Popliteal node metastasis in infra-popliteal melanoma: Arcane or mundane?

Abstract

e21594 Background: Infra-popliteal melanoma constitutes a major percentage of cutaneous melanoma among women. The primary lymphatic drainage of is to the groin with incidence between 20 to 30%. The popliteal basin harbingers metastases in 1.5 - 10%. Popliteal metastasis though recognized as early as 1980 by lymphoscintigraphy studies, has still not seen the light of guidelines. The principles of sentinel node biopsy remain the same for both basins. Non-recognition of popliteal metastases can result in potential understaging. We aimed to quantify the popliteal node metastasis, compare clinicopathologic factors and analyze oncologic outcomes of popliteal node metastasis in infra-popliteal melanoma. Methods: A descriptive study of all patients with infra-popliteal melanoma treated at our Institute between January 2010 and December 2024 treated with curative intent with wide excision of lesion/residue or scar with sentinel lymph node biopsy of inguinal and popliteal basins. Results: Among 80 patients the prevalence of occult popliteal metastases in our cohort was 15% with male-to-female ratio 2:1 and mean age of 65 years, common among patients with T4b (100%), and hindfoot (87.5%) melanoma. A third had isolated popliteal involvement in the absence of inguinal node involvement. For every 3 patients with positive popliteal scintigram, 1 patient had histology proven metastasis. There were no popliteal metastases beyond the sentinel nodes. Inguinal node metastases occurred in 31 patients (38.75%) 84% clinically occult. Three (11.5%) patients had inguinal metastases beyond sentinel nodes mirroring MLST 2. The median follow-up duration was 81 months and the OS of pN+ patients was 30 months (95% CI 24 to 37 months) and that of pN- patients which was 108 months (95% CI 32 to 184 months). The median OS among patients with isolated popliteal metastasis (n = 4) was 18 months (95% CI 0 – 60 months) while that of patients with isolated inguinal metastasis (n = 22) was 30 months (95% CI 23 to 37 months). The dual node positive cohort (n = 8) had a median OS of 15 months (95% CI 0-44 months). The log-rank test did show a significant difference in between overall survival of 3 subgroups (p = 0.000). Conclusions: While the traditional nodes metastases in cutaneous melanoma is common (with an incidence of 38.75% highest recorded in our cohort), so is the true popliteal metastases (15% incidence). Interval nodes are not a rare manifestation of disease, but rather an event in the natural history with finite possibility and solid anatomical pathways with decussation, and isolation probably with an aggressive biology. Multiple case series and retrospective reports of popliteal metastasis in infra-popliteal melanoma advocate the sentinel node biopsy of traditional as well as occult nodes. Identifying occult nodal metastases in cutaneous melanoma is crucial now, more than ever, in the era of effective adjuvants (targeted therapy, immunotherapy).