A method of correcting dose distributions produced by IMRT planning systems

For some complicated cases, the conformality of dose distributions may be improved only with IMRT. Whereas the set treatment goals may be achieved by the inverse treatment planning systems, the dose distributions in the treatment area may not be satisfactory. According to authors´ experience, the dose distributions produced by IMRT planning systems (for both tomographic and “step and shoot” methods) demonstrated excessive radiation of posterior neck (in the case of a neck tumor), or high doses to the lungs (in the case of a thoracic spine tumor). Moreover, high dose regions surrounded the spinal cord in both cases. Due to inevitable errors in patient positioning, this may result in overdosage of critical structures. In this work, a method that consists of addition of “pseudo critical structure(s)”, that are positioned in areas that require dose reduction, was developed. The shape and size of these “pseudo structure(s)” depend on the position and volume of the areas that need this reduction. The results of our study have demonstrated that the proposed method significantly improved the dose distribution. In the case of a neck tumor, the plans containing “pseudo structure” demonstrated a significant reduction in the dose delivered to the posterior neck (from 90% or 80% to 30%) for both IMRT delivery techniques. For the thoracic spine tumor, the maximum dose to the lung reduced from 59 Gy to 51 Gy (tomographic delivery) and from 61 Gy to 46 Gy (“step and shoot” delivery)