Purpose: To develop a novel 4D dynamic liver blood flow model, capable of accurate dose estimation to circulating blood during radiotherapy.

Methods: Adult male and female liver phantoms with detailed vascular trees including the hepatic arterial, portal venous and hepatic venous trees were developed. A discrete time Markov Chain approach was applied to determine the spatiotemporal distribution of 10⁵ blood particles (BP) in the human body outside of the liver based on reference values for cardiac output and blood volumes. For BPs entering the liver, an explicit Monte Carlo simulation was implemented to track the propagation of individual BPs along 1996 distinct vascular pathways through the liver vasculature and time-dependent radiation fields. The model tracks accumulated dose from the time-dependent radiation fields with a 0.1s time resolution. The model was evaluated for 3 male and 3 female patients receiving photon (VMAT, IMRT) and proton (passive SOBP and active PBS) treatments. The impact of treatment modality, delivery time and fractionation on circulating blood was investigated and quantified using mean dose (), V_>0Gy, V_>0.125Gy and D_2%.

Results: The blood DVHs were successfully generated for all 6 patients planned for photon and proton treatments. Average reductions in integral dose, , V_>0Gy, V_>0.125Gy and D_2% of 45%, 6%53%, 19% respectively, were observed for proton treatments. Our simulation also shows that V_>0Gy, V_>0.125Gy, D_2% are highly sensitive to the beam-on time. Both V_>0Gy and V_>0.125Gy increased with beam-on times, whereas D_2% decreased with increasing beam-on times, demonstrating the tradeoff between low dose to large fractions of circulating blood and high dose to a small fraction depending on delivery speed.

Conclusion: Our 4D dynamic liver blood flow model enables realistic dose estimation to circulating blood during liver-directed radiotherapy. The assessment of different treatment modalities and delivery times shows that the time structure and beam on time is potentially more impactful on dose to the blood than overall integral dose. 

Short Bio: Stella Xing is a second-year physics resident at the Massachusetts General Hospital. During her first-year residency, she worked on developing a realistic blood flow model to calculate dose to circulating blood and lymphocytes in the liver with Dr. Harald Paganetti and Dr. Clemens Grassberger.  Stella is also part of the Science council associates mentorship program in AAPM.

Stella received her PhD in Medical Physics specializing in diffusion-weighted MRI and her BSc. and MSc. in Physics all from McGill University in Montreal, Canada. Besides her professional life, Stella is passionate about promoting education equity and helping those less fortunate through her work with non-profit organizations.