Localised nodular synovitis mimicking melanoma metastasis on PET-CT. MRI correlations

Clinical History

A 41-year-old woman came to our attention because, during a clinical breast examination, a suspicious nodular lesion was found in the left axillary region: the subsequent biopsy revealed metastatic melanoma cells, but the primary lesion was unknown. The patient underwent PET-CT scan for the primary tumour research.

Imaging Findings

PET-CT showed (Fig. 1) an oval markedly overactive lesion, located between the right small trochanter and femoral external obturator muscle. The woman was therefore sent for MRI in order to further characterize the lesion. MRI showed, in the PET-CT uptake region, a 2.6 x 2.5 cm lesion, without cleavage plane with the external obturator muscle, heterogeneously hypointense on T1-T2 weighted images (Fig. 2, 3, 4)and characterised by intense contrast enhancement (Fig. 5). No significant signal abnormalities compatible with femoral bone erosion were found. MRI could not exclude or confirm the malignant nature of the lesion (secondarism? synovitis nodule? fibromatosis nodule?). CT-guided biopsy was recommended to investigate the nature of the lesion.

Discussion

Melanoma is a malignant tumour primarily affecting individuals of both sexes between 30 and 60 years. A considerable number of patients presents with clinically evident regional lymph-node metastases and a fair percentage of patients (2%-13%) has occult primary tumour. Survival rates of patients with unknown and with known primary site are similar as well as prognostic factors. PET-CT is the examination of choice to research primary site of malignancy but sometimes it is not conlcusive, thus patients with unknown primary sites of melanoma should be treated as stage III melanomas.
In the presente case, the sole FDG uptake lead to MRI in order to characterize soft tissue lesion. However MRI findings were not univocal since location and morphology was not typical for primary site or eventual metastases and, furthermore, melanin containing lesion may show hyperintensity on T1w sequences for the shortening of T1 relaxation-time.
Second possible diagnosis was a benign lesion such as Pigmented villonodular synovitis (PVNS). PVNS represents an uncommon benign neoplastic process that may involve the synovium of the joint diffusely or focally or that may occur extra-articularly in a bursa or tendon sheath. It should be considered in younger patients (30-40 years) presenting with monoarticular joint symptoms, like limited joint mobility (absent in our case). Pathologic specimens of the hypertrophic synovium may appear villous, nodular or villonodular, and haemosiderin deposition, often prominent, is seen in most cases. PVNS is also referred to as Giant Cell Tumour of the tendon sheath (GCTTS): it has a predilection for lower extremities(knee and hip). Typical MRI findings are a relative low signal mass in T1-T2w sequences, sometimes with scattered areas of hyperintensity: low signal on all pulse sequences (in particular Gradient-Echo sequences) is a typical and nearly pathognomonic characteristic due to haemosiderin presence. After Gadolinium admnistration, PVNS can show intense (like in our case) or moderate enhancement. MRI is optimal for evaluating lesion extent, crucial information to achieve complete surgical resection. PVNS hypervascularity and heterogeneously distributed hypercellularity translate into increased PET-CT uptake.
MRI showed some signal characteristics in disagreement with malignant nature of the lesion, in particular hypo-intensity on T2w images (with and without fat signal suppression) and hypo-intensity on T1w images.
Patient's history of melanoma and PET-CT features raised the doubt of malignancy localization not confirmed at MRI. CT-guided biopsy was requested prior to treatment.
In this presente case, as previously said, patient was treated as stage III melanoma.

Differential Diagnosis List

Final Diagnosis

Pigmented villonodular synovitis (PVNS) in a patient affected by melanoma

URL:	https://www.eurorad.org/case/9039
DOI:	10.1594/EURORAD/CASE.9039
ISSN:	1563-4086