Hepatic metastasis disguised as fat spared area in the background of fatty liver: Detection on FDG PET/CT.

Area of fat sparing in fatty liver is known to pose a diagnostic challenge in an oncological setting, especially in cancers with higher propensity for liver metastases. We report an unusual appearance of hepatic metastases in a fat spared area, on both computed tomography (CT) and positron emission tomography (PET), in a combined 18 fluorine-fluorodeoxyglucose (18 F-FDG) PET/CT study done in metastatic adenocarcinoma of colon.

INTRODUCTION

Heterogenous nature of FDG uptake often poses a challenge in picking up diffuse metastatic disease, more so, when associated with fatty liver. We report a case of metastatic liver disease which apparently appeared normal, in background of fatty liver.

CASE REPORT

A 65-year-old gentleman was diagnosed with carcinoma of sigmoid colon 2 years back, for which he underwent sigmoid colectomy. Histopathological analysis revealed moderately differentiated adenocarcinoma. Patient then received adjuvant chemotherapy. Serial carcino-embryogenic antigen (CEA) levels and follow-up imaging were normal until 1 year, when metastatic lesion was seen in segment VII of liver. Patient underwent transarterial chemo-embolization (TACE) and radiofrequency ablation (RFA), which led to complete regression of liver lesion. Follow-up was uneventful for 6 months, after which a rising trend was seen in CEA levels. Whole body 18 F — FDG PET/CT study was done. Maximum intensity projection (MIP) image showed diffuse tracer uptake in left side of liver [Figure 1] – arrow] and a focus of abdominal uptake [Figure 1– arrowhead] in midline. Axial venous phase CT images showed fatty infiltration in entire right lobe of liver [Figure 2a — thick arrow], with associated post-treatment changes at the site of treated liver lesion. Left branch of portal vein was patent, with no evidence of tumor infiltration. Axial fused PET/CT image showed no tracer uptake in the right lobe. Diffuse pattern of intense tracer uptake was seen in a wedge shaped area in the left lobe of the liver [Figure 2b — arrow] which, however, showed normal enhancement on CT images [Figure 2a — arrow]. Maximum standardized uptake value (SUVmax in g/ml) was 17.3. No focal hypodense lesion was seen. Midline abdominal uptake corresponded to the abdominal wall metastatic nodule [Figure 2c and d — arrow]. Though FDG uptake pattern in liver favored metastasis in the background of rising CEA, the anatomical picture was not supportive. Hence, CT guided biopsy was performed, which was confirmatory for metastases from adenocarcinoma. Patient was started on alternative chemotherapy regimen, and follow-up PET/CT was done in 3 months. MIP images showed persistent hypermetabolism in the left lobe metastases [Figure 3 — arrow], with mild reduction seen in metabolic activity of abdominal wall deposit [Figure 3 — arrowhead]. Axial CT and fused PET/CT images showed no change in metabolic activity and extent of metastatic site in the left lobe of liver, max SUV being 16.7 [Figure 4a, b - arrows]. There was a mild reduction in metabolic activity of abdominal wall nodule, with size remaining unchanged, as seen on the axial CT [Figure 4c — arrow] and fused PET/CT [Figure 4d — arrow] images. In view of stable disease, with no new metastatic site, patient is being planned for left hepatectomy and abdominal nodule excision [Event chronology - Table 1].

Figure 1

Maximum intensity projection image showing diffuse hepatic tracer uptake (arrow) with focal uptake (arrow-head) corresponding to abdominal wall nodule

Figure 2

(a) Axial computed tomography (CT) image showing large area of fatty liver (thick arrow) with normally enhancing rest of the liver (arrow), (b) Fused axial positron emission tomography (PET)/CT image showing diffuse tracer uptake (arrow) in the area with normal enhancement on CT (a – arrow). Axial CT (c – arrow) and fused PET/CT images (d – arrow) showing ill-defined anterior abdominal wall nodule showing fluorodeoxyglucose uptake

Figure 3

Post-treatment status, maximum intensity projection image showing persistent diffuse fluorodeoxyglucose uptake in the left lobe of liver (arrow) with a mild reduction in tracer uptake in abdominal wall nodule (arrow-head, compared to Figure 1)

Figure 4

Axial CT (a) and fused PET/CT (b) images showing no change in size, metabolic activity, and enhancement pattern in metastatic lesion in the left lobe of liver (arrows), compared to Figure 2a and b. Axial CT (a) and fused PET/CT (b) images shows ill-defined nodule in anterior abdominal wall, showing a mild reduction in metabolic activity, with no change in size, compared to Figure 2c and d

Table 1

Chronology of disease – investigations done

DISCUSSION

Focal fat sparing in the presence of diffuse fatty liver is most commonly seen around the gall bladder fossa and appears as a spot or band on cross-sectional imaging.[1] However, it can also occur in other parts of the liver and in various shapes, such as a wedge shaped area, as seen in our case.[2] These patterns have been reported to mimic metastases on cross-sectional imaging.[34] Contrarily, a true hepatic lesions simulate fat sparing, is documented in a single case, where US and magnetic resonance imaging (MRI) was used for confirmation.[5] Fatty liver is seen as diffuse hypodensity on CT images, with attenuation values less than 40 Hounsfield units (HU) on unenhanced images.[6] As a result, there is an apparent hyper-attenuation in the fat spared region of liver. Similar finding was seen on the CT component of PET/CT images in our case. Also, intense tracer uptake seen in the fat spared area. However, the interesting fact was the pattern of FDG uptake — diffuse with involvement of entire left lobe with no focality. This was quite unlike, a metastatic liver lesion,[7] more so when the previous metastatic site in segment VII, which was subsequently treated, showed a characteristic rounded appearance. Both these findings — apparent hyper-attenuation on CT in absence of focality and diffuse homogenous tracer uptake pattern on PET in the fat spared area of liver, when seen together can be a misleading, and metastatic lesion can be ‘missed’. The clinching factor here is the intensity of FDG uptake, depicted by SUVmax value, which was high in our case. Since such a high grade of FDG uptake is characteristic of adenocarcinoma metastases,[8] the lesion most likely favored metastatic involvement. This was subsequently confirmed on histology and follow up PET/CT study. Such pattern of liver morphology and metabolism in the presence of fatty liver, is a “red herring” on FDG PET/CT imaging and needs to be kept in mind. Correlation with tumor histology and intensity of radiotracer concentration serve as useful tools in reaching proper diagnosis in these cases.