Late enhancement and washout maps for distinguishing tumor progression from radiation necrosis in malignant brain tumors: an exploratory study

BACKGROUND: Distinguishing between tumor progression and radiation necrosis during brain tumor follow-up is challenging. Advanced MR imaging techniques, such as perfusion, diffusion-weighted imaging (DWI), and spectroscopy, can help differentiate between these conditions. In our clinic, we established a novel technique, the rapid washout and late-enhancement map. This study investigated the value of this map in distinguishing true progression from radiation necrosis. METHODS: Patients with a histologically confirmed initial diagnosis of glioblastoma, high-grade astrocytoma, primary central nervous system lymphoma, or brain metastasis who were undergoing radiochemotherapy and participated in our MRI protocol, including MR perfusion and rapid washout imaging, between January 1, 2024, and December 31, 2025, were included in the study. We volumetrically measured late enhancement and washout in brain tumors at all follow-up examinations and analyzed the resulting volumes. Additionally, we evaluated diffusion-weighted imaging and MR perfusion images. RESULTS: A total of 33 patients (25 with glioblastoma, two with astrocytoma grade 4, two with astrocytoma grade 3, one with CNS lymphoma, and three with brain metastasis) were included. Diagnostic confirmation was obtained by histopathological assessment in 20 patients, MRI follow-up in 13 patients, with additional fluorodeoxyglucose-positron emission tomography in two. Among these patients, 23 were diagnosed with tumor recurrence, four with mixed pathology comprising "radiation necrosis with scattered active tumor cells", and six with radiation necrosis. The mean washout ratio (± standard deviation) was 31 ± 18 % for progressive disease, 35 ± 20 % for mixed pathology, and 10 ± 7 % for radiation necrosis, respectively. CONCLUSIONS: The washout ratio can enhance the differentiation between recurrent tumor growth and radiation necrosis during post-treatment monitoring of malignant brain tumors. As MR perfusion can yield inconclusive results in this setting, the proposed method offers complementary information that supports initial diagnostic decision making. Diagnostic discrimination may be strengthened by combining the relative cerebral blood volume with washout ratio analysis, rather than relying on a single imaging marker.