Aspiration cytology of liver abscess uncovering metastatic rectal mucosal melanoma—A case report

Aspirates of liver abscess are frequently encountered in routine practice and are often of a low index of suspicion. However, necrotic liver metastasis clinically and radiologically mimics liver abscesses, and malignant cells can be obscured in an inflammation‐rich background on cytology. It is important to recognise malignant neoplasms in this scenario, in particular uncommon conditions such as metastatic mucosal melanoma.


| INTRODUC TI ON
Fine needle aspiration cytology (FNAC) is a robust technique employed in the diagnosis of hepatic lesions. 1 FNAC is simultaneously diagnostic and therapeutic for specific conditions such as abscesses or haemorrhagic cysts. 2 Abscess aspirates constitute a large proportion of liver aspirates, and occasionally can harbour a malignant diagnosis.
We report a rare case of liver metastasis from rectal mucosal melanoma presented as a liver abscess.This case highlights the difficulty in recognising uncommon cytologically difficult malignant neoplasms that are obscured by inflammation-rich background and illustrates the importance of maintaining vigilance despite low clinical suspicion.

| C A S E REP ORT
A 43-year-old female presented with fever and right upper quadrant pain.Portable ultrasound showed a necrotic liver mass.The mass was aspirated, yielded 200 mL of blood-stained material and sent to the cytology laboratory labelled as liver abscess aspirate.The aspirate was highly cellular and composed of rich inflammatory cells, including polymorphs, lymphocytes, and macrophages associated with abundant necrotic material.Scattered discohesive atypical cells were noted.The atypical cells were hyperchromatic with irregular nuclear contours and occasional prominent nucleoli.The provisional cytological diagnosis was atypia, suspicious of malignancy (Figure 1).The patient was subsequently admitted, and a liver function test revealed increased alanine transaminase (86 U/L) and mildly elevated alkaline phosphatase (147 U/L) levels.An urgent computed tomography (CT) of the abdomen with contrast was performed, and an 11 cm cystic liver lesion with fluid level and nodular component was seen (Figure 2).The patient's medical history was reviewed.It was discovered that the patient had received a diagnosis of rectal mucosal melanoma from a colonoscopic polypectomy from another institute 1 year prior.The patient was further treated by local excision with close margins, followed by abdominoperineal resection.Preoperative staging positron emission tomography (PET) was negative for metastatic lesions, and there was no evidence of residual disease or lymph node involvement in the resection specimen.The patient received adjuvant nivolumab and paclitaxel.The latest PET-CT before this episode was dated 4 months previously, and at that juncture no sign of recurrence or metastasis was seen.
A liver biopsy was obtained, which demonstrated lesional cells with similar morphology as the aspirate, consisting of clusters of discohesive hyperchromatic cells with frequent mitosis.
Immunohistochemistry was performed and the lesional cells were positive for melan-A, SOX10, and PRAME (Figure 3).A cell block was prepared for the aspirate specimen, and for confirmation, the atypical cells were also immunoreactive to SOX10 (Figure 3).The case was then signed out as consistent with metastatic malignant melanoma.The patient was managed with intravenous antibiotics and stabilised.Unfortunately, the malignant melanoma progressed despite a switch to pembrolizumab plus ipilimumab regimen and she succumbed 4 months later.

| DISCUSS ION
Liver abscess can be caused by bacterial, fungal, viral, and parasitic organisms, with specific forms such as tuberculosis, aspergillosis, candidiasis, cytomegalovirus infection, and amoebiasis, which are readily recognised on FNAC with/without immunocytochemistry. 3 Liver abscesses are fluid-filled and yield large amounts of material for cytological and microbiological analyses, thus suited for aspiration.Although the use of FNAC has decreased relative to that of core biopsies, due to the improving techniques in image guidance and biopsy equipment, 4 liver abscess is very commonly encountered in liver aspirates, and is one of, if not the most, frequently issued diagnoses in liver aspirates of our institute.This is because not only can FNAC diagnose liver abscesses without core biopsy, aspiration is highly effective in treating liver abscesses. 2inically and radiologically, liver abscess and metastasis share overlapping features, including symptoms such as fever, abdominal pain, and constitutional upset, and CT findings such as peripheral enhancement and central necrosis. 5Liver function test is not useful in distinguishing these two entities, nor are tumour markers such as carcinoembryonic antigen and alpha-fetoprotein which can be elevated in patients with liver abscesses. 6There are cases reported in the literature where liver abscesses were being mistaken as metastatic malignancies. 7Similarly, liver metastases, most commonly pancreatobiliary carcinomas and colorectal carcinomas, can be difficult to distinguish from infective/pyogenic liver abscesses. 8This case reinforces the importance of actively looking for clinical information in all patients at the time of specimen analysis.
FNAC of the liver is sensitive for the detection of malignant lesions, including primary and metastatic malignant neoplasms, typically hepatopancreatobiliary and metastatic colorectal carcinomas. 1,9However, the identification and typing of rare lesions may not be straightforward in cytological preparations. 10In the case of liver abscesses, inflammatory pseudotumors are examples of cytological mimickers of the inflammatory cell-rich picture seen in abscesses.As in the current case, the malignant melanoma cells were discohesive, cytoplasmic melanin pigments were not apparent in either the liver biopsy or the aspirate, and confirmation of melanocytic differentiation relied upon melan-A, SOX10, and PRAME immunostains.Sufficient clinical information was not available on laboratory referral for the current case, and with the relatively young age and dispersed discohesive lesional cells among inflammatory cells and debris, a specific diagnosis of malignancy was not reached at the initial cytological assessment.
Malignant neoplasms on occasion masquerade as clinically benign conditions.There are significant clinical and radiological overlaps for liver abscesses and liver metastasis.Despite the majority of liver abscess aspirates being infective in nature, caution should be

F I G U R E 1
Cytology of the liver aspirate.(A) Atypical cells among a background of abundant inflammatory cells and necrotic material (cytospin, Pap stain, 100×).(B) Discohesive hyperchromatic irregular cells among polymorphs and necrotic material (cytospin, Pap stain, 400×) F I G U R E 2 Computed tomography showing an abscess-forming lesion in the liver with fluid level