FREQUENT NON-PARENCHYMAL METASTASES

Bone metastases

Bone metastases are the most common non-parenchymal metastases. The three most common primary cancers are breast, lung and prostate.

Depending on the anatomy, skull-base metastases may present as five clinical syndromes (Table 2 and Figure 1). The most common mechanism is haematogenous spread, and another possible mechanism is retrograde venous spread via the valveless Batson's venous plexus.

Key imaging:

Bone metastases may present as diffuse bone involvement or focal lesions, where the hyperintense signal from fatty marrow on T1WI sequences without fat suppression is replaced by a hypointense lesion (Figure 2, 3, 4).

Lytic bone metastases show restriction in the DWI (Figure 3, 4), especially of breast and lung cancer origin, but not useful for prostate bone metastases. Bone metastases show variable contrast enhancement. In addition, post-contrast sequences allow the assessment of whether there is associated dural invasion (Figure 5).

Tips:

• Use fat saturation pulses when obtaining post-contrast T1WI (Figure 3).
• The use of subtraction sequences is helpful (Figure 3).

Leptomeningeal carcinomatosis (LMC)

LMC is a rare but serious complication that occur in about 5-8% patients with solid tumours. It is caused by the spread of tumour cells into the leptomeninges and subarachnoid space. The most common primary is breast cancer, followed by lung cancer and melanoma.

Multifocal clinical deterioration is highly suggestive of LMC, but patients may present with isolated or subtle neurological symptoms or signs and even have a normal neurological examination.

Key imaging:

It is recommended to perform an MRI before conducting a lumbar puncture, if possible, as the procedure can lead to dural enhancement.

MRI findings may include linear or nodular enhancement of the cerebellar folia and cerebral sulci, enhancement of the basal cisterns, and enhancement of the cranial nerves, brain surface or nerve roots (Figure 6). Note that MRI may be normal. MRI findings are not specific, so their interpretation must be made in the specific clinical context.

MRI findings can be divided into five subtypes (Figure 7). EANO-ESMO proposes a classification based on clinical, MRI and CSF cytology, which has prognostic value and guides treatment decisions.

Tip: Using post-contrast FLAIR and black blood sequences can enhance the sensitivity for detecting LMC (Figure 8).

RARE NON-PARENCHYMAL METASTASES

Dural metastases

They are very rare (1% of intracranial tumours) and can occur through direct spread from a bone metastasis, via haematogenous spread, or in a small percentage by intraoperative dissemination (Figure 9). The most common primaries are breast, prostate and lung.

MRI findings:

• Diffuse dural thickening and enhancement (Figure 10), which can appear smooth or nodular.
• Focal dural mass mimicking meningioma. In these cases, it is not easy to differentiate between the two entities on conventional MRI.

Tips:

• PET imaging with gallium-68-DOTATATE (Figure 11): Meningiomas usually show high uptake because they contain somatostatin receptors. However, caution is advised, as metastases from neuroendocrine tumours also show high uptake (Figure 12).
• Post-contrast FLAIR (Figure 11): Meningiomas are typically characterised by a complete ring or >50% enhancement on the post-contrast FLAIR sequence ('CE-FLAIR rim sign').

Pituitary metastases

Pituitary metastases are rare, comprising 0.4% of intracranial metastases and about 1-3.6% of operated pituitary lesions. The most common primaries are the breast, lung, kidney and prostate.

Most are asymptomatic and are discovered incidentally on imaging. The most common clinical presentation is diabetes insipidus (DI) and panhypopituitarism, reflecting a predilection for involvement of the neurohypophysis and pituitary stalk.

MRI:

• T1 and T2 signal is variable.
• They usually enhance avidly, although they may be heterogeneous, unlike adenomas.
• They may present as dumbbell-shaped pituitary/pituitary stalk lesions with loss of neurohypophyseal signal hyperintensity (Figure 13, 14).

Tip:

• It is challenging to differentiate pituitary metastases from other lesions on imaging alone.
• The presence of a pituitary lesion with DI is suggestive of metastasis.
• The presence of a rapidly growing pituitary mass in a patient with a known carcinoma.
• Hypothalamic/infundibular involvement supports pituitary metastasis (Figure 13, 14).

Pineal gland metastases

They occur in 0.4-3.8% of cancer patients, with the most common primaries are the lung and breast. Most pineal metastases are asymptomatic, although they may clinically present with headache, encephalopathy, and hydrocephalus (Figure 14). Identifying pineal metastases is crucial, as they can lead to leptomeningeal dissemination, which occurs in up to 67% of cases (Figure 15).

Tips:

• Like pituitary metastases, they are not easy to differentiate from other lesions.
• Suspect when there are new pineal lesions in patients with lung and breast cancer.

Ventricular/choroid plexus metastases

Accounts for <1% of intracranial neoplasms. The most common primaries are kidney, lung, colon and melanoma. It is usually asymptomatic. The clinical presentation is variable due to increased intracranial pressure secondary to CSF flow or obstructive hydrocephalus. The most common location is the lateral ventricle.

On MRI (Figure 16), they appear as T1 hypointense and T2 hyperintense tumours with intense contrast enhancement, which may be homogeneous or heterogeneous. When they become large, they may be associated with vasogenic oedema or infiltration of adjacent structures.

It is challenging to differentiate diagnosis from other lesions (papillomas or ventricular meningiomas). The most crucial point is to have a previous history to know that these are patients with underlying cancer and cases of rapidly growing ventricular masses.

EXTRACRANIAL METASTASES

Orbital metastases

Orbital metastases are rare, accounting for 1-13%. The most common route of spread to the orbit is by haematogenous spread. Among the most common primaries, breast cancer is the most common (almost half of the cases), followed by prostate, melanoma and lung.

Clinical manifestations may include proptosis, ocular motility disorders, pain, diplopia and decreased vision. A rare sign is paradoxical enophthalmos, which occurs in 10% of orbital metastases of scirrhous breast carcinoma.

There are various imaging patterns (Figure 17):

• Diffuse infiltrative lesions with infiltration of fat and muscle.
• Focal intra- and/or extraconal orbital masses
• Bone metastases
• Involvement of the extraocular orbital musculature.

These lesions show diffusion restriction and contrast enhancement.

Choroidal metastases are the most common ocular metastases. They present as well-defined, isointense T1, hypointense T2, and contrast-enhancing choroidal lesions/masses. Exophytic growth into the vitreous can cause retinal/choroidal detachment. The main differential diagnosis is uveal melanoma, which would be hyperintense on T1, except for haemorrhagic metastases of mucinous adenocarcinoma.

Muscular metastases

Most are asymptomatic and go unnoticed clinically and radiologically. The most common primary tumours are lung cancer, gastrointestinal tumours, renal cancer, and breast cancer. Radiologically, a muscle metastasis is indistinguishable from a primary tumour (Figure 18).

Soft tissue metastases (cutaneous/subcutaneous)

Cutaneous and subcutaneous metastases are rare, accounting for approximately 0.7-10% of cases, but usually indicate widespread metastatic disease and a poor prognosis.

Primary sites vary according to gender, with the most common sites in women being breast, lung, gynaecological and gastrointestinal, in decreasing order, and in men being lung and gastrointestinal.

They may present as generally small nodules (<2 cm) or infiltrative masses with homogeneous or heterogeneous enhancement (Figure 19). Imaging is not specific but must be considered in the context of a history of systemic cancer, and biopsy is mandatory in cases where there is no known primary tumour.

Intramedullary metastases in the spinal cord

Intramedullary metastases are rare, but their incidence is increasing. The most common primary site is the lung, followed by the breast.

They are often underdiagnosed because they occur at late stages of the disease; the most common site is the thorax, but up to 25% of metastases may occur in the cervical region.  

MRI findings:

• Almost all metastases usually appear as focal enhancing lesions associated with extensive longitudinal edema (Figure 20, 21). The presence of intratumoral haemorrhage and intra- or peritumoral cystic/necrotic changes is uncommon and should lead us to consider other diagnostic possibilities.
• Two specific signs of intramedullary metastases from non-CNS tumours have been described: the rim and flame signs (figure 21). The rim sign consists of a ring of more intense enhancement at the lesion's periphery, and the flame sign is described as an ill-defined flame-shaped enhancement at the superior/inferior margin.