Cavity volume changes after surgery of a brain metastasis-consequences for stereotactic radiation therapy

For a large or symptomatic brain metastasis, resection and adjuvant radiotherapy are recommended. Hypofractionated stereotactic radiotherapy (HFSRT) is increasingly applied in patients with a limited number of lesions. Exact target volume definition is critical given the small safety margins. Whilst technical advances have minimized inaccuracy due to patient positioning and radiation targeting, little is known about changes in target volume. This study sought to evaluate potential changes in the resection cavity of a brain metastasis.

In all, 57 patients treated with HFSRT after surgical resection of one brain metastasis between 2008 and 2015 in our institution were included in this study. Gross tumor volume (GTV) of the initial metastasis and the volume of the resection cavity in the post-operative, planning, and follow-up MRIs were measured and compared.

The mean cavity size decreased after surgery with the greatest change of -23.4% (±41.5%) occurring between post-operative MRI and planning MRI (p < 0.01). During this time period, the cavity volume decreased, remained stable, and increased in 79.1, 3.5, and 17.4%, respectively. A further decrease of -20.7% (±58.1%) was perceived between planning MRI and first follow-up (p < 0.01). No significant difference in pattern of change could be observed depending on the volume of initial GTV, size of the post-operative resection cavity, initial or post-resection FLAIR (fluid-attenuated inversion recovery) hyper-intensity, postsurgical ischemia, or primary tumor. The resection cavities of patients with post-operative ischemia were significantly larger than resection cavities of patients without ischemia.

The resection cavity seems to be very dynamic after surgery. Hence, it remains necessary to use very recent scans for treatment planning.