Tumor Motion Management in Radiotherapy Using 4D-MRI

Respiration-induced patient motion has become a major obstacle for achieving high-precision radiotherapy of cancers especially in the thorax and upper abdomen. As the target is continuously moving, an additional margin has to be added to the clinical target volume to compensate for the uncertainty in the tumor and organ motion, causing toxicity to the normal tissue and limiting the dose delivered to the target. To account for the tumor motion, surrogate tracking methods are commonly used in clinics during radiotherapy. However, the relationship between the surrogate and tumor motion is hard to generalize as it depends on individual patients, tumor location, treatment fractions, and sometimes shows complex patterns or transient, unpredictable changes. Hence, there is an urgent need to better scrutinize the current surrogate-based motion management strategies. Moreover, the most robust motion management strategy for the given patient should be determined in the pre-treatment setting but the investigators currently lack a sufficient tool to provide this information.

4D-CT is typically used to characterize the tumor motion over the course of the radiotherapy. However, 4D-CT is an oversimplified snapshot representation of a single-breathing cycle with low soft tissue contrast while imparting a considerable amount of radiation dose to the patient. Consequently, the limitations of 4D-CT prevent applicability in acquiring information over timescales that represent a treatment session. MRI is highly advantageous as it is non-ionizing and provides excellent soft tissue contrast. Although real-time 3D dynamic MRI is limited by low image quality and temporal resolution, 2D dynamic MRI techniques have high fidelity and spatio-temporal resolution requisite for real-time tracking of the moving target. Furthermore, a respiration-correlated 4D-MRI can be reconstructed from multi-slice 2D dynamic MR images, enabling volumetric image processing and analysis. Therefore, 4D-MRI is an attractive solution to address breathing motion and tumor tracking obstacles in radiotherapy.

The main goal of this research is to characterize patient-specific respiration-induced tumor and surrogate motion to evaluate the accuracy and effectiveness of the surrogate-based motion management strategies currently used in clinics. Specifically, the investigators hypothesize that dynamic MRI obtained over a temporal duration consistent with radiotherapy treatments will provide spatio-temporal information of both the tumor and surrogate, and therefore can serve as a means to assess the quality of the tumor motion tracking with the surrogate. To test their hypothesis, the investigators specifically propose to 1) track and characterize the tumor and surrogate motion with 4D-MRI and 2) evaluate surrogate-based motion tracking in a cohort of patients with thoracic tumors.

External and internal surrogate-based strategies commonly used in clinics have not been appropriately validated. With the increasing adaptation of these surrogate methods for motion management, the proposed research addresses these urgent issues in clinical radiotherapy while providing a means to achieve patient-specific motion management.