Inclusion of organ deformation in dose calculations

Abstract

A previously described system for modeling organ deformation using finite element analysis has been extended to permit dose calculation. Using this tool, the calculated dose to the liver during radiotherapy can be compared using a traditional static model (STATIC), a model including rigid body motion (RB), and finally a model that incorporates rigid body motion and deformation (RBD). A model of the liver, consisting of approximately 6000 tetrahedral finite elements distributed throughout the contoured volume, is created from the CT data obtained at exhale. A deformation map is then created to relate the liver in the exhale CT data to the liver in the inhale CT data. Six intermediate phase positions of each element are then calculated from their trajectories. The coordinates of the centroid of each element at each phase are used to determine the dose received. These intermediate dose values are then time weighted according to a population-modeled breathing pattern to determine the total dose to each element during treatment. This method has been tested on four patient datasets. The change in prescribed dose for each patient's actual tumor as well as a simulated tumor of the same size, located in the superior, intermediate, and inferior regions of the liver, was determined using a normal tissue complication model, maintaining a predicted probability of complications of 15%. The average change in prescribed dose from RBD to STATIC for simulated tumors in the superior, intermediate, and inferior regions are 4.0 (range 2.1 to 5.3), -3.6 (range -5.0 to -2.2), and -14.5 (range -27.0 to -10.0) Gy, respectively. The average change in prescribed dose for the patient's actual tumor was -0.4 Gy (range -4.1 to 1.7 Gy). The average change in prescribed dose from RBD to RB for simulated tumors in the superior, intermediate, and inferior regions are -0.04 (range -2.4 to 2.2), 0.2 (range -1.5 to 1.9), and 3.9 (range 0.8 to 7.3) Gy, respectively. The average change in the prescribed dose for the patient's actual tumor was 0.7 Gy (range 0.2 to 1.1 Gy). This patient sampling indicates the potential importance of including deformation in dose calculations.

Potential Use of Ultrasound Speckle Tracking for Motion Management During Radiotherapy

“…Although these numbers are very small for comparison, the fact that the liver has the highest variation is not really a surprise because it is well known that there is actually tissue deformation occurring in the liver when it is being displaced by respirations. [31][32][33][34] This deformation would add variability to the liver displacement measures, even though this variability would be a reflection of the true liver motion. The angular variations in our prostate scans are solid-body motions, which would have no such variations, while the lung motion is a complex volumetric motion, 39 and we are only imaging the surface.…”

Evaluation of dosimetric misrepresentations from 3D conventional planning of liver SBRT using 4D deformable dose integration

“…This could result in significant underdosing of target volumes and increased doses to healthy tissues, particularly for highly conformal techniques applied to thoracic and abdominal malignancy treatments 1 , 2 , 3 . Attempts to contend with organ deformation in image‐guided and adaptive radiotherapy often involve the implementation of deformable image registration (DIR) algorithms 4 , 5 , 6 , 7 , 8 .…”

Compensation method for respiratory motion in proton treatment planning for mobile liver cancer

“…This is the same for the respiration‐induced anatomical change because it can also lead to relative change in density distribution and, consequently, to miscalculation of proton dose distribution. ( 12 , 13 ) To date, however, a proper dose calculation method, such as a four‐dimensional dose (4D dose) calculation combined with deformable image registration, ( 14 , 15 ) has not been developed in proton planning systems.…”