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Subcutaneous endometriosis mimicking melanoma metastasis

Detection of high-risk melanoma entails the use of imaging procedures such as ultrasound examination, computerized tomography and magnetic resonance imaging (MRI) [1]. These procedures can reveal lesions indicative of melanoma, but such lesions have several differential diagnoses. Here we report on the difficulty of diagnosis during melanoma follow-up. A 33-year-old female patient was diagnosed with an ulcerated melanoma. The tumor on her back was 1.1 mm thick (stage IIA, pT2b pN0 cM0, AJCC 2017) [2]. After tumor resection and a tumor-free sentinel lymph node biopsy from the left axilla, we administered adjuvant low-dose interferon therapy. Except for giving birth to a healthy child by caesarean section five years previously, the patient had no relevant medical or surgical history. At her six-month follow-up, the CT scan revealed a solid, ill-defined, irregular 2 × 1 cm subcutaneous lesion in the lower right abdomen which was attached to the abdominal wall fascia but had not penetrated it (Figure 1a). Subcutaneous metastasis was suspected, and a differential diagnosis of subcutaneous reaction following interferon injections was also considered. Ultrasonography (US) revealed a hypoechoic, indistinctly demarcated lesion with rare central echoes (Figure 1b), and duplex US revealed some central perfusion (not shown). However, sonography of melanoma metastases typically reveals sharply demarcated, echo-free lesions with defined borders and peripheral perfusion; a differential diagnosis of melanoma metastasis was therefore unlikely. The patient initially reported no symptoms from the lesion, which was barely discernible clinically. During the diagnostic process she mentioned a slight swelling associated with her menses, suggesting differential diagnosis of extragenital endometriosis. However, the gynecologist who was consulted considered that unlikely. To further analyze the indeterminate mass, MRI of the abdomen and pelvis was performed. The signal intensity of the lesion was low with T2-weighted imaging (WI), and small hyperintense foci were found (Figure 2a). With T1WI, signal intensity of the lesion was intermediate and isointense with skeletal muscle before contrast was administered (Figure 2b). Contrast enhancement was moderate after administration (Figure 2c). MRI findings indicated an endometriotic implant. Because of the patient's previous history of caesarean section, scar endometriosis was likely. The patient underwent surgery in order to conclusively exclude melanoma metastasis. Histopathology eventually confirmed subcutaneous endometriosis (Figure 2d). Whereas genital endometriosis is common, extragenital endometriosis is a rare condition; this often results in misdiagnosis [3–5]. Both types of endometriosis are benign, non-neoplastic disorders in which endometrial tissue occurs outside the uterine cavity, typically in or close to scars or in the umbilical region [6, 7]. Symptoms can include cyclic pain or swelling [8], but the disorders can remain almost asymptomatic, as in our patient. After a caesarean section, 0.1–0.8 % of women develop endometriosis of the abdominal wall or the scar region. The risk of developing endometriosis continues to increase over time, with a median onset of six years after pelvic surgery [9–11]. In our patient, the proximity of the lesion to the caesarean section scar was not obvious, but her medical history made extragenital endometriosis a potential differential diagnosis.