Anatomic site of primary melanoma is associated with depth of invasion

Background: The prognosis of melanoma is correlated to the stage of the primary lesion. Tumor site has also been implicated, with locations such as the trunk carrying a worse prognosis than others (such as limbs). Hypothesis: To determine if tumor thickness correlates with location in a nonvisible body area. Design and Setting: Retrospective medical record review of all patients with stage I melanoma treated in our medical center between 1986 and 1994. Demographic data as well as primary tumor characteristics were recorded. Methods: The skin's surface was divided into occult and exposed areas. Exposed indicated visible to the patient during routine activities; occult areas included posterior aspect of the neck, back, posterior aspect of the thigh, calf, and plantar region. Data were analyzed for significance using the χ² test. Results: Of the 178 patients with stage I melanoma analyzed, 51.1% had limb, 35.9% trunk, and 12.9% head and neck lesions. Depth of invasion was less than 0.76 mm in 32.6%, 0.76 to 1.5 mm in 25.2%, and more than 4 mm in 7.9%. In 87 patients the tumor occurred in occult areas and in 91 patients in exposed areas. Comparing the depth of invasion in these 2 groups disclosed that thin (<0.76 mm) melanomas occurred in 20.7% of occult areas compared with 44% of exposed areas (P<.05). Deeper melanomas (>2.50 mm) occurred in 28.7% of occult areas compared with 12.1% of exposed areas (P<.05). Conclusions: Tumors in less-visible body areas are significantly thicker at the time of diagnosis than those occurring in more highly visible areas. Delayed detection may be responsible for this finding.