Contradiction of clinical expectations in lymphoscintigraphy sentinel node mapping in detecting microscopic melanoma metastasis

A 51-year-old man presented with lower back skin pigmentation tag that he had had for 2 years. Physical examination showed a 10 x 10 mm ulcerated, protruding pigmented skin mass at the midline of back at the 4th lumbar spine level. Skin biopsy disclosed a 5 mm thickness nodular melanoma, which had invaded the reticular dermis. In addition, a 20 x 20 mm lymph node in the right inguinal region was noted. A pelvic computed tomography scan revealed no definite evidence of lymph node enlargement in the left inguinal, bilateral iliac or para-aortic region. However, the 99mTc-sulfurcolloid dynamic lymphoscintigraphy revealed a sentinel lymph node (SLN) in the left inguinal area. The histological examination of the 4-mm SLN demonstrated a small cluster of metastatic melanoma, which was confirmed using HMB-45 immunohistochemical stain. Meanwhile, no melanoma cells were seen in the right enlarged inguinal lymph node. Subsequently, complete left inguinal lymph node dissection was performed, which confirmed that there was no melanoma metastasis in any of remaining 14 nodes. The patient underwent regular follow-up for 9 months and had no evidence of malignancy recurrence to date.