SU-E-J-175: A Patient-Specific 4D Digital Phantom for Dynamic Dose Estimation of External Ionizing Radiation.

PURPOSE To develop a realistic yet flexible 4D digital phantom based on patient anatomy for dynamic estimation of organ doses due to external ionizing radiation. METHODS In this work, we propose to build a 4D digital phantom based on patient CT anatomy and time-tagged CT information. A regional 4D-CT is first acquired on the patient around lesion with a RPM system. By binning CT information at various phases of respiratory motion, we can create a regional 4D phantom. By applying non-uniform rational B-spline (NURBS) surface technology, we can easily accommodate the size-specific and gender-specific differences in the organ shapes and body contours, often observed during a long course of radiotherapy treatments. Based on the ICRP reference phantoms, we can extend the patient-specific 4D regional phantom into a 4D whole-body phantom unique to each patient. The resultant 4D phantom will serve as the digital representative of the patient himself or herself in the virtual environment and is ready for Monte Carlo particle transport. RESULTS Considering the newborn skin thickness (0.663 mm), 0.5 mm is defined as the maximum voxel resolution available in our 4D digital phantom. The resultant 4D digital phantom is essentially a high-resolution voxel phantom (with voxel dimensions down to 0.5 mm × 0.5 mm × 0.5 mm) capable of non-uniform scaling of organs and body contours based on NURBS surface technology. CONCLUSIONS It is feasible to build a 4D patient-specific digital phantom suitable for Monte Carlo radiation transport simulation. With a digital phantom for each patient, an accurate estimation of the organ doses in 4D will be made possible, which will contribute to a better tumor control because of the detailed knowledge of dose distributions in patient anatomy not only spatially but also temporally. The proposed 4D phantom will also allow for deformable dose registration for adaptive radiotherapy.