[Evaluation of compensator used in radiation therapy]

A radiation beam incident on an irregular or sloping surface produces skewing of isodose curves and may give rise to unacceptable nonuniformity of the dose distribution within the target volume or cause excessive irradiation of sensitive tissues. The concept of using a compensator, first introduced by Ellis, to circumvent the skewness of isodose curves was elegant. Other workers followed suit elaborating further and extending the procedures. The treatment of chest wall tumors is of special interest and compensators were frequently used in their management. The compensators of aluminum and copper were systematically examined for 6 and 10 MV X-rays. Compensating thickness ratios was determined from equivalent dose measurement at compensating depth by placing compensator at the regular blocking tray for maintaining the skin sparing feature of high energy photon beams. The compensator must compensate not only for the reduction of the primary beam attenuation due to tissue deficit, but also for the loss of scattering by the missing tissues. It was found that the compensating thickness ratios (CTR) was not a constant value and was dependent on the tissue deficit, field size, compensating depth and the material of compensator, but was independent of the energy. As the tissue deficit is increased, the CTR decreases, since the loss of scattering for the deficit volume can be compensated by the lowered attenuation of the compensator. As the field size is increased, the CTR decreases. From the study of central axis depth dose for various deficit thickness, normalized at the median plane of no tissue deficit, it was found that compensator can compensate the dose accurately to one depth only and the compensation to other depth is approximate, and that the compensating error is increased with greater tissue deficit and further depth. The effectiveness of the compensating system with a stair-step polystyrene phantom representative of varying tissue deficits over a small treatment field (20 cm x 20 cm) was also studied with film dosimetry and the error was within 3%. The compensator thickness ratios must be measured in different radiation beams from different treatment machines and in different irradiation conditions because there are major difference between the CTR data obtained directly from experiments and the data besed on calculation from attenuation coefficient mu, physical density rho or electronic density, rho e. The specific CTR reported for aluminum and copper in the literature should only be used as a guideline.