(P033) MRI Resection Cavity Dynamics Following Brain Metastasis Resection and Permanent Iodine-125 Brachytherapy

David R. Raleigh, MD, PhD, Zachary A. Seymour, MD, Bryan Tomlin, PhD, Michael W. McDermott, MD, Manish K. Aghi, MD, Philip V. Theodosopolous, MD, Mitchel S. Berger, MD, Penny K. Sneed, MD; University of California, San Francisco; California State University Channel Islands

PURPOSE/OBJECTIVES: Surgical resection and permanent iodine-125 (125I) brachytherapy provide good local control for brain metastases. However, resection cavity remodeling has been postulated to alter treatment efficacy and toxicity. The purpose of this study was to investigate cavity volumetrics following surgery and 125I brachytherapy for brain metastases.

MATERIALS AND METHODS: A total of 96 patients with 106 brain metastases, treated from September 1997 to July 2013, were retrospectively identified. The efficacy and toxicity of 125I brachytherapy in this cohort have previously been reported. In brief, the median age at surgery was 59 years, with a median overall survival of 12 months and overall local control of 92%. The overall risk of necrosis was 15% and trended lower without prior stereotactic radiosurgery (SRS) to the surgical site or with activity ≤ 0.73 mCi per source. All patients underwent magnetic resonance imaging (MRI) prior to surgery and were followed with surveillance MRIs beginning a median of 1 day after implantation (MRI1). Volumetric data were calculated from three-dimensional contours on T1-weighted postcontrast images by a single radiation oncologist (DRR). Cavities were censured from volumetric analyses at the time of tumor progression or necrosis.

RESULTS: A total of 476 brain MRIs were analyzed (median 3 per patient; range: 0–22) with a median imaging follow-up of 4 months (range: 0 mo–13.6 yr). Median metastasis volume was 13.5 cm³ (range: 0.21–76.2 cm³), and median cavity volume on MRI1 was 5.2 cm³ (n = 101; range: 0.3–23.2 cm³). At a median of 1.7 (n = 32), 3.6 (n = 46), 5.9 (n = 38), 11.7 (n = 30), and 20.5 (n = 22) months after surgery, cavity volumes decreased by 25%, 35%, 42%, 47%, and 60% relative to MRI1, respectively. Metastasis size was the strongest predictor of cavity volume and shrinkage on both univariate and multivariate linear regression modeling (P < .0001). Factors that were associated with an increase in cavity volume included prior SRS to the surgical site, periventricular location, and lobar tip location (P < .05). The cavity-to-metastasis volume ratio decreased with increasing metastasis size, and the rate of cavity shrinkage was greater for metastases > 13.5 cm³ (P < .0001). Despite these findings, multinomial logistic regression modeling, with or without adjustment for source activity and prior SRS, failed to predict the likelihood of local failure or necrosis using either metastasis or cavity volume.

CONCLUSIONS: Surgical resection with 125I brachytherapy is an effective strategy for local control of brain metastases. Although metastasis volume significantly influences resection cavity size and remodeling, volumetric parameters do not appear to influence local control or necrosis.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org