Where to draw the line: the error in marking surgical excision margins defined.

We studied the accuracy of marking surgical margins for excision surrounding skin lesions, to assess if the percentage error would differ for varying increments in margin measurements. In designing an experiment to test this hypothesis, surgeons of differing experience (n=19) marked excision margins of 2, 5 and 10 mm around a standard circular lesion drawn on paper. Use of surgical markers, rulers and loupe magnification were all permitted, with five attempts for each margin. The percentage error found was 35, 14 and 4% for the 2, 5 and 10 mm margins, respectively (regardless of the grade of surgeon). Repetition of the experiment on volunteer skin demonstrated a percentage error of 45, 16 and 8% for 2, 5 and 10 mm margins (significantly greater than the corresponding errors on paper, p<0.001 in all cases). These findings indicate that for these surgically marked margins, the term "accurate excision margins" may be inaccurate to a significant degree. This study has defined the errors inherent in the marking of surgical margins, and these should be taken into account when assessing studies that report margins around tumours. Specifically, comparisons between surgical and histological margins would aid assessment of reports detailing surgical margins dawn around skin tumours.