Technical Note: Radiotherapy dose characterization of gel dosimetry using shear wave elasticity imaging

Radiotherapy is an effective treatment for many types of cancer in clinical settings. Gel dosimetry has the potential to record three-dimensional (3D) dose distribution compared to a conventional ion chamber. As the elasticity of the gel is altered after irradiation due to gel polymerization, we aim to measure the dose recorded in gel dosimetry with ultrasonic shear wave elasticity imaging (SWEI), a nondestructive and quantitative elasticity imaging tool.

In this study, a cylindrical N-isopropylacrylamide (NIPAM) polymer gel with a diameter of 10 cm and a height of 10 cm and with cellulose as an ultrasonic scatterer was irradiated by a linear accelerator with the irradiation parameters of 6 MV x-ray, dose rate of 100 cGy/min and field size of 10  $$  20 mm² . The six gel phantoms were irradiated with the dose of 0, 1, 3, 5, 8, or 10 Gy. The gel phantoms were measured with SWEI at 24, 36, and 48 h after x-ray irradiation. The two-dimensional (2D) shear wave velocity and Young's modulus maps corresponding to x-ray dose distribution were reconstructed following a time-of-flight reconstruction from a set of time-series displacement maps. The spatial resolution of the reconstructed SWEI image is ~1 mm.

Our results show that the elastic modulus increases linearly as irradiation dose increases (R²  = 0.94 at 24 h, R²  = 0.98 at 36 h, R²  = 0.98 at 48 h), suggesting that the gel elasticity is highly associated with x-ray irradiation dose at 36 h post irradiation, and the dose resolution was 0.66 kPa/Gy. From the 3D elastic modulus maps, the dose distribution along the depth and lateral direction can be reflected in the NIPAM gel dosimetry using SWEI as well.

In this study, the irradiated NIPAM gel phantom was quantitatively measured with SWEI for the first time to read the dose distribution recorded in the gel dosimetry. The results suggest that the gel elasticity is highly associated with x-ray irradiation dose. In the future, 2D/or 3D dose distribution from intensity modulated radiotherapy (IMRT) or other potential particle radiotherapy will be measured and reconstructed with SWEI and compared with the dose map from a treatment planning system (TPS) in the clinic.