ESTRO 2021 Abstract Book

S55

ESTRO 2021

Figure 2: A) Blood propagation through the major vessels. B) the dose blood counts of the three simulated modalities. C) Dose volume histogram of the blood resulting from different radiation modalities.

Conclusion A computational model able to estimate the dose accumulated by the circulating blood for different radiation modalities and various dose rates and fractionation schemes was developed. Said model enables us to analyze a radiation modality’s potential impact on the circulating lymphocyte count, which may be especially important given the growing interest in immunotherapy. OC-0082 Impact of intra-fraction deformations on proton therapy measured with a 3D silicone-based dosimeter S. Vindbæk Jensen 1 , P. Balling 2 , L. P. Muren 3 , J. B. B. Petersen 4 , L. B. Valdetaro 1 , P. R. Poulsen 1 1 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark; 2 Aarhus University, Department of Physics and Astronomy, Aarhus, Denmark; 3 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark; 4 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark Purpose or Objective Pencil beam scanning (PBS) proton therapy offers more individualized dose distributions with less excess dose to healthy tissue than conventional radiotherapy. However, PBS is more sensitive to intra-fractional motion, which might degrade the delivered dose distribution. Dosimeters that can measure the impact of more complex motion such as deformations in the thorax and abdomen are lacking. A promising candidate is silicone-based deformable radiochromic three-dimensional (3D) dosimeters that may be cast and deformed to mimic the shape and complex motion of organs. The aim of this study was to establish a method to reconstruct intra-fraction deformations and correlate them to the delivery of proton spots. Materials and Methods Cylindrical deformable 3D dosimeters (50 mm x 50 mm Ø) were prepared using silicone elastomer, curing agent, chloroform, and the radiation-sensitive component leucomalachite green and left to cure for 48 h prior to irradiation. The optical density of the dosimeters was read-out pre- and post-irradiation using an optical CT scanner, with 1000 projections and an ordered subsets convex total variation reconstruction. During proton PBS delivery a motion-stage with a 20 mm peak-to-peak sinusoidal motion compressively strained one of the dosimeters between two acrylic plates (P2 and P3 in Fig 1) while a spring allowed it to translate. The dosimeters were irradiated with a four energy layer spread-out Bragg peak on a Varian ProBeam. A camera recorded the entire setup from above with 120 Hz. Video tracking was used to find the motion and deformation of the dosimeter while synchronizing it to the beam delivery using a low dose spot hitting a scintillating crystal before and after each energy layer (Fig 1A). For comparison, a stationary dosimeter compressively strained to 45 mm received the same plan. To account for quenching the measured dose distributions were corrected using an established linear energy transfer (LET) dependent calibration model for this dosimeter system. The calibrated dose distributions were compared with Monte Carlo (MC) simulated dose distributions using a 3%/3 mm 3D gamma comparison only including voxels above 10% of the maximum simulated dose. Results Camera monitoring allowed pinpointing the timing of each proton spot delivery to the deformational motion. The motion-stage’s movements were found to be divided equally between translation and compressive strain. The deformations resulted in the dosimeter length to vary sinusoidally between 35 mm to 45 mm (Fig 1B). The 3%/3mm 3D gamma pass rate was 94% for the static experiment and 90% for the dynamic experiment (Fig 2).