Synthetic CTs for MRI-only brain RT treatment: integration of immobilization systems

Abstract

Purpose Auxiliary devices such as immobilization systems should be considered in synthetic CT (sCT)-based treatment planning (TP) for MRI-only brain radiotherapy (RT). A method for auxiliary device definition in the sCT is introduced, and its dosimetric impact on the sCT-based TP is addressed. Methods T1-VIBE DIXON was acquired in an RT setup. Ten datasets were retrospectively used for sCT generation. Silicone markers were used to determine the auxiliary devices’ relative position. An auxiliary structure template (AST) was created in the TP system and placed manually on the MRI. Various RT mask characteristics were simulated in the sCT and investigated by recalculating the CT-based clinical plan on the sCT. The influence of auxiliary devices was investigated by creating static fields aimed at artificial planning target volumes (PTVs) in the CT and recalculated in the sCT. The dose covering 50% of the PTV (D50) deviation percentage between CT-based/recalculated plan (∆D50[%]) was evaluated. Results Defining an optimal RT mask yielded a ∆D50[%] of 0.2 ± 1.03% for the PTV and between −1.6 ± 3.4% and 1.1 ± 2.0% for OARs. Evaluating each static field, the largest ∆D50[%] was delivered by AST positioning inaccuracy (max: 3.5 ± 2.4%), followed by the RT table (max: 3.6 ± 1.2%) and the RT mask (max: 3.0 ± 0.8% [anterior], 1.6 ± 0.4% [rest]). No correlation between ∆D50[%] and beam depth was found for the sum of opposing beams, except for (45° + 315°). Conclusion This study evaluated the integration of auxiliary devices and their dosimetric influence on sCT-based TP. The AST can be easily integrated into the sCT-based TP. Further, we found that the dosimetric impact was within an acceptable range for an MRI-only workflow.

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“…The fact that the neck support is not visible on sCT may cause problems while troubleshooting patient positioning issues in treatment room (e.g., correct position of cervical vertebrae). A possible solution has been suggested by Masitho et al [34] , where silicon markers were used during MR scan to place a predefined auxiliary structure template for the immobilization device and the couch top during treatment planning. However, this method does not account for difference in setup for different patients.…”

Improved automatic segmentation of brain metastasis gross tumor volume in computed tomography images for radiotherapy: a position attention module for U-Net architecture

“…Additionally, the specialized MRI simulation equipment needed for consistent positioning is not universally accessible, particularly in resource-constrained environments, restricting synthetic CT’s widespread adoption. These obstacles need to be overcome in order for synthetic CT to have comparable reliability in radiotherapy to that of traditional CT, which continues to be the standard care of treatment planning ( 31 ). Enhancing CT-based automatic segmentation algorithms could thus ensure broader access to high-quality care and promote global equity in care.…”