Brain tumors frequently return after treatment, and often do so at a higher grade. In addition, the treatment itself, especially RT, provokes changes in brain tissue that are difficult to distinguish from tumor recurrence using MRI. Radiation necrosis, local response to immuno-therapy, as well as the tumor itself all can show contrast enhancement. MRSI improves specificity by differentiating radiation necrosis or enhancement phenomena after local immuno-therapy, from tumor recurrence [32, 92-94].
Using the ratio of choline at the biopsy site to creatine in normal brain tissue > 1.3, Rabinov et al. 22 successfully identified glioma as opposed to radiation effects in 16 of 17 cases. In a study by Ng et al.  patients with stable disease after RT of cerebral gliomas showed lower choline to creatine ratios (mean 1.2, range: 0.4 - 2.1) compared to those with tumor recurrence (mean 3.0, range: 0.9 - 9.5). However,
22 Rabinov et al.  used 3T, PRESS, MRSI, TR/TE=1500/144 ms, peak areas determined by an iterative fit assuming Lorentzian line shapes.
there was considerable overlap between these two groups and therefore Cho/Cr ratios were considered to be "inconclusive for determining if tumor re-growth or a stable process was occurring" (p. 708). Further complicating the picture is that choline levels may also increase during RT, associated with gliosis .
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