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

Background Postoperative stereotactic radiosurgery (SRS) to the cavity following resection of brain metastases (BM) improves local control (LC). We hypothesized that significant cavity constriction would take place from the immediate postoperative period to the time of SRS and aimed to elucidate optimal treatment timing. Methods We retrospectively reviewed seventy-nine consecutive patients with eighty-five resection cavities treated with SRS following gross total resection of a BM. The 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 then analyzed. Results Median immediate postoperative cavity volume was 7.5 cc and median SRS cavity volume was 8.7 cc. Median time from surgery to SRS was 20 days. The median volumetric cavity change was an increase of 28%. Thirty-four cavities (40%) increased in size > 2 cc, while only eight cavities (9%) decreased in size > 2 cc. Forty-three cavities (51%) had ≤ 2 cc 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 following BM resection increased a median of 28% from immediate postoperative scan to time of SRS. Greater than 90% of postoperative cavities either increased > 2 cc or remained within 2 cc of their immediate postoperative cavity volume. Early postoperative SRS within two to three weeks may be appropriate to minimize cavity growth. Delaying postoperative SRS beyond three weeks time in hopes of significant cavity contraction is not warranted.