Hi and welcome,
I'm actually having difficulty with your question, a brain tumor is an abnormal mass of tissue which may be solid or fluid-filled that are either benign which is generally harmless or malignant which is cancerous. Brain lesions refers to any type of abnormal tissue in or on brain tissue, when used in reference to MS though a lesion usually means scar, Multiple Sclerosis literally means multiple scars, MS lesions also show up as bright areas or spots on T2 MRI's..
Typically a single MS brain lesion (scar) would be distinguishable from a tumor (mass) but demyelinating diseases like MS can actually cause tumor-like (tumefactive) demyelinating lesions, tumefactive demyalinating lesions (TDL) are rarely seen and cause diagnostic challenges. Tumefactive MS is one of the 'rare' variants of MS and there atypical imaging and clinical features may cause misdiagnosis of tumor or abscess...
"The clinical presentation of patients with TDL is variable and atypical for demyelinating disease due to the differences in size and location of the lesion. In this article, we aim to explore TDL comprehensively and provide an evidence-based approach for diagnosis and treatment. This will result in recommendations that may improve the diagnostic accuracy and treatment outcomes. Detailed history, physical examination, and several MRI imaging can spare patients the need for a brain biopsy." https://www.ncbi.nlm.nih.gov/pubmed/28619436
There are challenges in diagnosing Tumefactive MS because of it's rarity, the multiple MRI's and LP you've had so far would be typical in trying to determine if you do actually have MS or not, unfortunately this is the type of MS that is hardly seen so it may take some time to work out, hang in there!!
Hope that helps a little......JJ