in soft tissue adjacent to bone measured by induced conductivity in polyethylene films Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pz50h038n

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  • The X-ray dose absorbed in soft tissue located near plane bone interfaces and in parallel-slab bone cavities was measured experimentally with a polyethylene-film dosimeter. The dose absorbed in soft tissue adjacent to plane bone interfaces was measured by placing a 12.7 micron film of polyethylene, upon which were deposited graphite electrodes, in contact with bone-equivalent disks of Shonka plastic. The dose was measured both as a function of X-ray quality and distance of the tissue-equivalent polyethylene away from the bone interface. The dose in finite soft tissue layers next to bone was also measured as a function of X-ray quality. The results indicated that the soft tissue dose in finite layers next to the bone was greater than that remote from the bone. At the lower effective X-ray energies the dose very close to the bone was considerably higher than for the higher effective X-ray energies. At higher effective X-ray energies, however, the increased dose effect extended to a greater depth in the soft tissue. At effective energies greater than 177 keV electronic equilibrium was obtained, and the dose was the same for the adjacent and distant soft tissue. Comparisons were made of the experimentally measured doses in soft tissue near plane bone interfaces with calculated doses for monoenergetic photons. It was observed that at effective X-ray energies below approximately 60 keV the experimental results gave higher dose values than the theoretical values. However, at higher effective X-ray energies there was good agreement between the experimental and theoretical results. It was concluded that X-ray spectral considerations were mainly responsible for the deviations at lower effective energies. The results of this study indicated that when one wishes to calculate the dose absorbed in soft tissue adjacent to plane bone interfaces at low effective X-ray energies, the theory cannot be relied upon and experimentally determined doses such as those shown herein should be used instead. The X-ray dose in soft tissue elements interposed between parallel slabs of bone was measured in the same manner as the dose in soft tissue adjacent to plane bone interfaces. However, due to experimental difficulties in simulating this bone-tissue orientation, it was not possible to make reasonable dose measurements.
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