Clinical considerations in the use of missing tissue compensators for thoracic cases

The irregular shape or contour of the patient's surface in the treatment field can alter the dose distribution resulting in non-uniformity of dose in the treatment volume. Missing tissue compensators have been most commonly used to improve this non-uniformity, especially in head and neck, breast, lung and supraclavicular regions. Two or three dimensional compensators have been typically designed to make the dose uniform at a specific depth. This compensation shifts the dose distribution within the treatment volume so that some structures may be under or over compensated. This study will examine how various thoracic sites are affected by compensators, with and without heterogeneity corrections. We have also analyzed the uncertainty in compensated dose due to the daily variations in patient repositioning. Computer isodose plans using Cobalt-60 gamma rays and 6 and 18 MV X-rays were generated using sagittal contours. Results show that superficial sites, like the supraclavicular nodes and the spinal cord, do not receive as consistent a dose compensation as the midplane sites. In addition, the compensated lung dose increases. Finally, patient movement or positioning errors of +/- 1.0 cm causes only a slight change in dose distribution.