ESTRO 2021 Abstract Book

S54

ESTRO 2021

Conclusion This study represents a first step towards a full evaluation of the biological efficacy of VHEE beams. The biologically relevant information obtained from these theoretical MC simulations could be used to complement further experiments which explore the radiobiological response to VHEE treatment.

OC-0081 Blood dose calculation model: proton therapy versus photon therapy A. Hammi 1 1 TU Dortmund University, Medical Physics, Dortmund, Germany

Purpose or Objective To minimize cumulated radiation dose to circulating lymphocytes during the fractionated radiotherapy, a dynamic model is required that considers the time-dependent whole-body blood flow recirculation and the temporal structure of the treatment, to estimate dose to circulating lymphocytes. In this work dynamic computational model was developed to calculate dose to the circulating blood for a variety of radiation To describe the 4D propagation of blood particles through organs and radiation fields, an explicit Monte Carlo blood flow model was developed based on the whole-body hemodynamics (fig 1 A). For head-and-neck patients, the large-scale architecture of cerebral arteries is extracted from MRA images (fig 1 B). The propagation of the blood was described as laminar along the vasculatures and through radiation fields (fig 2 A). A dynamic delivery model was implemented to simulate different radiation modalities such as proton pencil beam scanning (PBS) and passive scattering (PS) as well as photon therapy. The model included dynamic details of clinical delivery system, such up-/down-ramping of the current as well as the dead time during steering between spot positions and switching between energy layers were all taken into account (fig 1 C/D). treatment modalities. Materials and Methods

Figure 1: A) The blood circulation model. B) Reconstructions of cerebral vasculature from MRA. C) The energy profile of each beamlet and the corresponding delivery time. D) The synchronization of the radiation delivery and the time-dependent blood flow.

Results For a 2 Gy fraction, PBS reduced the average dose to the circulating blood by 1.9% compared to PS and by 130% compared to photon therapy (fig 2 B/C). The fraction of blood receiving any dose was only 1.1% for PBS, compared to 7.3% and 18.1% for PS and photon therapy, respectively. The blood volume receiving at least 0.07 Gy, which is known to deplete the lymphocyte count by 2 percentage points, increased from 0.65% for PBS to 1.4% for PS and 2.1% for photon therapy.