Teaching Case

Metastatic lymph nodes in a patient affected by melanoma

Patient with a brownish, raised spot of the upper back (1,3x1,2cm) showing a nodular, not homogenous morphology and irregular margins; the lesion was surrounded by pigmented nodules miming angiomas, with a maximum diameter of 0,5 cm and located 1,5 cm far from the main lesion (Figure-1). The lesion pattern oriented for melanoma with loco-regional satellitosis and the spot and surrounding skin and derma were thus surgically removed. Microscopic examination confirmed the presence of a not ulcerated melanoma with superficial involvement and neoplastic satellitosis. One month after surgery, the patient underwent US examination (high frequency probe/10-13MHZ) of the axillary and inguinal regions; in the left axilla, US revealed an increased lymph node (18x10 mm), which superior pole resulted completely altered, appearing as a hypoechoic area with lobulated margins, suspected for metastatic infiltration of the node; Power-Doppler showed moreover an anarchic distribution of the vessels (Figure-2). Near the first node, US showed also a second node (16x5mm) with a locally thick cortical, suspected for focal metastatic nodularity (3x2 mm) (Figure-3). . To depict any other metastatic nodal involvement, the patient underwent a whole-body PET-CT with 18-FDG, that evidenced only a pathological uptake in the left axillary region (SUV=3,8) (Figure-4). A radical lymphadenectomy of the left axillary region was performed; the pathology confirmed the presence of the two suspected metastatic lymph nodes, of which the biggest resulted to be completely infiltrated while the other showed infiltration of a limited area of the cortical, as it had been suspected at US (Figure-5).

Typical therapeutic approach in case of suspected melanoma consists in spot ablation; if pathological exam confirms the presence of tumor, therapeutic strategy includes the enlargement of the previous surgical field (removing the surrounding skin and subcutaneous tissue) and the examination of the sentinel lymph node. The study of the sentinel node starts injecting a specific radio-drug (generally Tc99m bound to colloid) in the region of pathological spot: some time after it is possible to detect the first drainage node of the lesion area (so called sentinel lymph node), which is surgically removed and analyzed by the pathologist. If histological analysis excludes neoplastic nodal infiltration, metastatic spreading is usually considered very unlikely, while if the sentinel node results infiltrated, all nodes of the same chain are removed (radical lymphadenectomy). In our case, the patient underwent US examination of axillary and inguinal regions instead of the lymphscintigrapy, because these superficial regions, frequent site of metastatic nodal involvement in cases of melanoma of the back, can be accurately evaluated also by imaging; US examination was anyway completed by PET-CT. Imaging role in patients affected by melanoma consists mainly in neoplasm staging, and depicting nodal and/or distant metastatic localizations. High frequency US allows in fact an accurate evaluation of the node structure. Typical US characteristics of a normal node are: -oval morphology, with a short/long axis rate >2; -thin but well defined cortical; -well defined hilum; -vessels mainly distributed at hilum (evaluated at Power-Doppler). The pattern of a metastatic node includes, instead, these imaging characteristics: -round morphology, with a short/long axis rate<2; -irregular margins with thickness of the cortical and possible eccentric, inhomogeneous areas; -frequent hilum lack or dislocation due to neoplastic infiltration; -aberrant vessels (passing through the cortical, with an irregular calibre and final stop at Power-Doppler). CT instead, does not represent a sensitive method to detect metastatic nodes, while the unique criterion that can lead to the suspicion of a metastatic lymph node is nodal enlargement (short axis measuring > 1cm). The possibility of acquiring very thin CT slices (<1mm) and create multiplanar reconstructions allows anyway to determine all nodal diameters and so to express an evaluation of nodal malignancy basing on the shape roundness. Nevertheless, in case of a not dimensionally significant lymph node at CT, it is not anyway possible to exclude certainly microscopic metastatic foci. Lately, also MR has been introduced in analyzing lymph nodes, because of its ability in depicting melanin, usually concentrated in normal and metastatic nodal cortical. Melanin determines a T1 time shortness, with a consequent hyperintensity in T1 weighted images, while signal intensity in T2 w.i can be quite variable, resulting mostly hypointense, because of the concomitant shortening of T2 time. Moreover, the use of superparamagnetic contrast medium (iron ioxides particles) has resulted useful in differentiating hyperplastic from metastatic nodes: reactive nodes appear in fact hypointense in T2 w.i. after contrast medium administration, while metastatic nodes show hyperintense because of the lack of ioxides particles captation by the lymphcites.