An Algorithm for Lymphatic Node Placement in Hybrid Computational Phantoms-Applications to Radionuclide Therapy Dosimetry

The presence of lymphatic nodes within computational human phantoms is critical to the evaluation of tissue doses in lymphoma patients treated with targeted radionuclide therapy. Furthermore, the International Commission on Radiological Protection (ICRP) now considers the lymphatic nodes to be a radiosensitive tissue of the remainder tissues of the effective dose as defined in ICRP Publication 103. Lymphatic nodes have previously not been included within the structure of stylized phantoms and are only present within a few voxel-based phantoms through manual retagging of voxels of the residual soft tissues. In this paper, an automated methodology is introduced whereby lymphatic nodes are placed-in an anatomically realistic fashion-within the University of Florida adult male reference hybrid phantom. First, a total of 16 lymphatic node anatomical sites are defined within the phantom. These include the extrathoracic, cervical, thoracic (upper and lower), breast (left and right), mesentery (left and right), axillary (left and right), cubital (left and right), inguinal (left and right), and popliteal (left and right) nodal regions. Secondly, individual lymphatic node positions are randomly sampled within spherical tissue regions centered at each previously selected lymphatic node site. Thirdly, smaller voxelized tissue spheres representing individual lymphatic nodes are then created to populate each nodal region of the phantom using a nominal nodal diameter. This process is repeated until a total lymphatic node mass is achieved to match age-dependent total reference masses derived from ICRP Publication 66 and 89 data. An illustrative set of radionuclide S-values for 131 I are presented whereby regional lymphatic nodes are considered to be independent source and target regions of a reference lymphoma patient. As nodal size can easily be changed within individual nodal regions, the lymphatic node model suggested in this paper can be used to obtain realistic dosimetry for lymphoma patients treated with radionuclide therapy.