What is the microscopic tumor extent beyond clinically delineated gross tumor boundary in nonmelanoma skin cancers

Purpose: To quantify the microscopic tumor extension beyond clinically delineated gross tumor boundary in nonmelanoma skin cancers. Methods and Materials: A prospective, single arm, study. Preoperatively, a radiation oncologist outlined the boundary of a gross lesion, and drew 5-mm incremental marks in four directions from the delineated border. Under local anesthesia, the lesion was excised, and resection margins were assessed microscopically by frozen section. Once resection margins were clear, the microscopic tumor extent was calculated using the presurgical incremental markings as references. A potential relationship between the distance of microscopic tumor extension and other variables was analyzed. Results: A total of 71 lesions in 64 consecutive patients, selected for surgical excision with frozen-section-assisted assessment of resection margins, were accrued. The distance of microscopic tumor extension beyond a gross lesion varied from 1 mm to 15 mm, with a mean of 5.2 mm. A margin of 10 mm was required to provide a 95% chance of obtaining clear resection margins. The microscopic tumor extent was positively correlated with the size of gross lesion, but not with other variables. Conclusions: The distance of microscopic tumor extension beyond a gross nonmelanoma skin cancer was variable, with a mean of 5.2 mm. Such information is critical for the proper radiation planning of skin cancer therapy.