DOI:10.2214/AJR.04.1986
AJR 2006; 187:W117-W119
© American Roentgen Ray Society
Acne Vulgaris: False-Positive Finding on Integrated 18F-FDG PET/CT in a Patient with Melanoma
Timothy M. Pawlik1,
Jeremy J. Erasmus2,
Mylene T. Truong2,
Homer Macapinlac2,
Merrick I. Ross1 and
Jeffrey E. Gershenwald1
1 Department of Surgical Oncology, The University of Texas M. D. Anderson Cancer
Center, Unit 444, PO Box 301402, Houston, TX 77230-1402.
2 Division of Diagnostic Imaging, The University of Texas M. D. Anderson Cancer
Center, Houston, TX 77030.
Received December 30, 2004;
accepted after revision March 15, 2005.
Address correspondence to J. E. Gershenwald
(jgershen{at}mdanderson.org).
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Keywords: acne vulgaris • cancer • melanoma • PET
Introduction
Malignant melanoma was diagnosed in approximately 59,580 patients in the
United States in 2005. Appropriate treatment of patients with this tumor
depends on accurate staging at presentation. CT has been used in the
evaluation of melanoma at high risk of metastasis (i.e., thick, ulcerated
primary tumors [stage IIC] or stage III disease). Because the accuracy of CT
in the detection of occult metastatic lesions is limited, whole-body
18F-FDG PET has emerged as a complementary imaging technique and is
being used with increasing frequency in examinations of patients with advanced
melanoma [1,
2]. Although integration of PET
and CT findings increases diagnostic accuracy, use of this combination in the
detection of distant metastatic lesions is associated with a high
false-positive rate [3].
Awareness of potential diagnostic pitfalls is therefore needed to avoid
misinterpretation of integrated PET/CT images that can lead to inaccurate
staging. In particular, accurate identification of cutaneous and subcutaneous
metastasis of melanoma is important because the lesions may represent new or
additional regional or distant metastatic disease. Such findings may influence
the overall treatment approach. We report the case of a patient with melanoma
lymph node metastasis (American Joint Committee on Cancer stage III) who had
18F-FDG accumulation at the sites of acne vulgaris that might have
been misinterpreted as evidence of multiple distant cutaneous or subcutaneous
metastatic lesions, potentially altering the pre-PET surgical treatment plan
of regional lymph node dissection.
Case Report
A 26-year-old man with a 4.0-mm-thick ulcerated melanoma of the left lower
back (stage T4b according to the American Joint Committee on Cancer melanoma
staging system) had no signs of metastatic disease at clinical examination,
and the serum lactate dehydrogenase level was normal. An 18F-FDG
PET scan performed at another institution reportedly showed abnormal uptake in
the right axilla and right lung. Because the images were not available for
review, an integrated PET/CT study was performed at our institution before
planned surgery. The PET/CT study showed scattered focal increased
18F-FDG uptake (maximal standard uptake value, 3.5) in the
cutaneous and subcutaneous tissues with minimal associated soft-tissue
abnormality (Figs. 1A,
1B, and
1C). No other sites of abnormal
18F-FDG uptake were identified on the whole-body PET scan. In
particular, no increased 18F-FDG uptake was found in the primary
tumor site, draining inguinal nodal basin, right axilla, or right lung.
Comparison of imaging findings and clinical findings revealed that the focal
regions of increased 18F-FDG uptake corresponded to lesions of acne
vulgaris (Fig. 1D). Subsequent
sentinel lymph node (SLN) biopsy of the draining inguinal nodal basin revealed
a positive node with a 7.5 x 2.5 mm focus of metastatic melanoma.
Completion lymphadenectomy (levels I, II, and III) revealed three additional
metastatic foci in lymph nodes other than the SLN, the largest measuring 2.5
x 0.7 mm. With the resumption of the patient's topical medication, the
acne resolved, and no cutaneous or subcutaneous abnormalities were found at
postoperative examination or at a 3-month follow-up visit.
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Fig. 1B —26-year-old man with melanoma of back. PET image (B)
and integrated PET/CT image (C). Focal increased uptake of
18F-FDG in cutaneous and subcutaneous tissues of upper torso
(short arrows) and lower abdomen (long arrows) can be
misinterpreted as indicating metastatic lesions. PET images, including
B, were acquired during shallow breathing in 2D mode for 3 minutes per
bed position. Integrated PET/CT images, including C, were obtained with
Discovery ST-8 scanner (GE Healthcare). Images were obtained 90 minutes after
IV administration of 18.8 mCi (695.6 MBq) 18F-FDG.
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Fig. 1C —26-year-old man with melanoma of back. PET image (B)
and integrated PET/CT image (C). Focal increased uptake of
18F-FDG in cutaneous and subcutaneous tissues of upper torso
(short arrows) and lower abdomen (long arrows) can be
misinterpreted as indicating metastatic lesions. PET images, including
B, were acquired during shallow breathing in 2D mode for 3 minutes per
bed position. Integrated PET/CT images, including C, were obtained with
Discovery ST-8 scanner (GE Healthcare). Images were obtained 90 minutes after
IV administration of 18.8 mCi (695.6 MBq) 18F-FDG.
|
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Discussion
Detection of occult metastatic lesions is important for appropriate
planning of the treatment of patients with melanoma. Consensus has not been
reached, however, on which imaging studies should be used routinely as part of
the preoperative screening and staging evaluation. Chest radiographs are
frequently obtained, but the rate of detection of occult asymptomatic
metastatic lesions is low, and false-positive rates are high. CT also has a
low sensitivity and a high false-positive rate in the detection of occult
metastatic lesions. Interest has been increasing in the use of
18F-FDG PET as a noninvasive and potentially accurate technique for
preoperative staging of melanoma.
Methodologic limitations in earlier PET studies have made it difficult to
develop guidelines for the use of 18F-FDG PET in the evaluation of
melanoma. Several prospective studies have shown that 18F-FDG PET
evaluation of regional lymph node basins in patients with stage I, II, and
IIIA melanoma is not sensitive in the detection of occult nodal metastasis
diagnosed with SLN biopsy
[3-5].
In a study involving patients with early-stage melanoma, Wagner et al.
[5] reported that
18F-FDG PET had a sensitivity of only 16.7% and a positive
predictive value of 50% compared with SLN biopsy in the detection of SLN
biopsy-diagnosed occult nodal metastasis. In a more recent morphometric
analysis of the metastatic load in SLNs, Mijnhout et al.
[6] found that
18F-FDG PET did not depict most metastatic deposits in SLNs in
patients with melanoma because most such metastatic lesions were too small. In
the current case, 18F-FDG PET did not depict metastatic lesions in
the draining inguinal nodal basin. The false-negative 18F-FDG PET
finding probably was caused by the limited spatial resolution of the PET
scanner and tumor volumes below the threshold of detection.
18F-FDG PET may be more useful in the detection of distant
metastasis in patients with clinically detectable regional nodal metastasis
(stage IIIB and IIIC disease) and those with distant metastatic disease (stage
IV). Tyler et al. [3] reported
that 18F-FDG PET had a positive predictive value of 78.6% in the
detection of distant metastasis and that the detection rate improved to
approximately 90.6% when imaging findings were correlated with clinical
information. However, the false-positive rate was 56.5%. As we did, Tyler et
al. found that a careful history interview and physical examination were
useful in clarifying the cause of increased 18F-FDG uptake in
almost two thirds of patients. A prospective analysis of 18F-FDG
PET and CT in the detection of metastasis in patients with pathologically
confirmed stage IV melanoma being evaluated for resection of metastatic
lesions showed that the two types of images interpreted together had higher
sensitivity in lesion-by-lesion analysis than did either technique alone
[1]. Although it is clear that
PET and conventional imaging are complementary in the evaluation of advanced
melanoma, the true role of 18F-FDG PET in this group of patients
has yet to be determined. As this case shows, even integrated PET/CT can lead
to erroneous clinical staging if not correlated with a thorough history
interview and physical examination.
Increased glucose metabolism of inflammatory tissues represents the main
source of false-positive 18F-FDG PET findings in oncology.
Postsurgical inflammation, other inflammatory lesions, and some benign tumors
have been shown to be associated with false-positive 18F-FDG PET
results. Specifically, false-positive findings on PET scans have been
associated with an inflamed epidermal cyst, Warthin's tumor of the parotid
gland, surgical wound inflammation, leiomyoma of the uterus, suture granuloma,
and endometriosis [7]. The
current case illustrates that, like other inflammatory processes, acne
vulgaris is associated with 18F-FDG uptake and therefore must be
considered in the differential diagnosis of melanoma when
18F-FDG-avid cutaneous lesions are identified on PET scans.
18F-FDG PET does not have a role in the evaluation of
early-stage melanoma. Although whole-body 18F-FDG PET is being used
increasingly to detect occult disease in patients with advanced melanoma,
false-positive uptake of 18F-FDG can lead to inaccurate clinical
staging and inappropriate therapeutic recommendations. This case illustrates
that acne vulgaris, an inflammatory process, can cause increased uptake of
18F-FDG and lead to an erroneous interpretation of metastatic
disease on PET scans [8].
Correlating the sites of increased 18F-FDG uptake with the findings
in the clinical history and at physical examination can be important in
improving the accuracy of detection of distant metastasis.
References
- Finkelstein SE, Carrasquillo JA, Hoffman JM, et al. A prospective
analysis of positron emission tomography and conventional imaging for
detection of stage IV metastatic melanoma in patients undergoing
metastasectomy. Ann Surg Oncol 2004;11
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scanning in malignant melanoma. Cancer2000; 89:1019
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Oncol 1999; 17:1508
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case of lymph node staging in melanoma. J Clin Pathol2003; 56:283
-286[Abstract/Free Full Text]
- Holder WD Jr, White RL Jr, Zuger JH, Easton EJ Jr, Greene FL.
Effectiveness of positron emission tomography for the detection of melanoma
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Introduction
Case Report
