Keywords Anorectal melanoma - pattern of fluorodeoxyglucose uptake - positron emission tomography–computed
tomography
Introduction
Malignant melanoma is tumor of melanocytes which are present in the skin, mucosa,
and ocular layer. Non-cutaneous malignant melanomas are rare including ocular – eyelid,
uvula, mucosa – hard palate, sinonasal, oral cavity, gastrointestinal, and genitourinary
tract, etc., Usually, malignant melanoma is high-grade fluorodeoxyglucose (FDG) avid
and contrast hyper-enhancing tumor, but on the contrary, here presented two cases
with different FDG uptake and computed tomography (CT) morphological features which
could be misleading in positron emission tomography (PET)–CT-based diagnosis/interpretation.
Case Reports
Case 1
A 59-year-old male patient had complained of intermittent per rectal bleeding for
2 months and weight loss 2–3 kg. Video-colonoscopy found eccentric growth in the anal
canal and serum carcinoembryonic antigen (S.CEA) level was normal. Biopsy with immunohistochemistry
was suggested melanoma. The patient was referred for fluorodeoxyglucose (FDG) PET-contrast-enhanced
CT (CECT) scan. The finding was predominantly low-grade FDG avid heterogeneous mild-to-moderately
enhancing endoluminal growth in the anal canal, anorectal junction, and lower rectum
[Figure 1 ]. A non–FDG-avid tiny left mesorectal lymph node was found [Figure 2 ]. The patient was operated, and the histopathology report suggested Mucosal Amelanotic
Melanoma with one metastatic lymph node.
Figure 1 Maximum intensity projection images, axial (upper) and sagittal (lower) fluorodeoxyglucose
positron emission tomography-computed tomography fusion images show low-grade fluorodeoxyglucose
avid heterogeneously enhancing endo-luminal growth in the anal canal, anorectal junction,
and lower rectum
Figure 2 Axial fluorodeoxyglucose positron emission tomography-computed tomography fusion
images show nonfluorodeoxyglucose avid enhancing mesorectal metastatic lymph node
Case 2
A 65-year-old male patient had complained of upper abdominal pain and weight loss.
USG found multiple liver lesions and guided biopsy diagnosed as metastatic melanoma.
The patient had no cutaneous lesion and hence, referred for PET-CT scan to identify
the primary lesion. Whole-body 18 F-FDG PET-CECT scan done and found high-grade FDG avid polypoidal ulcerated contrast
hyper-enhancing lesion in the lower rectum, anorectal junction, and extends in the
upper part of the anal canal with metastatic liver and lung lesions [Figure 3 ],[Figure 4 ],[Figure 5 ].
Figure 3 Maximum intensity projection images of case 2 show high-grade fluorodeoxyglucose
avid primary lesion in the lower rectum with metastatic liver and lung lesions
Figure 4 Fluorodeoxyglucose positron emission tomography-computed tomography fusion axial
and coronal images show high-grade fluorodeoxyglucose avid ulcerated contrast hyper-enhancing
lesion in the lower rectum, anorectal junction, and extends in the upper part of the
anal canal
Figure 5 Fluorodeoxyglucose positron emission tomography-computed tomography fusion axial
and coronal images show high-grade fluorodeoxyglucose avid metastatic liver and lung
lesions
Scans were done on SIEMENS Biograph TruePoint 16-slice PET-CT scanner (SIEMENS AG,
Wittelsbacherplatz 2, DE-80333 Muenchen, Germany) after 60 min of 370 MBq 18 F-FDG intravenous injections.
Discussion
Anorectal mucosal melanoma is rare among all malignant melanomas (about 1.3%) and
16.5% of mucosal melanomas and also rare among all anorectal malignancies associated
with poor prognosis. In India, it is more prevalent in males, whereas more prevalent
in females in other countries.
Because of its lower incidence rate and confusing histological features, it is often
misdiagnosed as lymphoma, carcinoma, or sarcoma.[1 ],[2 ] Patients present with local symptoms such as lower abdominal pain, per rectal bleeding,
or bowel alteration. Up to 67% of cases initially present with metastasis at the time
of diagnosis.[3 ] Melanoma metastasizes hematogenous or lymphogenous route to locoregional lymph nodes,
liver, lung, bone, or brain.
Falch et al., reviewed novel staging method for anorectal melanoma (ARM) and observed
FDG PET-CT scan is a useful tool for staging and treatment planning.[4 ]
FDG PET-CT scan in recommended imaging tool in Stage III and IV as provides additional
impactful information over CT scan. FDG PET-CT scan is superior to CT scan for the
diagnosis of nodal involvement and distant metastasis.[5 ]
ARM usually appears as polypoidal or ulcerated contrast hyperenhancing lesion with
high-to-moderate grade FDG avidity. Few variants of ARM are amelanotic, epithelioid,
spindle cell, lymphoma-like, and pleomorphic type.[6 ] However, as of now, the management and prognosis of malignant melanoma are not dependent
on its subvariant.
Very few articles had been published related to ARM and PET scan specifically for
a direct intent of evaluating the pattern of FDG uptake and other PET-CECT-related
features and hence have to gone through articles statements, legends and given PET
or PET-CT fusion images visual assessments to evaluate the pattern of FDG uptake in
ARM cases. After reviewing such articles, it has been found that most of the cases
had high-grade FDG avid primary ARM; however, only one case[7 ] had low-to-moderate grade FDG-avid lesion based on given images [Table 1 ].
Table 1 Pattern of fluorodeoxyglucose uptake in anorectal melanoma mentioned in various articles
Here, in the mentioned first case, the lesion was well-defined endoluminal growth
causing luminal narrowing without extramural extension with low-grade FDG avidity
and minimal enhancement. Such a less-known variant of low-grade FDG avid and low contrast-enhancing
well-circumscribed lesion could be missed as low-grade non-Hodgkin's lymphoma or mucinous
adenocarcinoma. However, the final histopathology report mentioned as mucosal malignant
melanoma. There was no distant metastatic lesion, and this is a possible similarity
of low-grade FDG-avid lesions [Table 1 ].
In the above-mentioned second case, the lesion was an endoluminal growth with ulceration
and showed high contrast-enhancement and FDG avidity. There were FDG-avid liver and
lung metastatic lesions also. Based on cases of review of articles and here reported
two cases as mentioned in [Table 1 ], it has been noted that all cases of ARM with distant metastases were high-grade
FDG avid. This might raise possible positive correlation of FDG avidity and the presence
of distant metastasis, and thus, the intensity of FDG uptake may give indirect biological
insight into disease aggressiveness.
Conclusion
In general, melanomas are considered as high-grade FDG avid malignancy, but here we
reported two cases of anorectal malignant melanoma with two different FDG PET-CECT
scan patterns in terms of variety of FDG uptake, contrast enhancement, morphological
appearance-like will defined or ulcerated lesion and the presence of metastatic lesions
and might raise possibilities of (a) correlation of imaging features with aggressiveness
of disease in terms of nodal and distant metastasis and (b) correlation of imaging
features with histological subvariant of melanoma which can be assessed on larger
scale and multicentric studies.
Declaration of patient consent
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In the form the patient(s) has/have given his/her/their consent for his/her/their
images and other clinical information to be reported in the journal. The patients
understand that their names and initials will not be published and due efforts will
be made to conceal their identity, but anonymity cannot be guaranteed.
