Dr. Sawaya’s review provides a comprehensive overview and excellent summary of current knowledge on an important topic. Brain metastases far outnumber primary brain tumors; their diagnosis and treatment are bread-and-butter for both medical and neurosurgical oncologists. Additionally, unlike primary brain tumors, a definable at-risk population exists for brain metastases, making prevention a potentially achievable goal.
As Dr. Sawaya points out, however, many brain metastases are asymptomatic. Diagnosis and treatment before symptoms develop are important. When brain imaging is included in a "staging" work-up, magnetic resonance imaging (MRI) with and without gadolinium should be used rather than computed tomography (CT) because of its greater sensitivity. Most asymptomatic metastases are less than 1 cm in diameter, and many are only a few millimeters in size. Although MRI with "triple-dose" gadolinium is considered by some to be even more sensitive than standard contrast imaging, we have used standard contrast MRI with careful attention to the imaging sequences that highlight edema around small metastases. The fluid-attenuated inversion recovery (FLAIR) sequence is probably best for this purpose.
When a metastasis has a ring-enhancing appearance with surrounding edema, brain abscess is an important differential consideration. The characteristic bright appearance of abscesses on diffusion-weighted imaging is helpful in this regard, because the necrotic centers of most ring-enhancing metastases will show low or intermediate signal intensity.
We find that most brain metastases can be confidently diagnosed without obtaining tissue. In some situations, such as a single brain lesion with no known systemic cancer or with a chest primary of unknown histology that cannot be biopsied without thoracotomy, biopsy of a brain metastasis is necessary for diagnosis. Interstitial brachytherapy can be performed at the same sitting in this situation, using a probe introduced stereotactically into the center of the tumor to deliver in a few minutes a therapeutic single-fraction treatment equivalent to radiosurgery.
As Dr. Sawaya notes, corticosteroids are indicated for most patients with brain metastases, especially those with neurologic symptoms. An exception occurs when brain lymphoma is a diagnostic consideration, because even a brief course of steroid treatment can cause complete regression of these lesions, thus thwarting biopsy.