Quantifying setup uncertainty between computed tomography guidance and magnetic resonance guidance in intramuscular
Travis Salzillo1, Surendra Prajapati1, Zhiqian Henry Yu1
1Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas, USA.
Background And Purpose:
MR-Linac (MRL) is well-suited for treatment of oligometastatic disease (OMD) in muscle due to its superior soft tissue contrast. The purpose of this study was to assess dosimetric differences and quantify margins when CT-guidance (CTgRT) is used for OMD treatment compared to MR-guidance (MRgRT) on the MRL.
Materials And Methods:
Five patients with intramuscular oligometastasis were treated with the MRL using daily MRgRT. To simulate CT-based alignment, an in-house deep learning model was used to generate synthetic-CT images from MR images for each fraction. The synthetic-CTs were independently aligned to the corresponding simulation-CTs by seven physicists using rigid registration. The difference between CTgRT and MRgRT alignment was calculated to quantify the inter-fractional image-guidance uncertainty and estimate PTV margins. Dose from the clinical beam sets was recalculated on the synthetic-CTs for comparative analysis.
Results:
The CTgRT alignment compared to MRgRT were -0.59 ± 3.49 mm, 2.04 ± 3.96 mm, and 1.23 ± 1.20 mm in left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions, respectively. Compared to clinical plans, the dosimetric parameters of recalculated synthetic-CT plans, V95 and D95, were significantly lower, while the homogeneity index was significantly higher (p < 0.001 for each metric). Compared to MRgRT, CTgRT required additional treatment margins of 10.0 mm in LR, 11.5 mm in SI, and 4.6 mm in AP directions.
Conclusions:
We quantified the dosimetric difference in treating intramuscular metastases using MRgRT versus CTgRT and showed that treatment margin can be reduced by at least 5 mm in each direction with MRgRT.
