Effect of Target Changes on Target Coverage and Dose to the Normal Brain in Fractionated Stereotactic Radiation Therapy for Metastatic Brain Tumors

PURPOSE: We evaluated the dosimetric effect of tumor changes in patients with fractionated brain stereotactic radiation therapy (SRT) on the tumor and normal brain using repeat verification magnetic resonance imaging (MRI) in the middle of the treatment period. METHODS AND MATERIALS: Fifteen large intracranial metastatic lesions with fractionated SRT were scanned employing standardized planning MRI (MRI-1). Repeat verification MRI (MRI-2) were performed during the middle of the irradiation period. Gross tumor volume (GTV) was defined as the volume of the contrast-enhancing lesion on T1-weighted MRI with gadolinium contrast agent. The doses to the tumor and normal brain were evaluated on the MRI-1 scan. Beam configuration and intensity on the initial volumetric modulated arc therapy plan were used to evaluate the dose to the tumor and the normal brain on MRI-2. We evaluated the effect of D(98%) (percent dose irradiating 98% of the volume) on the GTV using the plans on the MRI-1 and MRI-2 scans. For the normal brain, the V(90%), V(80%), and V(50%) (volume of the normal brain receiving >90%, 80%, and 50% of the prescribed dose, respectively) were investigated. RESULTS: Three (20% of the total) and 4 (26% of the total) tumors exhibited volume shrinkage or enlargement changes of >10%. Five (33% of the total) tumors exhibited volume shrinkage and enlargement changes of <10%. Three tumors (20% of the total) showed no volume changes. D(98%) of the GTV increased in patients with tumor shrinkage because of dose inhomogeneity and decreased in patients with tumor enlargement, with a coefficient of determination of 0.28. The V(90%), V(80%), and V(50%) increase with decreasing tumor volumes and were linearly related to the tumor volume difference with a coefficient of determination values of 0.97, 0.98, and 0.97, respectively. CONCLUSIONS: Repeat verification MRI for brain fractionated SRT during the treatment period should be considered to reduce the magnitude of target underdosing or normal brain overdosing.