A model to accumulate fractionated dose in a deforming organ

Purpose: Measurements of internal organ motion have demonstrated that daily organ deformation exists throughout the course of radiation treatment. How ever, a method of constructing the resultant dose delivered to the organ vo lume remains a difficult challenge. In this study, a model to quantify inte rnal organ motion and a method to construct a cumulative dose in a deformin g organ are introduced. Methods and Materials: A biomechanical model of an elastic body is used to quantify patient organ motion in the process of radiation therapy. Intertre atment displacements of volume elements in an organ of interest is calculat ed by applying an finite element method with boundary conditions, obtained from multiple daily computed tomography (CT) measurements. Therefore, by in corporating also the measurements of daily setup error, daily dose delivere d to a deforming organ can be accumulated by tracking the position of volum e elements in the organ. Furthermore, distribution of patient-specific orga n motion is also predicted during the early phase of treatment delivery usi ng the daily measurements, and the cumulative dose distribution in the orga n can then be estimated. This dose distribution will be updated whenever a new measurement becomes available, and used to reoptimize the ongoing treat ment. Results: An integrated process to accumulate dosage in a daily deforming or gan was implemented. In this process, intertreatment organ motion and setup error were systematically quantified, and incorporated in the calculation of the cumulative dose. An example of the rectal wall motion in a prostate treatment was applied to test the model. The displacements of volume elemen ts in the rectal wall, as well as the resultant doses, were calculated. Conclusion: This study is intended to provide a systematic framework to inc orporate daily patient-specific organ motion and setup error in the reconst ruction of the cumulative dose distribution in an organ of interest. The re alistic dose distribution in an organ of interest gives the true dose-volum e relationship, and may play an important role in the evaluation of the dos e response of human organs. Dose reconstruction during the course of treatm ent delivery can also be used as an important feedback for the online optim ization of individual treatment plans. (C) 1999 Elsevier Science Inc.