In addition to neuropsychological testing, biomarkers may be a useful research and prognostic tool. Elevated levels of certain proteins or neurotransmitters in the blood or urine may be indicators of CNS damage caused by invasion of brain metastases and/or radiation induced damage. Much of the work on biomarkers for CNS injury has been done in stroke patients. These studies have identified multiple markers of blood brain barrier disruption and neuronal damage. The various categories include markers of endothelial damage, excitotoxicity, inflammation and angiogenesis (Table ).
Two serum markers that have potential as screening tools for endothelial and neuronal damage are neuron-specific enolase (NSE) and S100B. NSE is a glycolytic enzyme found in the CNS, which is expressed by neural and neuroendocrine cells [75
] and can be used as a marker of neuronal damage. Elevated levels have been found in patients with brain metastases from both small cell lung cancer and non-small cell lung cancer (NSCLC) [76
]. A multi-center retrospective study involving 231 NSCLC patients demonstrated that high serum levels of NSE indicated shorter survival and was a specific marker of metastases [77
S100B is a nervous system specific cytoplasmic protein found in astrocytes and is released into circulation when the blood brain barrier is breached [78
]. It is elevated in stroke patients and its levels have been shown to correspond to infarct volume [79
]. In a study looking at the presence of S100B in the serum of 38 patients with lung carcinoma, an elevated S100B level was either associated with brain metastases or with the presence of imaging changes suggestive of chronic, diffuse cerebral microvascular disease [80
]. S-100 levels have also been shown to be a predictive marker of melanoma brain metastases [81
Neuronal damage can lead to excitotoxicity where excess neurotransmitters such as glutamate and GABA are released. This increase in neurotransmitters causes an influx of Ca2+
leading to Ca2+
mediated cell death [82
]. Excitotoxicity is seen in traumatic brain injury, ischemic stroke and neurodegenerative diseases. In addition, glutamate and GABA have been measured in the blood of patients who have had a stroke [83
]. The release of neurotransmitters has never been studied in patients with brain metastasis or in patients with CNS damage caused by radiation but they also may be potential markers.
Radiation stimulates the inflammatory pathway and leads to the release of various cytokines, adhesion molecules and chemokines. Animal models have shown that radiation induced damage to the brain up regulates expression of TNF alpha, ICAM-1 and Il-1 [85
]. These inflammatory markers already have been detected in the blood of patients who received radiation [86
]. Radiation as well as CNS injury of any kind can cause release of these inflammation molecules. For example TNF alpha, ICAM and Il-1 all have been measured in the plasma of patients with stroke induced brain injury [87
]. These never have been measured in patients receiving WBRT but they may be potential markers of CNS damage.
Angiogenic proteins released by metastatic cancer cells also may be used to monitor disease status and assist in predicting recurrence. Angiogenic factors have been investigated as possible tumor markers in various malignancies [89
]. Vascular endothelial growth factor (VEGF) and matrix metalloproteinases (MMPs) have been shown to have prognostic value in various tumor types. A number of studies have demonstrated the role of VEGF and MMPs in breast [90
], lung [91
] and melanoma [93
] metastases, but none specifically have examined blood or urine levels in patients with brain metastases. MMPs are not only involved in tumor invasion but can also be a sign of CNS vascular injury as indicated by an increase in plasma levels of MMP9 and MMP13 in stroke patients [94
The NCI Radiation Oncology Branch protocol mentioned above that evaluates neuropsychological function also includes the collection of serum, plasma and urine specimens. The objective is to identify and evaluate the above biomarkers and to investigate the ability of these biomarkers to predict neuropsychological decline after WBRT and to predict progression of disease. The study will collect specimens before WBRT, at the completion of WBRT, and then at monthly intervals each coinciding with neuropsychological testing.