Hepatic metastases are 18-40 times more common than primary liver tumours 6. Ultrasound, CT, and MRI are all useful for detection of hepatic metastases and evaluation across multiple postcontrast CT series, or MRI pulse sequences are necessary. The most common sites of primary malignancy that metastasize to the liver are 2:
- gastrointestinal tract (via portal circulation)
- breast cancer
- lung cancer
- genitourinary system
- essentially all metastatic solid malignancies
The demographics of patients with liver metastases will mirror that of the underlying primaries. Incidence therefore increases with increasing age.
Liver metastases are usually asymptomatic and found during work up of a malignancy which has presented in other ways. If hepatic metastatic burden is large then the presentation or symptoms related to the liver disease may include:
- localised pain and tenderness due to capsular stretching
- disordered liver metabolic function
- low grade fever 2
One of the main difficulties in liver imaging for metastatic disease is the high prevalence of benign liver lesions that can be misinterpreted as evidence of metastatic disease, thus dramatically changing a patients stage, and therefore treatment options. Liver haemangiomas, and to a lesser degree focal nodular hyperplasia (FNH), are the main sources of confusion 3. Additionally, pseudolesions (e.g. transient hepatic attenuation differences (THADs), focal fatty sparing / focal fatty change) may further muddy the waters. Therefore, an understanding of the various appearances of metastatic disease is crucial.
Routine greyscale ultrasound, contrast enhanced ultrasound, and intra-operative ultrasound all have roles to play.
Unfortunately, not only do metastases have a wide range of appearances, but background echogenicity changes of the liver due to fatty change make absolute statements difficult to make. In general, however, metastases may appear as 3:
- rounded and well defined
- positive mass effect with distortion of adjacent vessels
- hypoechoic: most common ~65% and is a concerning feature 8
- hypoechoic halo due to compressed and fat spared liver
- cystic, calcified, infiltrative and echogenic appearances are all possible: see liver metastases ultrasound appearances
Liver metastases are typically hypoattenuating on unenhanced CT, enhancing less than surrounding liver following contrast 1. If there is concomitant hepatic steatosis, then the lesions may be iso- or even slightly hyperattenuating. Enhancement is typically peripheral, and although there may be central filling in, on portal venous phase, delayed phase will show washout; helpful in distinguishing a metastasis from a haemangioma 1.
The appearance of liver metastases on MRI is also variable, but MRI is more sensitive than CT for the detection of liver metastases 5. MRI examination of the liver may involve numerous sequences (see liver MRI protocol), and choice of the gadolinium contrast agent (extracellular contrast agent or Eovist) is an important consideration.
Most frequent appearances are 5:
- T1: moderately hypointense
- T2: mildly to moderately hyperintense
T1 C+ (Gd): enhancement may be lesional or perilesional 7 (enhancement outside the confines of the T1 delineated lesion)
- small lesions (<1.5 cm) tend to uniformly enhance.
- larger lesions (>1.5 cm) tend to show transient rim enhancement (i.e. with wash-out); helpful feature in distinguishing a metastasis from a liver haemangioma.
- perilesional enhancement is most commonly seen in colorectal and pancreatic adenocarcinoma metastases 5.
T1 C+ (Eovist):
- Eovist is often useful for detection and confirmation of metastatic disease
- on the delayed phase, metastatic lesion do not retain any Eovist and essentially appear as "holes" in the liver
Fluid-fluid levels are considered a specific finding for neuroendocrine tumour metastases 9.
Treatment and prognosis
Hepatic metastases from colorectal adenocarcinoma can potentially be treated with hepatic metastasectomy, since they may be the only site of metastatic disease. Up to 20% of patients undergoing metastasectomy for this indication remain disease-free 10. Multiple staging systems for disease free survival after metastasectomy have been proposed and are being refined. One of the more frequently used systems (Clinical risk score (CRS), "Fong" score) includes variables such as: 11
- node-negative primary
- single hepatic metastasis
- hepatic metastasis size <5 cm
- CEA <200 ng/ml
- disease-free interval >1 year
These variables suggest a better metastasectomy disease-free survival.
Transarterial chemotherapy and radioembolization are other options for treatment for hepatic metastases.
General differential imaging considerations include:
- hepatic haemangioma: usually hyperechoic, geographic, no mass effect, discontinuous peripheral nodular enhancement and 'filling in' 1
- multiple hepatic cysts: no enhancement or mural nodules; can be hyper-dense/intsense due to blood or infection
- hepatocellular carcinoma (HCC): hypervascular, more often solitary, possible hypoechoic US halo, cirrhotic liver, vascular invasion
focal nodular hyperplasia (FNH)
- often younger patients
- central scar, with persistent delayed enhancement
- hepatic adenoma
- transient hepatic attenuation differences (THAD): often in cirrhosis
- multifocal fatty infiltration/focal fatty sparing: periligamentous, perivascular distribution; MRI in-out phase signal changes, vessels course through "lesions"; no mass effect
- multiple biliary hamartomas
- cholangiocarcinoma: delayed enhancement, capsular retraction
- multiple liver abscesses: possible CT cluster sign and right pleural effusion
- hepatic peliosis
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