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AJR Integrative Imaging LIFELONG LEARNING FOR RADIOLOGY |
1 Both authors: Department of Diagnostic Radiology, Mayo Clinic, 13400 Shea Blvd., Scottsdale, AZ 85259.
Received September 12, 2006; accepted after revision November 11, 2006.
Address correspondence to A. C. Silva.
Keywords: abdominal imaging • dynamic MRI • liver • oncologic imaging
Clinical History
A 67-year-old man presented with the new onset of generalized abdominal pain. CT of the abdomen revealed a 3-cm mass at the root of the mesentery, for which he underwent exploratory laparotomy. Tissue biopsy of the mass revealed high-grade gastrointestinal stromal tumor. The mass was unresectable because of encasement of the superior mesenteric vessels. Despite neoadjuvant chemotherapy, the patient developed liver metastases. His chemotherapy regimen was altered, and he subsequently underwent MRI of the abdomen to evaluate his response to treatment.
Radiologic Description
Unenhanced axial T1-weighted in-phase imaging (Fig. 1A) shows multiple lesions, including a 2.3-cm hypointense mass in segment VIII of the liver This lesion is hyperintense on the axial intermediate-echo T2-weighted image (Fig. 1B), which persists on the long-echo T2-weighted image (Fig. 1C). Steady-state free precession imaging (Fig. 1D) shows well-defined high signal intensity in the lesion equal to that of the left portal vein. After gadolinium contrast administration, this mass shows intense peripheral nodular enhancement in a continuous ringlike pattern during the arterial phase (Fig. 1E) and progressive centripetal enhancement on delayed images (Fig. 1F). Follow-up MRI revealed the decreased size of all lesions (Figs. 1G and 1H).
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The differential diagnosis includes hemangioma, focal nodular hyperplasia, adenoma, hepatocellular carcinoma, and hypervascular metastases.
Diagnosis
The diagnosis is chemotherapy-treated metastasis from gastrointestinal stromal tumor of the mesentery.
Commentary
Hemangiomas are common benign liver lesions that can have a variable appearance on MRI. However, a mass that maintains high signal intensity on a long-echo T2-weighted sequence and shows early peripheral nodular enhancement with centripetal progression and coalescence of nodules after contrast administration has been considered virtually pathognomonic for hemangioma [1–3]. Although this has been shown to be the most common pattern, small hemangiomas can display rapid intense homogeneous arterial enhancement, whereas large hemangiomas can exhibit persistent central hypointensity [3].
The discrimination of hemangiomas from hepatic metastases is a common clinical indication for MRI. Because most liver metastases are hypovascular, and thus hypointense on the portal venous phase, these lesions are easily differentiated from hemangiomas on contrast-enhanced studies [4]. On the other hand, distinguishing hypervascular metastases from the early-enhancing type of hemangioma can be problematic because both types of lesions are rapidly enhancing on arterial phase contrast-enhanced images. However, on the portal venous and delayed phases, hypervascular metastases typically show early washout, becoming isointense or hypointense to the surrounding liver parenchyma. In addition, the peripheral washout of contrast material on serial delayed contrast-enhanced images (target lesion) has been shown to be highly specific for hypervascular metastases [5]. Thus, the portal venous and delayed phase retention of contrast material by hemangiomas is key to differentiating them from other hypervascular masses.
In regard to T2-weighted sequences, the markedly hyperintense signal of hemangiomas typically exceeds that of most metastases, which are more heterogeneous and intermediate in signal. The use of heavily T2-weighted [6] and steady-state free precession [7] sequences will help improve differentiation. However, a number of different liver metastases have been reported to have high T2 signal, which is thought to relate to the increased fluid content of these tumors. These lesions include leiomyosarcoma, carcinoid, melanoma, renal cell cancer, insulinoma, glucagonoma, and hepatocellular carcinoma [2, 8–10]. Thus, the T2-weighted imaging characteristics alone cannot reliably be used to differentiate hypervascular metastases from hemangioma.
Another problematic issue in the differentiation of hemangiomas from other hepatic masses occurs after treatment with chemotherapy. The neovascularity of the primary tumor and its metastases is the basis for differentiating an underlying mass from normal background organ parenchyma on contrast-enhanced MRI. For example, the ring-enhancing pattern frequently seen in metastatic lesions is attributable to the increased density and permeability of neovessels at the tumor periphery [11]. After initiation of chemotherapy, metastases can exhibit a less aggressive pattern, mimicking hemangiomas, including early peripheral nodular enhancement and delayed retention of contrast material. This has been postulated to be a result of chemotherapy-induced antiangiogenesis, with the early peripheral nodular enhancement due to altered vascularity, and the retention of contrast material on the 10-minute contrast-enhanced images reflecting an enlarged extracellular space or decreased venous drainage [12]. In a previous study describing chemotherapy-treated metastases mimicking hemangiomas, none of the metastases had the classic appearance of rounded lobules of contrast enhancement in a dis continuous ring with gradual coalescence [12]. Thus, although variations in the morphology and the rate of progression of enhancement in hemangiomas can occur, a nodular, but continuous, rim enhancement should favor the diagnosis of treated metastasis.
With regard to the other differential considerations, these can be excluded in this patient for the following reasons. Focal nodular hyperplasia is usually isointense, or nearly isointense, on T1- and T2-weighted imaging, except for a T2 hyperintense central scar. On delayed contrast-enhanced images, focal nodular hyperplasia will exhibit washout, except for a late enhancing scar, visible in up to 78% of cases undergoing MR [13, 14]. Adenomas and hepatocellular carcinoma (HCC) can show variable T1 and T2 signals, depending on the amount of fat, hemorrhage, or cellular atypia present. However, these two lesions do not typically exhibit the degree of increased T2 signal present in this patient. In addition, both adenomas and HCCs also will classically show washout on delayed contrast-enhanced images, with a late enhancing peripheral rim (capsule) [15, 16]. The lack of underlying chronic liver disease also makes HCC less likely. Both the early-enhancing type of hemangioma and hypervascular metastases can present as hypervascular masses with well-defined, persistent increased signal on heavily T2-weighted and steady-state free precession sequences. Typically, the later contrast-enhanced images are the key sequences in differentiating a persistently enhancing hemangioma from the contrast washout of hypervascular metastases. However, it is the intact, continuous rim of enhancement on the arterial phase images that suggests the correct diagnosis of metastases. Table 1 describes the characteristic MR signal and enhancement patterns of common focal liver lesions.
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Clinical history and prior imaging often are critical in the discrimination of these lesions. In this case, the lesions were not present on the patient's initial diagnostic CT examination, but grew in size over the course of the year until the chemotherapy regimen was altered, at which time the lesions began to regress in size and number, indicating a favorable response to treatment.
Objective
The educational objective of this article is to emphasize potential imaging similarities of chemotherapy-treated liver metastases and hemangiomas.
Conclusion
Liver metastases treated with chemotherapy can show MRI characteristics similar to those seen in benign hemangiomas. One discriminating feature of treated metastases is a nodular, but continuous, rim of enhancement in the arterial phase, as opposed to the discontinuous peripheral nodules of enhancement typically seen in hemangiomas. A history of chemotherapy should be sought in patients undergoing evaluation for liver metastases to avoid inaccurate staging and ensure appropriate treatment planning.
References
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  C.-H. Liu, D. C. Rivard, A. C. Silva, K. J. Burkholz, and S. D. Beaty Imaging of Focal Hepatic Lesions: Self-Assessment Module Am. J. Roentgenol., June 1, 2008; 190(6_Supplement): S65 - S68. [Abstract] [Full Text] [PDF]
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