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Session: FLASH Radiotherapy - Radiobiology, Treatment Planning and Dosimetry - II [Return to Session]

The Impact of Ultra-High-Dose Rate (FLASH) Proton Therapy On Radiation Dose to Circulating Lymphocytes

A Hammi1*, (1) TU Dortmund University, Dortmund, North Rhine-Westphalia, DE

Presentations

TH-A-202-6 (Thursday, 7/14/2022) 7:30 AM - 8:30 AM [Eastern Time (GMT-4)]

Room 202

Purpose: To investigate the potential sparing effect of ultra-high-dose rate radiation therapy (FLASH-RT) on circulating lymphocytes (CLs) utilizing intensity modulated proton therapy (IMPT) as delivery mode.

Methods: An in-house software was developed to model the radiation field's spatially-varying instantaneous dose-rate and the spatio-temporal allocation of lymphocytes in the primary- and secondary- lymphoid organs. An additional cardiovascular system was implemented to simulate the dynamic flow pattern of blood and CLs across the entire body. The blood-pool counts 50*10⁶ blood cells.For brain cancer cases, vasculature networks were extracted from 3T MR-angiography data of 15 subjects. A Fractal law was implemented to mimic arterial bifurcations and extend the macrovascular network with arterial trees. Three different hypofractionated courses were simulated at the same physical dose: 1 x 50Gy, 2 x25Gy and 5 x 10Gy. FLASH-RT IMPT-plans were simulated using a maximum beam current of 300nA. The FLASH deliveries were compared to three different conventionally fractionated treatment modalities (25 x 2Gy) at standard dose rates, namely IMPT, passive scattering PT (PS) and intensity-modulated (photon) RT (IMRT). The linear-quadratic model was used to compute the depletion of CLs cells.

Results: The FLASH-RT dose rate was set to be >40 Gy/s. The irradiated CL subsets (V>0Gy) ranged between 0.8-2% by FLASH-RT and reached a maximum of 95%-99.4% for IMRT. The sparing effect on CLs decreased with dose/fraction was highest for FLASH-RT at 1 x 50Gy (0.2%) and, 2.4% and 5.4% at 2 x 25Gy and 5 x 10Gy, respectively. This sparing effect reduced further at conventional dose rates: 51.7%, 31.7% for and 25.9%, for IMPT, PS and IMRT, respectively. The maximum accumulated dose to a single CL was 47.2Gy, during single-fraction FLASH-RT.

Conclusion: We have assessed a significant sparing effect on CLs by FLASH-RT as compared to standard dose-rate radiation therapy, which supports the reported FLASH effect.

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