Experimental dosimetric evaluation of inhomogeneity effects caused by thoracic vertebrae

The presence of tissue inhomogeneities alters the absorbed dose distribution, which magnitude depends on the physical properties of these tissues and the quality of the radiation. Incorrect assessment of dose distribution may affect local tumor control or increase the normal tissue complication probabilities. The aim of this study was to investigate the influence of the thoracic vertebrae inhomogeneous effects on the dose absorbed by the surrounding soft tissue. The values predicted by the treatment planning system (TPS) were compared to the experimental measurements with EBT-2 radiochromic films positioned on a simulator consisting of only water and inserted axially into a thoracic phantom made of synthetic bone and water. There was a significant change in the dose distribution pattern, increased absorbed dose at the bone-soft tissue interface and high point doses adjacent to the bone compared to the results obtained for the films in homogeneous medium and TPS. The experimental measurements in the water agreed with the TPS within 1.0% with respect to the modal dose whereas the biggest difference found for the medium containing the vertebrae was of 4.6%, however, both values are within the experimental uncertainty. (author)