What are the imaging characteristics and differential considerations for radiation necrosis on MRI, and how can MR spectroscopy aid in its differentiation from recurrent neoplasm?

Radiation necrosis typically occurs 6 to 24 months post-radiation therapy and presents as an enhancing lesion with mass effect and ring enhancement on MRI. It can also appear as multiple foci of enhancement, often mimicking recurrent neoplasm. Key diagnostic features include elevated lactate and lipid peaks (0.9 to 1.3 ppm) on MR spectroscopy, along with reduced levels of choline, creatine, and N-acetylaspartate. Clinical pearls include recognizing that radiation necrosis may present with edema and mass effect similar to tumor recurrence. A common pitfall is misinterpreting the enhancement pattern as tumor progression; thus, MR spectroscopy is crucial for differentiation.