Hi Cai, in some cases (especially in the late stage), it may be difficult to differentiate from PML. In the early stage like this case, it would be unusual to see PML without significant FLAIR signal abnormality. The PML's restricted diffusion is more prominent at the edges whereas MTX toxicity's pattern is more evenly throughout. Another thing is location. MTX toxicity loves centrum seminovale, whereas PML likes to go to subcortical or periventricular white matter and subcortical u fibers, etc. When I was a trainee at Children's Hospital saw a case of MTX toxicity for the first time, I was confused about this pattern of "white matter acute infarction". My attending came in and right away asked if the patient was on intrathecal MTX. I was like, how did you know that? It's a pattern recognition thing. Thanks.

@@neuroradish Thank you so much for tutoring. Great teacher.