Polymer gel dosimetry with MRI-readout for 3D dose verification - detector characteristics and clinical applications

Abstract: Background & Purpose: Radiation therapy is an essential treatment for cancer patients with the ultimate goal to deliver radiation doses to the tumour with precision while minimizing exposure to surrounding healthy tissues. Treatment verification is crucial to ensure the accuracy of radiation delivery, for instance by measuring the radiation dose distribution. Among various dosimetry techniques, gel dosimetry has emerged as a promising method due to its ability to measure 3D dose distribution with high spatial resolution. The overall aim of this thesis is to assess the applicability of normoxic polymer NIPAM gel dosimeters with MRI readout in clinical practice. Specific goals include 1) Investigating and optimizing the gel dosimetry workflow containing new laboratory equipment, new MRI sequences during readout and novel software for analysis 2) Evaluating different characteristics of the dosimeter for various types of radiation qualities and 3) Using the dosimeters for a 3D dose verification of a clinically novel radiotherapy treatment techniques. Material & Method: The NIPAM gel dosimeter was produced by dissolving gelatin in deionized water. N’-methylenebisacrylamide (BIS) and n-isopropyl acrylamide (NIPAM) together with antioxidant tetrakis (hydroxymethyl) phosphonium chloride (THPC) were added. The gel was then exposed to different radiation qualities (220 kV photon, 6 and 10 MV photon, and 120 MeV proton beams) with varying setups to investigate the dose-response for clinically relevant dose range, the resolution, the inter-batch & intra-batch variation and the dose rate & energy dependency. Readout was done using a clinical MRI (3 T). In the application of 3D measurement, the NIPAM gel dosimeter was used to verify a multiple brain metastases stereotactic HyperArc photon treatment. The dose distribution acquired from the gel measurement was compared with the dose distribution from the treatment planning system using different gamma criteria. Result: NIPAM-gel dosimeter exhibited an approximately linear dose response for various types of radiation, including 220 kV photons, 6 and 10 MV photons, and 120 MeV proton at the plateau region. The gel dosimeter demonstrated a high spatial resolution. Intra-batch variation showed good consistency with a standard deviation within 1%. The inter-batch deviation was found with a maximum of 7% at 26 Gy. No dose rate dependency was found for dose rates of 600 MU/min and 300 MU/min within the 0-15 Gy dose range. Similarly, no energy dependency was observed for 6 MV and 10 MV within the same dose range. When measuring the depth dose curve of the proton beam, the dosimeter exhibited a quenching effect of 40% to 45% in the Bragg peak due to higher linear energy transfer. The result from verification of multiple brain metastases stereotactic HyperArc photon treatment showed pass rates of 99.94% and 99.87% (5%/5mm); 96.46% and 91.91% (3%/3mm) and 92.21% and 83.44% (3%/2mm) for 50% of dose and 90% of dose (global gamma), respectively when compared to treatment planning system. Conclusion: The work within this study has developed and improved a workflow for gel dosimetry, incorporating the use of new laboratory equipment, new MRI sequences during readout, and novel software for data analysis. Further, the results of this study enhance the understanding of NIPAM-gel dosimeter characteristics and showed potential use for clinical applications in accurately assessing 3D dose distribution.