Five-Fraction Stereotactic Radiosurgery With Non-Contrast-Enhanced MRI-Based Target Definition and Moderate Dose Spillage Margin for Limited Brain Metastases With Impaired Renal Function

In stereotactic radiosurgery (SRS) planning for brain metastases (BMs), the target volume is usually defined as an enhancing lesion based on contrast-enhanced (CE) magnetic resonance images (MRI) and/or computed tomography (CT) images. However, contrast media (CM) are unsuitable for certain patients with impaired renal function. Herein, we describe two limited BM cases not amenable to CM, which were treated with five-fraction (fr) SRS, without whole brain radiotherapy (WBRT), through a target definition based on non-CE-MRI. These included synchronous and partly symptomatic four BMs from esophageal squamous cell carcinoma (Case 1) and one presymptomatic regrowing lesion after WBRT for BMs from lung adenocarcinoma (Case 2). In both cases, all BMs were visualized as well-demarcated mass lesions almost distinguishable from the affected parenchyma on non-CE-MRI, particularly on T2-weighted images (WI). The gross tumor volume (GTV) was defined mainly based on T2-WI under a comprehensive comparison of non-CE-T1/T2-WIs and CT for SRS planning under image co-registration and fusion. Stereotactic radiosurgery was implemented with volumetric modulated arcs using a 5-mm leaf width multileaf collimator, for both of which 5 fr was selected, considering the maximum tumor volume and the effects from WBRT, respectively. Dose distribution was designed to ensure a moderate dose attenuation margin outside the GTV boundary and a concentrically-laminated steep dose increase inside the GTV boundary. Specifically, the peripheries of the GTV and 2 mm outside the GTV boundary were covered by ≥43 Gy with <70% isodose relative to the maximum dose and ≥31 Gy, respectively. The not-too-steep dose spillage margin can cover potentially invisible tumor invasion outside the GTV and other inherent uncertainties regarding target definition and irradiation accuracy. Post-SRS tumor responses were excellent clinically and/or radiographically with mild adverse radiation effects in Case 2. In limited BM cases unsuitable to CM, multi-fraction SRS with non-CE-MRI-based GTV definition and sufficient GTV dose along with moderate dose spillage margin would be a valuable treatment option for selected cases, with the entire GTV boundaries being almost visible on non-CE-MRI.