CNS metastasis

CNS metastasis is the spread and proliferation of cancer cells from their original tumour to form secondary tumours in portions of the CNS. Typically, this progression begins when tumour cells separate from the primary tumour and enter the bloodstream or the lymph system via intravasation. Intravasation into the circulatory system allows tumour cells to travel and colonise distant sites, such as the brain, a major structure of the CNS, forming a secondary brain tumour. Subsequently, metastasised cells assume new genomic phenotypes while dropping unfavourable characteristics once they disassociate from the primary lesion. This is particularly crucial for the formation of CNS metastasis, as tumour cells require characteristics that favour the disruption of the blood-brain barrier, allowing them to transverse. Recent evidence demonstrates that the dissemination of cells from the primary tumour is not sequential but consists of overlapping processes and routes. Up to 30% of adult cancer cases harbour CNS metastasis, although this statistic is reportedly underdiagnosed because of the fallibility of medical diagnostic methods. Since most cancers can progress towards CNS metastasis despite multimodal treatments, it is a significant risk for patients with systemic cancer. == Symptoms ==

Metastasis occurrence indicates stage 4 cancer progression and carries a poor prognosis. Clinically, CNS metastasis is known to cause haemorrhage or obstruction in the cranial portion of the CNS, leading to hydrocephalus. Generally, other symptoms include cystic degeneration, necrosis, and CNS haemorrhage, commonly within the brain. These conditions lead to the long-term degradation of neurocognition, speech, coordination, and behaviour, thereby altering patients' quality of life. == Risk factors ==

Since CNS metastasis is the pathway of the natural progression of primary cancers, hence, main risk factors include modifiers of cancer risk. These modifiers include the accumulation of genetic, epigenetic, and environmental factors resulting in chromosomal and genomic aberrations and instability. Research has demonstrated that 80-90% of malignant tumours are caused by external environmental factors such as carcinogens. Clinically, research evidence demonstrated that the primary tumours that have the greatest association with brain metastasis consist of lung, breast, melanoma, and colon cancers. Despite knowledge of the sources, there is a lack of understanding of why these sources have increased predilection, nor is there an understanding of the differences in mechanisms behind each metastasis process. == Diagnosis ==

CNS metastases can be diagnosed through various imaging approaches and clinical manifestations. These techniques allow doctors to detect abnormalities and identify the location and extent of the metastatic spread. Magnetic resonance imaging (MRI) and computed tomography (CT) are two representative imaging procedures for this purpose. MRI scans use strong magnetic fields and radio waves to create an image, while CT scans use X-rays. MRI scans produce more detailed images of bodily structures, particularly soft tissues, including the brain, Alternative techniques When a lesion is suspected of having CNS metastases and its primary site is unknown, additional imaging and biopsies may be necessary for an accurate diagnosis. Biopsies involve the surgical removal of the suspected tissue, but can be invasive. They warrant a thorough evaluation of their necessity and the patient's ability to withstand the side effects. A less intrusive alternative imaging technique is magnetic resonance spectroscopy (MRS), which is used to determine the chemical composition of cells. However, it is not as reliable as biopsies. These techniques are also relevant if a singular metastasis site is inadequate to explain the patient's symptoms. In this case, additional screenings would be warranted to locate the other lesions and the tumour source. With this information, doctors aim to determine the metastasis lineage and accurately identify the underlying cancer. Modern clinical screening allows the detection of numerous serum levels of circulating tumour cells. However, a disproportionate amount of metastasis is still undetectable, causing under-diagnosis. == Therapeutic methods ==

The best treatment approach for patients depends on a comprehensive assessment of several factors, including the primary cancer type, tumour location, prognosis, and patient preference, among others. Some of the main treatment methods are surgery, radiotherapy, chemotherapy, immunotherapy, and other system-targeting therapies. If cancer recurs or progresses, the therapeutic methods are adjusted, and varying combinations of all available options are explored. Coping with successive disease progression can be challenging due to the taxing side effects, which can take a physical and mental toll on patients. Consequently, the prognosis for further attempts may not be as promising as it was initially. == Recent development ==

Diagnostic techniques for CNS metastasis are a major area of ongoing research, as detecting metastatic lesions early is crucial for timely treatment and better patient outcomes. Biomarkers need to be uniquely representative of CNS metastasis. Otherwise, there could be high incidences of false-positive results, rendering the method less precise. Both methods require a better understanding of the molecular determinants of CNS metastasis. Knowing these biomolecular factors could also lead to the development of preventative methods, a heavily underdeveloped area in CNS metastasis. == References ==