Brain metastases are a common complication of cancer and the most common type of brain tumor. Anywhere from 10% to 26% of patients who die from their cancer will develop brain metastases (Nayak 2012). While few cancers that metastasize to the brain can be cured using conventional therapies, long-term survival and palliation are possible with minimal adverse effects to patients. Increasingly, neuro-cognition and quality of life are being recognized as an important endpoint for patients as survival continues to increase.
Primary cancers such as lung, breast, and melanoma are most likely to metastasize to the brain. Small-cell lung cancer has a high propensity to spread to the brain such that prophylactic treatment (cranial irradiation) is considered the standard of care. Other malignancies such as prostate and head and neck cancers rarely result in brain metastases. It can be difficult to predict which patients will develop brain metastases other than by using tumor type and subtype.
Brain metastases are the most common type of intracranial tumor. In the United States, an estimated 98,000 to 170,000 cases occur each year. The incidence of brain metastases is increasingly likely as a result of several factors (Smedby 2009). Patients with a systemic metastatic disease have a longer survival with new systemic therapies (including immunotherapy) that have recently seen more widespread use. Furthermore, the growing use of sensitive MRI techniques has contributed to better detection of small asymptomatic brain metastases.
Metastatic cancer passes through the bloodstream and enters the central nervous system through a breakdown of the blood-brain barrier. Clonal cells then proliferate, causing local invasion, displacement, inflammation, and edema. Distribution throughout the central nervous system is more common in areas of high blood flow; however, different histological subtypes tend to have different distributions of location within the brain (Quattrochi 2012).
A detailed history and physical should be performed, focusing on symptoms, duration, and intensity. Focused questions about headaches, blurry vision, and nausea should be asked. A complete neurologic examination should be performed. This examination should include assessment of strength, sensation, coordination, reflexes, cerebellar function, proprioception, cranial nerve function, speech, thought, vision, and memory. An ophthalmic examination should be performed to evaluate for papilledema. Additional information including age, performance status, and status of systemic cancer burden should be gathered to understand the disease course and guide future therapeutic intervention.
A head CT allows for a quick examination, although fine-slice MR of the brain with contrast is the gold standard for neuroimaging in cases of suspected brain metastases. MR allows for a determination of the number and anatomical location of tumors and degree of associated edema. Basic laboratory assessment including CBC, metabolic panel, and liver function test should be performed.
The first step in the management of newly diagnosed brain metastases is the treatment of intracranial edema. Oral or intravenous steroids (such as dexamethasone) are commonly used. A loading dose of 10 mg intravenous (IV) dexamethasone followed by 4 mg IV every six hours is one dosing regimen. After the initial clinical response, which can occur rapidly, the dose may be tapered to avoid many of the adverse effects of long-term high dose steroid administration.
Following initiation of steroids, definitive management may be initiated. Treatment options include surgical resection (for limited brain metastases in patients with good performance status and surgically accessible lesions), whole brain radiotherapy, and stereotactic radiosurgery. Whole brain radiotherapy is given by daily radiotherapy treatments (usual 10 to 15) targeting the whole brain. Radiosurgery is a more precise form of radiotherapy which delivers a large dose only to the area of the brain metastasis, usually in a single fraction. Each of these treatments has distinct advantages as well as a unique side effect profile. A multidisciplinary treatment team of a neurosurgeon, radiation oncologist, and neuro-oncologist should participate in the formulation of the treatment plan together with the patient.
The historical standard in patients with good performance status has been surgical resection. Local recurrence following surgical resection remains high, with one trial recently reporting a 12-month freedom from local recurrence of 43% following surgical resection and observation (Mahajan 2017). Local control can be improved with post-operative radiosurgery or whole brain radiotherapy (Mahajan 2017, Brown 2017). Postoperative therapy should remain an individualized treatment recommendation, taking into account the number of non-resected metastases, tumor histology, follow up, and patient preference. Whole brain radiotherapy following surgical resection of brain metastases can increase intracranial control compared to postoperative stereotactic radiosurgery but results in poorer neuro-cognitive outcomes (Brown 2017).
For patients either not eligible for surgical resection of brain metastases or who elect for non-surgical therapy, stereotactic radiosurgery offers an excellent option for controlling a limited number of intracranial metastases. Although first used in combination with whole brain radiotherapy as a way to intensify local treatment (Andrews 2004), stereotactic radiosurgery is now commonly used as a stand-alone therapy. While ultimate control of brain metastases varies with dose and lesion size (Amsbaugh 2016), lesions less than one centimeter have high local control with single fraction radiosurgery (Chang 2003). For larger lesions, multi-fraction treatments are sometimes employed (Kirkpatrick 2017). Stereotactic radiosurgery is considered standard for patients with one to four brain metastases, but emerging data indicate it may be an acceptable treatment for patients with up to ten brain metastases (Yamamoto 2016).
For patients with poor performance status or many brain metastases, the standard of care is whole brain radiotherapy. Whole brain radiotherapy provides control of individual brain metastases as well as reduces the risk of failure in the brain at a new site. These benefits must be weighed against its potential neurocognitive side effects which occur for many patients to varying degree. Emerging data suggest that for patients with extremely poor performance status, whole brain radiotherapy may have a minimal benefit over steroids alone (Mulvenna 2016). Therefore, in the management of brain metastases, treatment decision will need to be made on an individual patient level, taking in to account the goals of treatment in a particular situation as well as the acceptable side effect profile.