MRI Morphologic Alterations After Liver-SBRT: Assessment of Correlation With Dose Based on Deformable Matching

Abstract

CT-morphologic alterations after SBRT are described in the literature and they can be correlated to histopathologic alterations. In this work we analyzed the correlation of MRI-morphologic alterations with radiation doses to assess the potential for MR-based in-vivo dosimetry in healthy liver tissue. MRI data of 22 patients with liver metastases before and after 7±3 weeks after image guided SBRT in computer-controlled deep inspiratory breath hold (DIBH) were analyzed retrospectively. T1- and T2-weighted MRI sequences were matched to the planning CT with the corresponding dose distribution with a commercially available deformable registration algorithm. Isodose lines were directly correlated to the radiologic changes. Absolute doses were converted to BED2 with an α/β-value of 2 and 3 for healthy liver tissue and 10 for tumor tissue (BED2=Dx (d+α/β)/(2+α/β)). Nominal prescription dose was 48.9±14.7Gy (median 60Gy) in single doses of 14.1±6.7Gy (median 12Gy). BED2 was 93.9±26.8Gy (median 110Gy) for the PTV (α/β10). Central necrosis was observed within the isodose lines of nominally 48.8±14.7Gy (median 60Gy), BED2 (α/β10) 93.9±26.7Gy (median 110Gy). A surrounding contrast medium accumulation was observed within the isodose lines of nominal 47.4±14.9Gy (median 58.5Gy), BED2 (α/β10) 91.8±27.4Gy (median 105.8Gy). Outside the high-dose volume, in the region of radiation beam entrance, characteristic sharply defined radiological changes were observed which could be directly correlated to the isodose lines: T1 hypo- or hyperintensity occurred starting at isodose lines of nominally 22.2±6.8Gy (median 24Gy), BED2(α/β2) 43.4±7.8Gy (median 42Gy), BED2 (α/β3) 39.3±6.8Gy (median 37.6Gy). T2-hyper/or hypointensity was observed within isodose lines of nominally 22.4±6.7Gy (median 24Gy), BED2 (α/β2) 43.1±11.6Gy (median 41.5Gy), BED2 (α/β3) 39.6±9.8Gy (median 38,7Gy). No high grade toxicity or changes in liver laboratory values were observed. (1) A sharply defined pattern of radiological changes indicates a precise dose application in repetitive breath hold during hypofractionated SBRT. (2) Characteristic radiogenic changes occur in the first post-SBRT MRI. Within the high-dose region of the PTV, tumor necrosis and peripheral contrast medium accumulation can be observed. Beam entrance doses of 37-42Gy (BED2) induce MRI morphologic changes which can be correlated to isodose lines. These changes have not resulted in clinical or biochemical toxicity in the studied cohort. Based on deformable matching, these data allow a direct spatial and dosimetric correlation and quantification of SBRT-induced changes in healthy liver tissue.