Differentiating Radiation Effects From Tumor Progression After Brain Tumor Radiosurgery: Does ?t1/t2 Matching? Correlate With Other Imaging Methods And Clinical Findings?

Keywords: brain metastasis, gamma knife, Imaging, radiation injury, recurrent disease

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     After stereotactic radiosurgery (SRS), the differentiation between tumor progression and radiation effects (RE) can be difficult using current clinical criteria supplemented by computed tomography (CT) or magnetic resonance imaging (MRI). Metabolic imaging using single-photon emission computerized tomography (SPECT) or positron emission tomography (PET) also fail to provide reliably predictive information.
     We sought to develop a simple approach using conventional MRI sequences that could help differentiate RE from tumor progression after SRS.
     We correlated post-operative imaging and histopathology in 68 patients who underwent delayed resection of a brain metastasis after SRS. Resection was required because of clinical and imaging evidence of lesion progression 0.3 to 27.7 months after SRS.
     At the time of SRS, the median target volume was 7.1 ml (range, 0.5-26 ml) which increased to 14 ml (range, 1.3-81 ml) at the time of surgery. After initial SRS, routine contrast-enhanced MRI was used to assess tumor response and to detect potential adverse radiation effects. We correlated these serial MRIs with the postoperative histopathology to determine if any routine MRI features might differentiate tumor progression from RE.
     The median time from SRS to surgical resection was 7 months. A shorter interval from SRS to resection was associated with a higher rate of tumor recurrence (p=0.014). A correspondence between the contrast-enhanced lesion on T1-weighted images and the low-signal defined lesion margin on T2-weighted images (“T1/T2 match”) was associated with tumor progression at histopathology (p<0.0001). Lack of a clear and defined lesion margin on T2-weighted images compared to the margin of contrast-uptake on T1-weighted images (“T1/T2 mismatch”) was significantly associated with a higher rate of RE in pathological specimens (p<0.0001). The sensitivity of the T1/T2 mismatch in identifying RE was 83.3% and the specificity was 91.1%.
     This is a retrospective study.
     We found that time to progression and T1/T2 mismatch were able to differentiate tumor progression from RE in most patients.
     This approach can help clinicians make management decisions regarding the need for further observation, medical therapy, or tumor resection.


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