There are no well-defined clinical factors to predict the risk of occult invasion in melanoma of the lentigo maligna type (LM) prior to complete histopathologic analysis.

To evaluate whether clinical size was a predictor of invasion in LM and subclinical extension.

Consecutive cases of LM were recorded in a prospectively maintained database from 2006 to 2019. Patient and tumor data were recorded during initial evaluation. 'LM clinical area' was calculated in square millimeters (length x width). All patients were treated with staged excision.

We included 600 patients. Mean age was 65.9 years (SD 12.3; range 27 - 95 years); 62.8% (n=377) were males. The mean LM clinical area was 128.32 mm² for in situ lesions vs 200.14 mm for invasive lesions (p=0.1). Based on quantile regression, the median margin required for complete removal increased with LM clinical area.

study performed in a tertiary cancer center with possible referral bias and more complex cases.

LM can present with variable clinical size which may correlate with subclinical extension; however, the presence of invasion is not well estimated by LM clinical area.