This trial entered 2001 patients but only the results relating to 1269 patients with intermediate thickness tumours (1.2 mm – 3.5 mm) have been published. See diagram below
Study Design of MSLT-1
All patients had wide local excision of the primary tumour and were then randomised (40% versus 60%) either to the observation arm where (delayed) lymphadenectomy was performed only if metastatic regional lymph nodes became palpable or to the biopsy arm where all patients had sentinel lymph node biopsy and immediate (or early) lymphadenectomy was performed if the sentinel node contained melanoma.
In a subgroup analysis the authors compared the survival of the 122 patients whose sentinel nodes contained melanoma and who underwent early lymphadenectomy with the survival of the 78 patients in the observation arm who had delayed lymphadenectomy and claimed a 20% survival advantage in favour of early lymphadenectomy (see diagram above).
Within a randomised controlled trial, such a survival comparison is statistically invalid. The difference in survival is entirely explained by a prognostic difference in the two groups of patients compared. The incidence of prognostic false-positivity in the 122 patients with melanoma in the sentinel node was 24%.
An in depth commentary on this subject can be read in Nature Oncology