Purpose: The purpose of this study is to develop a dynamic blood flow model based on realistic vasculature and blood flow rates in the human brain to calculate lymphocyte dose-volume histograms depending on tumor location and dose rate.
Methods: The 4D dynamic brain blood-flow model is based on adult-male and adult-female brain computational phantoms including all relevant vascular structures in the brain starting with the left and right Internal Carotid Artery (ICA) and ending with the left and right Transverse Sinus in the head and neck transition region. The blood circulation in the brain was explicitly modeled using a Monte Carlo simulation to track the propagation of discrete blood particles (BP) along the vascular pathways with velocities derived from published literature on relative blood vessel flow rates and radii. The BP movement along the vasculature trees were simulated with a spatial resolution of 0.1 cm and a time resolution of 0.01s, and radiation treatments were modeled field-by-field using varying dose rates.
Results: The anatomical vascular model yielded 4,047 distinct vascular pathways throughout the brain with blood flow velocities from 1-25 cm/s (mean: 5 cm/s), covering the entire brain volume. Mean dose to circulating lymphocytes varies for similar tumor volume by location: peripheral tumors lead to an average of 34% less dose to lymphocytes compared to central tumors for conventional beam-on-time (BOT) of 60s. The percentage of lymphocytes receiving a high dose is highly sensitive to the BOT: reducing BOT by 25% or 50%, the D2% to lymphocytes increased from 4.5 Gy to 6.0 and 9.0 Gy respectively for base of skull tumors.
Conclusion: Accurate estimation of dose to circulating lymphocytes during brain irradiation reveals complex dependencies on tumor location and the time structure of delivery. Dynamic simulations are required to investigate the impact of delivery techniques on radiation-induced lymphopenia.