Changes in Target Coverage and Dose to the Normal Brain during Fractionated Stereotactic Radiotherapy for Metastatic Brain Tumors

Abstract

Fractionated stereotactic radiotherapy (SRT) has been applied to large brain metastases to reduce the risk of radiation necrosis. For fractionated SRT, potential of interfractional tumor changes during the treatment period, such as tumor size, shape, and geometry, must be considered to improve the tumor local control. Our center performs adaptive re-planning for large brain metastases based on repeat MRI verification with a contrast agent in the middle of the treatment period. Purpose of this study is to evaluate the dosimetric impact of the changes in tumor size, shape, and geometry on the doses to the targets and normal brain in patients with brain metastases undergoing fractionated SRT. Fifteen solitary large intracranial metastatic lesions treated with fractionated SRT were investigated. Standardized planning MRI (MRI-1) and repeat verification MRI (MRI-2) were performed during the middle of the irradiation period. The GTV on the MRI-1 and MRI-2 scans was contoured by the same oncologist. The PTV was created by adding an isotropic margin of 1 mm from the GTV in all directions. Volumetric modulated arc therapy (VMAT) with beam energies of 6 MV (flattening filter-free mode) was used and plans were normalized such that PTV D95% or D98% was equal to the prescribed dose. Beam configuration and intensity on the initial VMAT plan were used to evaluate the dose to the tumor and the normal brain on MRI-2. We evaluated the impact of D98% on the GTV using the plans on the MRI-1 and MRI-2 scans. For the normal brain, the V90%, V80%, and V50% were investigated. The median GTV changed from 9.8 cc (range of 3.2-33.0 cc) to 9.7 cc (range of 2.8-36.5 cc) (p = 0.482). Three and four tumors exhibited volume shrinkage and enlargement changes of >10%. Five tumors exhibited volume shrinkage and enlargement changes of <10%. Three tumors showed no volume changes. Of the 15 large brain metastases, 12 tumors required treatment plan modification. The dosimetric parameters of the GTV, PTV, and normal brain did not significantly differ between the MRI-1 and MRI-2 scans. Regarding the tumor dose, the D98% to the GTV increased in patients with tumor shrinkage because of dose inhomogeneity and decreased in patients with tumor enlargement. The V90%, V80%, and V50% increase with decreasing tumor volumes and were linearly related to the tumor volume difference, with a coefficient of determination of 0.97, 0.98, and 0.97, respectively. Our study demonstrated the usefulness of repeat verification MRI for adaptive radiotherapy in the middle of the treatment period due to changes in tumor size, shape, and geometry in patients with brain metastases. Repeated MRI should be considered to evaluate the dose to the target and normal brain, which improves tumor local control and reduces brain necrosis, to reduce the magnitude of underdosing to the target or overdosing to the normal brain during the treatment period.