Introduction: Radiosurgery or brachytherapy to a surgical cavity after resection of brain metastases are promising treatments for local control. It is not known, however, how the initial resection cavity changes in the interval between resection and subsequent treatment, a factor that can affect prescription doses.
Methods: We conducted a retrospective review of patients with a brain metastasis treated with surgical resection followed by radiosurgery or brachytherapy to the resection cavity. Post-operative and pre-radiosurgical/brachytherapy magnetic resonance images were reviewed and the resection cavity volume was measured using volumetric analysis.
Results: We identified 39 consecutive patients with brain metastases treated with surgical resection followed by radiosurgery or brachytherapy within 3 months. Mean age was 52.6±12.3 years and median time between post-operative and pre-radiosurgical/brachytherapy MRIs was 38 days (range 7-85). The mean resection cavity volume after surgical resection was 8.3±9.3cm3 and shrank to a mean of 4.7±5.6cm3 at time of repeat imaging for radiosurgical and brachytherapy planning. Univariate analysis showed that the greatest reduction in resection cavity volume was seen between 1 and 2 months post-resection (compared to <1 and 2-3 months, p=0.0082). Using stepwise regression analysis, we constructed a model to predict the change in volume between scans incorporating post-operative cavity volume (p<0.0001), time interval between scans (p=0.004), and the square of the time interval between scans (p=0.004; overall model R2=0.76, p<0.001).
Conclusions: The surgical cavity after brain metastasis resection constricts with time at a rate that is associated with initial cavity volume and a quadratic function of time. The optimal time to perform radiosurgery or brachytherapy to a resection cavity may be between 1-2 months when the reduction in cavity volume is greatest. These findings also suggest that resection cavities may begin to expand after 2 months.
Patient Care: These findings identify factors associated with resection cavity volume dynamics after surgical resection of brain metastases. Timing radiosurgery or brachytherapy to a resection cavity to coincide with time of maximal volume constriction can minimize the necessary radiation dose and injury to adjacent brain.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) identify factors associated with resection cavity constriction after surgical resection of brain metastases and 2) identify the optimal timing for post-surgical radiosurgery or brachytherapy based on maximal resetion cavity restriction.