To investigate the need of a margin other than for accuracy reasons in stereotactic radiosurgery (SRS) of brain metastases by means of histopathology.

Evaluation of 45 patients from two pathology departments having had brain metastases and an autopsy of the brain. Growth patterns were reviewed with a focus on infiltration beyond the metastases boundary and made visible with immunohistochemical staining: the metastasis itself with tumor-specific markers, surrounding normal brain tissue with a glial marker, and a possible capsule with a soft tissue marker. Measurements were corrected by a tissue-shrinkage correction factor taken from literature. Outcomes parameters for infiltration were mean and maximum depths of infiltration and number of measured infiltration sites.

In 48 of 76 metastases, an infiltration was present. The largest group of metastases was lung cancer. Small-cell lung cancer (SCLC) and melanoma showed a maximum depth of infiltration of > or =1 mm, and other histologies <1 mm. For non-small-cell lung cancer (NSCLC), melanoma, and sarcoma, the highest number of infiltrative sites were observed (median, 2; range, 1-8). SCLC showed significantly larger infiltrative growth, compared with other diagnostic groups. In NSCLC, the highest percentage of infiltration was present (70%).

Infiltrative growth beyond the border of the brain metastasis was demonstrated in 63% of the cases evaluated. Infiltrative growth, therefore, has an impact in defining the clinical target volume for SRS of brain metastases, and a margin of approximately 1 mm should be added to the visible lesion.