Results From 7 Tesla 1-h Mr Spectroscopic Imaging Studies In A Regrowing Lesion After Radiosurgery

Keywords: gamma knife, Imaging, magnetic resonance imaging, outcome, recurrent disease

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     NAA/Choline and NAA/Cr ratios obtained using MR Spectroscopic Imaging (MRSI) at 1.5 Tesla (1.5T) have not demonstrated good sensitivity or specificity in differentiating tumor regrowth from pseudoprogression (presumably due to radiation injury) following radiosurgery to metastatic lesions.  .
     Lipid levels however, when elevated, have been reported to be indicative of tumor regrowth and increased spectroscopic resolution may also assist in the differentiation of the two diagnoses. We report our initial findings using 7 Tesla (7T) MRSI.
     We present a 63 year old female with metastatic Her2Neu positive breast carcinoma diagnosed initially in 1987.  
     She underwent whole brain radiation therapy to 30Gy in 2003 and then presented in 2008 with multiple new intracranial metastases for which she underwent Gamma Knife SRS to 6 lesions. All lesions received 20Gy to the edge of the tumor (isodose line range 45-50%). 12 months following SRS, the patient’s left frontal lesion was noted to increase in size with an increase in the amount of surrounding FLAIR signal abnormality. 1.5T MR diffusion imaging showed that the lesion was bright on ADC centrally with a dark rim suggestive of possible tumor regrowth. 1.5T MRSI and PET imaging results were indeterminate. 7T MRSI was then performed to attempt to determine the diagnosis.
     7T MRSI results from the left frontal lesion demonstrated globally decreased NAA, Cho and Cr peaks with ratios suggestive of necrosis but a notable elevation in the lipid peak concerning for tumor regrowth.   Given the discrepancy in imaging results, 7T MRSI guided stereotactic biopsy was performed for diagnosis. The 7Tesla MRSI images were co-registered with the 1.5T gadolinium images using the BrainLab system. Stereotactic biopsies were then performed at multiple sites guided by discrepancies between gadolinium-enhancement and areas of spectroscopic abnormality.   All biopsies were histopathologically confirmed to be consistent with radiation necrosis without any evidence of tumor.
     This is a retrospective study.
     The increased resolution of 7T MRSI and the detection of a marked lipid elevation in lesions previously treated with SRS may not always be indicative of tumor regrowth.  
     New metabolic parameters are needed to differentiate tumor progression from pseudoprogression.


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