Inomogeneidade de tecidos ósseos na irradiação de tumores no sistema nervoso central

Abstract

External radiation therapy is a clinical modality that uses high-energy photons in the treatment of malignant or benign diseases. The energy transfer process of the photons to the medium is influenced by various physical characteristics of human tissues. Inhomogeneities, as lung tissue, air cavities, bone or metallic implants, disrupt the transport of primary photons and secondary electrons produced in interactions, therefore alter the spatial dose distribution. It is essential that the dose absorbed by the tissues in the presence of these inhomogeneities is precisely predicted to maximize therapeutic benefit and to estimate the normal tissue complication probability. Although the presence of inhomogeneities, particularly bony structures, is common in clinical practice, it is observed that the dosimetric studies are still incipient in the evaluation of the absorbed dose in specific clinical situations. Thus, this study aimed to investigate the effect of bone heterogeneity through experimental dosimetric intercomparisons or simulations, using treatment planning systems (TPS) and physical measurement, in two distinct clinical situations: the irradiation of thoracic vertebrae and the whole brain irradiation. In the first case, the methodology consisted in develop a simplified anthropomorphic phantom constituted of materials equivalent to the thoracic vertebrae, muscles and soft tissues. The absorbed dose was evaluated by four radiochromic films inserted into the vertebrae, and four radiochromic films inserted in water, following the same spatial position. The measurements were compared with the dose distribution predicted by the TPS Soma Vision/CAD PLAN from Varian Medical Systems with no heterogeneity correction and with a mathematical equation that considers the physical properties of tissues to provide the absorbed dose. The method used in the second study situation assessed consisted of comparing the simulation of a two-dimensional (2D) and conformal three-dimensional (3D) planning for total irradiation of the brain in an equivalent individual condition for two fields technique and four field-in-field technique. These simulations were also performed with the inclusion of heterogeneity correction by the collapsed cone method of CAT3D from MEVIS Informática Medica. The results obtained in the evaluation of tissue inhomogeneity effects on the spinal cord showed that the measurements in the water agreed with the planning system in 1.0% with respect to the modal dose whereas the largest difference found for the medium containing the vertebrae was 4.6%. The theoretical result indicated an increase in the absorbed dose of 3.4%, however, both values were within the experimental uncertainty. Differences in the pattern of spatial dose distribution and the presence of statistically significant high dose points, greater than 16% of the reference dose that were not predicted by TPS were observed however. The experimental results of the field-in-field technique in 2D planning for total brain irradiation showed no significant differences between the absorbed dose in 2D and conformal 3D planning, presenting identical values of modal dose and a difference inferior to1% in the mean dose. The four field-in-field technique significantly improved dose distribution in brain volume compared to two fields technique for the proposed situation, interpreted by an improvement of 19% in the conformity index and reduction of 10% in the maximum dose. In this particular case, the differences of attenuation assigned to the tissue inhomogeneity differences are less relevant than the differences in the skull contour heterogeneity. It is concluded that the calculation algorithms present in the current planning systems are able to correctly calculate the mean and modal dose within a volume close to the bone inhomogeneity. However, the pattern of distribution of the dose is not properly interpreted in such situations. In order to adequately access the absorbed doses and to understand the clinical effect of the dose distributions in heterogeneous medium, the development of dosimetric experiments with anthropometric phantoms with varied study situations is of great value.