DENVER A margin of 10 mm beyond the gross tumor border of a nonmelanoma skin cancer is required to achieve a 95% probability of obtaining clear resection margins, C. Richard Choo, M.D., said at the annual meeting of the American Society for Therapeutic Radiology and Oncology.
In contrast, a 5-mm margin will fully cover the microscopic tumor extent in only 62% of cases, added Dr. Choo, a radiation oncologist at the Mayo Clinic, Rochester, Minn.
This sort of information is critical to the success of radiation therapy, a modality that does not provide resection margins. The radiation therapy volume selected must be sufficient to cover the potential microscopic tumor extent beyond the clinical lesion while avoiding treatment of normal tissue, he explained.
Dr. Choo and his coworkers quantified microscopic tumor extension beyond the clinical gross tumor borders of 71 nonmelanoma skin cancers from 64 consecutive patients. Thirty-eight lesions were sclerosing basal cell carcinomas, 19 were other types of basal cell carcinoma, and 14 were squamous cell carcinomas. Thirty-one were previously treated recurrent malignancies. Sixty were located on the face. The mean tumor size was 2.1 cm.
Preoperatively, the visible border of each lesion was marked with a fine felt-tip pen, and marks were placed at 5-mm intervals in four directions from the outlined borders. A plastic surgeon then excised the gross tumor under local anesthesia, and a dermatopathologist exam-ined frozen tissue sections. A positive resection margin led to further excision using thin slices until clear margins were achieved.
The mean distance of microscopic tumor extension beyond the clinically delineated border was 5.2 mm, with a maximum of 15 mm. The distance correlated positively with the size of the gross tumor, but not with histologic type, location, or history of prior treatment, perhaps due to the limited sample size.