Systemic Sarcoidosis with Neurosarcoidosis: The patient's presentation of a rim-enhancing mass with minimal edema and no significant mass effect, along with multisystem involvement (steroid-responsive colitis, skin and joint symptoms, hilar lymphadenopathy), and rapid improvement on low-dose steroids strongly supports this diagnosis.

Tumefactive Demyelination: Given the atypical features for GBM (minimal edema, no significant mass effect, and multiple lesions), tumefactive demyelination is a consideration, especially with the presence of enhancing lesions on MRI.
Inflammatory Pseudotumor: This condition can mimic tumors and presents with inflammation; the patient's response to steroids also supports an inflammatory process.

Glioblastoma Multiforme (GBM): Although the presentation is atypical, GBM remains a critical diagnosis to rule out due to its aggressive nature and the need for prompt treatment. Tissue diagnosis is essential to differentiate it from other conditions.
Lymphoma: Central nervous system lymphoma can present with similar imaging findings and requires consideration, especially in the context of systemic symptoms and lymphadenopathy.
Metastatic Disease: Despite the lack of primary malignancy found on initial CT scans, metastasis to the brain from an occult primary tumor is a possibility that should not be overlooked.

Granulomatosis with Polyangiitis (GPA): This condition can present with granulomatous inflammation and systemic symptoms, including neurological involvement, but it is less likely given the specific pattern of organ involvement and the absence of typical GPA symptoms like upper respiratory tract or renal involvement.
NeuroBehçet's Disease: This rare condition involves the central nervous system and can present with a variety of neurological symptoms, including mass lesions, but it is less likely given the lack of other typical Behçet's symptoms such as oral and genital ulcers.