Poster T5 - Optimization of Magnetic Hydrogels to Improve Anatomical Displacement during Radiotherapy

Introduction: When the bowel is located immediately adjacent to the radiotherapy target volume the radiation oncologist must either compromise the treatment dose or volume, thus risking loss of tumor control or incurring radiation induced bowel toxicity which can be a life-threatening complication. We investigate a novel method to reduce dose to proximal bowel, which relies on a magnetic hydrogel placed within the bowel, controlled by exogenous magnetic fields (Figure 1a). Favorable anatomical management is achieved by increasing the distance between treatment volume and proximal bowel (Figure 1b). Magnetic hydrogels have been proposed for intracavitary drug delivery and medical implants but have yet to be applied in a radiotherapy setting. High Z components of magnetic hydrogels and the presence of magnetic fields inside the body will affect radiation dose to surrounding tissue, the magnitude of which is currently unknown. Here, we investigate the dosimetric characteristics of various magnetic hydrogel formulations when used in the setting of photon external beam radiotherapy.

Materials and

Methods: Magnetic hydrogels were formulated following previously proposed formulations for biomedical devices [1], consisting of a dilatant non-Newtonian fluid, held together by the cross-linking of polymer chains (5% aqueous polyvinyl alcohol) with borate ions (5% aqueous borax). Magnetic particles (NdFeB; average diameter: 160 μm) were added to polyvinyl alcohol solution and mixed via ultrasonication. The borax solution was then added and mixed to form the hydrogel. Protype hydrogels of 15%, 30%, and 50% magnetic particles by total weight were created. Mass density of each formulation was measured using a scale and water displacement. The dosimetric effect of the protype hydrogels was quantified via experimental measurements and Monte Carlo simulation. Samples of the hydrogel were placed in a cylinder cavity (2 cm height, 1 cm radius) in a stack of radiologically water equivalent plastic sheets and irradiated with single en face 6 MV photon beam on a Varian TrueBeam linear accelerator. Ionization was measured with a parallel plate ionization chamber (PPC05) placed at varying distances posterior and lateral to the hydrogel. Identical measurements were made without the hydrogel and differences due to the hydrogel were calculated. Experimental measurements were used to validate a model of the magnetic hydrogel in TOPAS (v3.9): in which the same analysis was conducted. Measurement and simulation results were compared.

Results, Conclusions, and Discussions:

Results:
The density of the hydrogels was 1.00±0.07, 1.40±0.06, 1.70±0.06 g/mL for 15%, 30%, and 50% respectively. In experimental measurements, radiation dose had the greatest change immediately posterior and lateral to the hydrogel. The 15% magnetic hydrogel exhibited a 3.3% decrease and a 2.7% increase immediately posterior (Figure 2a) and lateral (Figure 2b) respectively. These values were 7.9% and 8.5% for the 30% magnetic hydrogel and 11.4% and 6.1% for the 50% magnetic hydrogel. The change in dose was non-negligible up to 25 mm posterior to the hydrogel but became insignificant at 15 mm lateral to the hydrogel.

Conclusion:
Magnetic hydrogels have a non-negligible effect on Megavoltage radiotherapy beams, indicating that tissues near the hydrogel will have significant dose differences that will need to be accounted for during the treatment planning process. Future work will include investigating different formulations of magnetic hydrogels such as utilizing PEG based gel and iron oxide magnetic particles. Additionally, utilizing the validated TOPAS model to evaluate the dosimetry of a variety of radiotherapy scenarios including the application of external magnetic fields.

Acknowledgements (Optional): Reference:
[1] M. Sun, C. Tian, L. Mao, X. Meng, X. Shen, B. Hao, X. Wang, H. Xie, L. Zhang, Reconfigurable Magnetic Slime Robot: Deformation, Adaptability, and Multifunction. Adv. Funct. Mater. 2022, 32, 2112508. https://doi.org/10.1002/adfm.202112508