Factors Affecting Postoperative Surgical Cavity Volume and Surface Area Dynamics Specific to Brain Metastases

Abstract

Cavity radiation therapy with stereotactic radiosurgery (SRS) is increasing in clinical practice. Changes in diameter, volume, and surface area (SA) from the pre-operative tumor to the post-operative treatment planning MRI are largely unknown. Our study aimed to analyze the potential changes in cavity diameter, volume, and SA following surgery and determine predictive factors. 61 cavities in 59 patients with brain metastases treated with surgery followed by fractionated SRS were identified from a prospective registry and retrospectively reviewed. In each case, the SA and segmented three-dimensional (3D) volume were determined on the pre-operative, post-operative treatment planning and 1st follow-up diagnostic MRI. At each time point, the volumes and SAs were compared. Predictive factors for change in volume and SA analyzed included: location, histology, age, dural involvement, and degree of surgical resection. The median age was 61 years (range 26-86). Most commonly, resections were gross total (90%), tumors were supratentorial in location (72%), non-small cell lung cancer in histology (41%), and >3cm in diameter (57%). The median time between the pre-operative and post-operative treatment planning MRI was 32 days (11-110 days), while the median time between the post-operative MRI to first follow up MRI was 74 days (49-202 days). The mean pre-operative maximum tumor diameter, volume, and SA were 3.2cm (1.2-7.5cm), 16.0cc (1.6-52.1cc) and 59.3cm2 (3.9-299.4cm2), respectively. As compared to the pre-operative tumor on MRI, the post-operative cavity volume was smaller by 22.5% (P=0.0001). Pre-operative tumor diameter >3cm was associated with a significant reduction in volume (24.4%, p<0.05). A trend to further volume reduction was observed as compared to the 1st follow-up MRI (18.3%, p=0.12). In the pre-operative tumors ≤3cm no significant changes in 3D volume or SA were observed when comparing the pre-operative dimensions to the post-operative treatment planning or 1st follow-up MRI. Tumors without dural contact (p=0.02) and older age (p=0.01) demonstrated significantly greater reduction in cavity volume when comparing the post-operative cavity volume to the pre-operative volume. Pre-operative tumors >3cm, tumors without dural contact, and older patients have significant reduction in the volume of the cavity at the time of treatment planning as compared to the pre-operative volume. SA did not change significantly between scans. This has implications on the selection and timing of cavity SRS.