Under what circumstances do you consider a biopsy for a patient with a new brain lesion who is undergoing active systemic cancer treatment?

Indications for biopsy would be rare:

First - if the brain lesion is dural-based and slow-growing, and the patient has excellent systemic control.

Second - if the brain lesions are progressing despite standard radiation therapy.

Third - if control of CNS disease is being done using CNS-penetrating t...