  Purpose: The XCAT phantom allows for highly sophisticated multimodality
imaging research. It includes a complete set of organs, muscle, bone, soft
tissue, while also accounting for age, sex, and body mass index (BMI), which
allows phantom studies to be performed at a population scale. At the same time,
the XCAT phantom does not currently include the lymphatic system, critical for
evaluating bulky nodal malignancies in lymphoma. We aimed to incorporate a full
lymphatic system into the XCAT phantom and to generate realistic simulated
images via guidance from lymphoma patient studies.
  Methods: A template lymphatic system was extended based on known anatomy was
used to define 276 lymph nodes and corresponding vessels using non-uniform
rational basis spline (NURBS) surfaces. Lymph node properties were modified
using the Rhinoceros 3D viewing software. The XCAT general parameter script was
used to input organ concentrations and generate binary files with uptake and
attenuation information.
  Results: Lymph nodes can be scaled, stretched, and translated within the
intuitive Rhinoceros interface, to allow for realistic simulation of different
lymph node pathologies. Bulky, heterogeneous PMBCL tumours were generated in
the mediastinum using expanded lymph nodes. Our results suggest that optimized
thresholding provides better accuracy for determining total metabolic tumour
volume (TMTV) of PMBCL tumours, while the gradient method was most accurate for
total lesion glycolysis (TLG).
  Conclusions: An upgraded XCAT phantom with a fully simulated lymphatic system
was created. Distributed to the research community, the XCAT phantom with the
new lymphatic system has the potential of enabling studies to optimize image
quality and quantitation, towards improved assessment of lymphoma including
predictive modeling (e.g. improved TMTV and radiomics research).
