Location and growth patterns
Metastatic involvement of brain tissue is most frequently detected at the gray-white matter junction (grey/white matter junction). However, deposits can occur in absolutely any part of the central nervous system: the cerebral hemispheres, brainstem, cerebellum, and can also spread to the meninges. Intraventricular location also occurs—metastases can affect the lateral ventricles, ependyma, and choroid plexus.
An essential concomitant sign is vasogenic edema (vasogenic oedema). It spreads into the surrounding white matter and often appears disproportionately massive compared to the size of the tumor node itself. The definitive radiological diagnosis is typically established based on the patient's history of a known systemic malignant neoplasm and the detection of multiple lesions.
Basic imaging on CT and MRI
On non-contrast computed tomography (CT) scans, most lung and breast cancer deposits are isodense to normal brain parenchyma. Exceptions include certain types of tumors, notably malignant melanoma metastases, which can be spontaneously hyperdense.
The primary diagnostic method is MRI. On T2-weighted images (T2WI), the node itself may demonstrate low signal intensity or visually blend with the surrounding extensive edema. This creates a picture of massive edema without a clearly visible node, complicating the search without contrast administration.
Upon intravenous contrast administration, most lesions show intense enhancement. The enhancement pattern can be homogeneous or ring-like. The latter variant is typical for situations where a rapidly growing tumor outgrows its blood supply.
Specific types of metastases
In approximately 10% of cases, metastases are complicated by hemorrhages. Blood degradation products yield high signal intensity on T1WI, whereas on T2WI the signal can be either high or low, depending on the age of the hemorrhage.
Melanoma metastases deserve special attention. Even without hemorrhage, they can present a specific imaging pattern due to the paramagnetic properties of melanin. Melanin causes T2 shortening, which makes multiple lesions on axial T2WI (e.g., in the frontal cortex or subcortical parietal region) appear hypointense.
Another classic example is a cystic breast cancer metastasis. When localized, for instance, in the thalamus, it appears on post-contrast T1WI as a lesion with peripheral enhancement and pronounced central necrosis.
Advanced neuroimaging techniques
Small metastases are easily masked by edema. To increase MRI sensitivity, it is necessary to increase the contrast dose, use high-relaxivity gadolinium-based agents, and employ volumetric (volumetric) post-contrast T1 scanning. The use of magnetization transfer (magnetisation transfer) enables the visualization of nodes down to 1 mm in diameter.
Advanced MRI techniques have limited specificity but are useful in complex cases:
- Diffusion-weighted imaging (DWI): Metastases of small cell lung cancer and neuroendocrine deposits may show lower diffusion compared to well-differentiated adenocarcinoma.
- Differentiation from abscesses: This is critical for cystic and necrotic metastases. Abscesses contain viscous pus, and therefore show pronounced diffusion restriction (bright signal on trace-weighted DWI, dark on ADC maps / ADC). Necrosis inside a metastasis or glioma is characterized by free diffusion (dark signal on DWI, bright on ADC).
- Perfusion (PWI) and spectroscopy (MRS): Evaluation of the perilesional (perilesional) zone helps in the differential diagnosis between secondary deposits and primary gliomas.
Briefly: answers to topical questions
What are the indications and doses for stereotactic radiosurgery in brain metastases?
Stereotactic radiosurgery is indicated as a local treatment method for solitary metastases, but data on the doses used are missing from the sources.
Indications for radiosurgery include:
- Solitary metastases — used as an independent local treatment method.
- Stable asymptomatic melanoma metastases — used in combination with targeted therapy (BRAFi + MEKi combination).
- Single metastasis — used as preoperative preparation before surgical resection of the lesion.
As an alternative, postoperative stereotactic radiotherapy to the resection cavity can be used. The addition of whole-brain radiation therapy to local treatment improves intracranial control but does not affect overall survival.
For the differential diagnosis of a cystic metastasis and an abscess, remember diffusion: pus in an abscess is viscous — diffusion is restricted (bright on DWI, dark on ADC). Necrosis in a metastasis is fluid — diffusion is free (dark on DWI, bright on ADC).
Class and exam questions
Why are small metastases difficult to spot on standard non-contrast MRI?
They can be completely masked by extensive vasogenic edema or merge with it in signal intensity on T2-weighted images.
How to improve the detection of small lesions up to 1 mm in size?
It is necessary to use volumetric post-contrast T1-weighted imaging, increase the contrast dose, and use high-relaxivity gadolinium-based agents.
What is the imaging characteristic of melanoma metastases?
Melanin has paramagnetic properties, which causes signal shortening on T2-weighted images, making lesions hypointense even without hemorrhage.
What else to study on this topic
- Anatomical localization of intraventricular tumors
- Ependymoma: imaging features and pathways of spread within the ventricles
- Central neurocytoma and the risk of obstructive hydrocephalus
- Choroid plexus tumors: from papillomas to carcinomas
- MR spectroscopy and perfusion in the differential diagnosis of gliomas
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