For the past several weeks, a 54-year-old man has had progressively worsening abdominal pain and general achiness. He has no nausea, vomiting, diarrhea, or fever and no history of trauma.
Sixteen months earlier, melanoma in the left antecubital fossa was diagnosed. When the lesion was resected, the sentinel node in the left axilla was positive, but the remainder of the lymph node basin was negative. No additional therapy was given.
The patient has well-controlled hypertension. He also has a 26 pack-year smoking history and drinks alcohol occasionally.
This thin man is in no acute distress. Temperature is 37.2°C (99°F); heart rate, 82 beats per minute; respiration rate, 17 breaths per minute; and blood pressure, 124/82 mm Hg. Head, neck, heart, and lungs are normal. There is reproducible focal tenderness in the midabdomen above the level of the umbilicus; however, no palpable masses are identified, and bowel sounds are normal.
You order a CT scan of the abdomen to evaluate the focal tenderness. An axial image at the level of the dome of the liver shows heterogeneity (Figure 1A); this appears to be related to the timing of the intravenous contrast bolus. A subcutaneous lesion is identified in the left posterior region; the differential diagnosis includes sebaceous cyst and metastatic melanoma.
An axial image at the level of the head of the pancreas and the hila of the kidneys strongly suggests the presence of a mass (Figure 2A). Another axial image, slightly inferior to the preceding one, is interpreted as normal (Figure 3A). Based on the findings in the 3 CT images, it is difficult to rule out a malignant lesion.
Which diagnostic test would you order next—and why?
WHICH TEST—AND WHY: Given the patient’s history of melanoma, you order apositron emission tomography (PET) scan. PET scans are superior to CT scans for identifying and staging melanoma metastases. Subcutaneous nodules are often overlooked on a CT scan—or are dismissed as benign sebaceous cysts. Although malignant lesions in this location can be very difficult to identify on CT, they are relatively obvious on PET; on a PET scan, only malignant lesions show up. The sensitivity of PET for metastatic melanoma is approximately 85% to 90%, while that of CT is about 60%. The specificity of PET is approximately 90%, while that of CT is about 70%. (There is some variation in the sensitivities and specificities cited in different studies; these figures represent an approximate average of the figures given in several studies.1,2)
Results of the scan. An image at the level of the dome of the liver (Figure 1B) shows increased activity in the posterior aspect of the left flank; this corresponds with the density seen on the CT image in the same location (see Figure 1A) and is most consistent with metastatic melanoma. An image at the level of the head of the pancreas (Figure 2B)—the correlate to the CT image at this level (see Figure 2A)—shows increased activity in a lesion in the head of the pancreas. Given the patient’s history and the presence of other lesions, this most likely represents metastatic melanoma as well.
An image (Figure 3B) just inferior to the previous one also shows a focal area of increased activity. When the corresponding CT image (see Figure 3A) is reviewed retrospectively, a soft tissue density is identified at that location; this was initially thought to be unopacified bowel but is now strongly suspected to be a focus of metastatic disease. Mesenteric lymph nodes are easily confused with unopacified bowel on CT, while these entities are distinguished with relative ease on PET.
Metastatic melanoma is diagnosed; the diagnosis is confirmed by biopsy of the subcutaneous nodule at the level of the dome of the liver. PET scanning also identified other foci of disease in the patient’s neck and lower extremities—regions not imaged on the CT scan. Another advantage of PET is that it is a whole body scan and thus offers greater coverage than CT.
Outcome of this case. The patient was referred for treatment of diffuse metastatic disease.
1. Eigtved A, Andersson AP, Dahlstrom K, et al. Use of fluorine-18 fluorodeoxyglucose
positron emission tomography in the detection of silent metastases from
malignant melanoma. Eur J Nucl Med. 2000;27:70-75.
2. Swetter SM, Carroll LA, Johnson DL, Segall GM. Positron emission tomography
is superior to computed tomography for metastatic detection in melanoma
patients. Ann Surg Oncol. 2002;9:646-653.