Objective: To identify a size cutoff below which it is safe to observe obscure brain lesions suspected of being metastases so that treatment of nonmetastases can be avoided.
Methods: Medical records from patients who underwent linear accelerator-based radiosurgery from August 1991 to October 2001 were reviewed. Inclusion criteria were defined as brain metastasis tumor volume less than 5 cm(3) (diameter, thick similar 2.1 cm) treated with a dose of 20 Gy or more. One hundred thirty-five patients had 153 evaluable brain metastases with follow-up imaging that met inclusion criteria. Median age was 54 years (range, 18-79 yr). Lesion primaries were non-small-cell lung (n = 39), melanoma (n = 44), renal (n = 37), breast (n = 18), colon (n = 3), sarcoma (n = 5), other (n = 5), and unknown primary (n = 2). Median tumor volume was 0.67 cm(3) (range, 0.06-4.58 cm(3)). The minimum peripheral dose was 20 Gy (n = 132) or 21 to 24 Gy (n = 21). At the time of analysis, the median follow-up for all patients was 10 months (range, 0.2-99 mo).
Results: The 1- and 2-year actuarial local control rates for all of the lesions were 69 and 46%, respectively. For lesions of 1 cm (0.5 cm(3)) or less, the corresponding local control rates were 86 and 78%, respectively, which was significantly higher than the corresponding rates of 56 and 24%, respectively, for lesions larger than 1 cm (0.5 cm(3)) (P = 0.0016).
Conclusion: A convincing brain metastasis measuring less than 1 cm should be pursued aggressively. If the suspected brain metastasis is ambiguous, observation is proposed up to a diameter of 1 cm. This is the first study in the literature to identify a 1-cm cutoff for radiosurgical control of small brain metastases, and validation by additional studies is required.