Assessment of Spatial and Dosimetric Accuracy of a 0.35 T MR-Linac using a Modular Phantom and PG Measurements

Lukas Nierer^*
*Correspondence: Lukas Nierer, Department of Radiation Oncology, University Hospital, LMU Munich, Marchioninistr 15, 81377 Munich, Germany, Email:

Keywords

Radiation • Polymer • MRI

Introduction

MR-guided radiotherapy (MRgRT) is an emerging treatment modality that combines the real-time imaging capabilities of MRI with the precise delivery of radiation therapy. This technique allows for the visualization of soft tissues and organs in real-time, which can help clinicians to adapt the radiation treatment plan to changes in the tumor and surrounding tissue. MRgRT is especially useful for tumors in organs that move, such as the lungs and liver and can reduce the risk of toxicity to nearby healthy tissue. To evaluate the accuracy of MRgRT, a multimodality phantom can be used. A multimodality phantom is a device that mimics the properties of human tissue and can be imaged using multiple imaging modalities, such as MRI, computed tomography (CT) and positron emission tomography (PET). By using a multimodality phantom, the accuracy of the MRgRT system can be evaluated and compared with other imaging modalities. Polymer gel dosimetry can be used in combination with a multimodality phantom to evaluate the accuracy of MRgRT. The gel can be placed inside the phantom and irradiated using the MRgRT system. The dose distribution can then be visualized using MRI and compared to the expected dose distribution. By comparing the measured and expected dose distributions, the accuracy of the MRgRT system can be evaluated [1,2].

Literature Review

Studies have shown that polymer gel dosimetry combined with a multimodality phantom can provide accurate and reliable measurements of the dose distribution in MRgRT. The technique has been used to evaluate the accuracy of MRgRT systems for various types of tumors, including lung, liver and prostate cancer. Polymer gel dosimetry combined with a multimodality phantom is a valuable tool for evaluating the accuracy of MRguided radiotherapy systems. The technique allows for the visualization and verification of the delivered radiation dose, which can improve the safety and efficacy of radiation therapy. As MR-guided radiotherapy becomes more widely adopted, the use of polymer gel dosimetry is likely to become more common in clinical practice [3].

Magnetic Resonance-guided Linear Accelerator (MR-linac) is an emerging technology that combines real-time imaging capabilities of MRI with the precise delivery of radiation therapy. The combination of these two technologies enables the adaptation of radiation therapy in real-time, which can improve the efficacy and safety of the treatment. However, the accuracy of MR-linac needs to be evaluated to ensure that the delivered radiation dose is within an acceptable range [4].

Discussion

One method to assess the accuracy of MR-linac is by using a modular phantom and polymer gel dosimetry (PG). A modular phantom is a device that mimics the properties of human tissue and can be used to simulate the treatment of different types of tumors. Polymer gel dosimetry is a threedimensional dosimetry method that uses a polymer gel as a dosimeter. The polymer gel changes its optical density upon exposure to radiation and the dose distribution can be visualized using MRI. A study was conducted to assess the spatial and dosimetric accuracy of a 0.35 T MR-linac using a modular phantom and PG measurements. The study used a modular phantom that consisted of multiple interchangeable modules that could be assembled to simulate the treatment of different types of tumors. The phantom was filled with polymer gel dosimeters, which were irradiated using the MR-linac [5].

The PG measurements showed high reproducibility, indicating that the method was reliable for measuring the delivered radiation dose. The study found that the MR-linac performed well in a non-adaptive setting in terms of spatial and dosimetric accuracy. However, larger mean deviations were found in the mid-dose range compared to the high-dose region. The study concluded that the combination of a modular phantom and PG measurements is a useful method for assessing the accuracy of MR-linac. The results of the study can be used to optimize the treatment planning process and improve the accuracy of MR-linac. The study also highlights the importance of evaluating the accuracy of new technologies such as MR-linac, to ensure that the treatment is safe and effective for patients [6].

The use of a modular phantom and PG measurements is a reliable and accurate method for assessing the spatial and dosimetric accuracy of a 0.35 T MR-linac. The method can be used to evaluate the performance of MRlinac and optimize the treatment planning process. As the technology of MRlinac continues to evolve, the use of modular phantom and PG measurements is likely to become more prevalent in clinical practice. The objective of this study was to validate the effectiveness of a new phantom for use with a 0.35 T MR-linac and measure its reproducibility and reliability. To achieve this, we conducted PG measurements on each phantom twice. In contrast to Pappas et al.'s approach of using separate phantoms for PG and gafchromic film measurements, we used the same phantom for PG and IC inserts, which eliminates any potential disparities in phantom geometry. This also allowed us to normalize the relative 3D dose obtained with PG to the absolute IC reference dose, which is only possible with dedicated phantoms that have exchangeable inserts. Additionally, we improved dosimetric accuracy by using a 1 × 1 × 1 mm³ dose calculation grid.

Conclusion

In this study, we assessed the suitability of a novel hybrid anthropomorphic phantom for use with a 0.35 T MR-linac. We observed good agreement in the high dose region used for normalization, but noted a mean error of 0.2 Gy (5.6%) in the intermediate dose region (3.2 Gy-4.8 Gy) for normalization N1 (IC measured dose). The 3D printed anthropomorphic phantoms' modular design allowed us to normalize the R2-map to the IC absolute dose reference, resulting in a 3D dose map that enabled us to analyze the dose distribution extensively, including organ at risk (OAR) and planning target volume (PTV) dose volume histograms (DVH). We evaluated the dosimetric and spatial accuracy of the MR-linac system for three similar cases in the cranial region. To validate the reproducibility and stability of PG dose measurements, we repeated measurements on each phantom using PG from different production batches and readout on two separate 1.5 T diagnostic MR scanners. This also helped to manage scanning times. Overall, we found high spatial accuracy for complex, non-adapted cases, but small setup deviations could potentially compromise the DVH of OARs close to or within steep dose gradients. None.

Acknowledgment

None.

Conflict of Interest

None.

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