Postsurgical Cavity Evolution After Brain Metastasis Resection: How Soon Should Postoperative Radiosurgery Follow?

Background: Postoperative stereotactic radiosurgery (SRS) to the cavity after resection of brain metastases improves local control. We hypothesized that significant cavity constriction would occur from the immediate postoperative period to the time of SRS and aimed to elucidate optimal treatment timing.

Methods: We retrospectively reviewed 79 consecutive patients with 85 resection cavities treated with SRS after gross total resection of a brain metastasis. Preoperative lesion, immediate postoperative cavity, and cavity at the time of SRS were contoured for each patient. Factors influencing cavity size and interval cavity change were analyzed.

Results: Median immediate postoperative cavity volume was 7.5 cm³, and median SRS cavity volume was 8.7 cm³. Median time from surgery to SRS was 20 days. Median volumetric cavity change was an increase of 28%. Of cavities, 34 (40%) increased in size >2 cm³, whereas only 8 cavities (9%) decreased in size >2 cm³; 43 cavities (51%) had ≤2 cm³ change. The largest postoperative cavities experienced the smallest percentage cavity change in the time interval to SRS (Spearman correlation -0.32, P = 0.003).

Conclusions: Cavity size after brain metastasis resection increased a median of 28% from immediate postoperative scan to time of SRS. Greater than 90% of postoperative cavities either increased >2 cm³ or remained within 2 cm³ of their immediate postoperative cavity volume. Early postoperative SRS within 2-3 weeks may be appropriate to minimize cavity growth. Delaying postoperative SRS beyond 3 weeks in hopes of significant cavity contraction is not warranted.