MR imaging of brain injury induced by carbon ion radiotherapy for head and neck tumors.

To clarify the characteristics of magnetic resonance (MR) imaging of radiation-induced brain injury following carbon ion radiotherapy and to observe the changes in lesions over time, we evaluated 40 patients with radiation-induced brain injury from carbon ion radiotherapy for head and neck tumors. Their primary lesions received a radiation dose of 48 to 70.4 Gray equivalent (GyE) in 16 to 18 fractions. MR imaging of radiation-induced brain injury was graded as follows: Grade 1: change in focal white matter; focal contrast enhancement and surrounding edema; Grade 2: nonenhanced area or cystic lesion in enhanced lesion; Grade 3: focal necrosis with mass effect; and Grade 4: mass effect requiring surgical intervention. Radiation-induced brain injury appeared as early as 2 months and as late as 57 months after carbon ion therapy (mean interval, 22.2 months). MR findings of initial lesion were Grade 1 in 26 cases (65.0%), Grade 2 in 13 (32.5%), and Grade 3 in 1 (2.5%). Brain injury was always found in the radiation field initially, but cystic lesion and edema later extended outside the field in 10 cases (25.0%). In follow-up MR studies, size of edema or enhanced lesion was reduced in 17 patients (42.5%) without treatment. Two cases with large cystic lesions required surgery. Improvement of radiation-induced brain injury was observed more often than had been previously described. Because edema and cystic lesion can occasionally extend outside the radiation field, such findings do not exclude the possibility of radiation-induced brain injury. Careful observation is recommended because cystic lesions can enlarge enough to require surgical treatment in some cases.