Treatment regimens
tumor, isodose, T1MRI, OSMS, SRS
SEPARATION, TUMOR, METASTASES
Concerning patient immobilization and treatment planning imaging, in Arm-A, a Leksell frame system was applied, and a volumetric MRI was performed. The target volume was defined as the contrast-enhancing tumor on the volumetric T1MRI. Prescription doses were 24 Gy for BMs ≤ 20 mm, and 20 Gy for lesions 21–30 mm, delivered at the 50% isodose line. No margins from target volume have been applied, and no image-guided performed before SRS treatment. The radiation dose was delivered with multi-isocenter plans so that the 50% isodose line conformed to 100% of the target volume. A basal CT scan without contrast and a post-contrast volumetric T1MRI were acquired for treatment planning in Arm-B. All the patients were immobilized with an open-face thermoplastic mask. The gross tumor volume (GTV) was defined as the contrast-enhancing tumor on the volumetric T1MRI. The planning target volumes (PTV) were generated by adding an isotropic margin of 2 mm from GTV. All plans were optimized with multiple isocentres (one per lesion) except for the cases where the separation between different metastases was inferior to 3 mm. In these cases, a shared isocentre was used. A single dose of 24 Gy was prescribed at the mean dose to PTV for metastases with a diameter ≤ 20 mm, while a dose of 20 Gy would have been prescribed for BMs with a diameter of 21–30 mm. The planning objective was that > 95% of PTV received 95% of the prescribed dose (PTV V95% > 95%) and that > 98% of GTV covered by 98% of the prescribed dose. SRS was delivered on an unit equipped with the high definition multileaf collimator with a leaf thickness at the isocentre in the target region of 2.5 mm. The Volumetric Modulated Arc Therapy technique was used, and flattening filter-free beams of 6 or 10 MV were selected for all patients. Image-Guided RT (IGRT) was performed with kV-CBCT, and patient repositioning was provided in six dimensions using an integrated six degrees of freedom treatment couch. The Optical Surface Monitoring Solution (OSMS, Varian Medical Systems, Palo Alto, US) performed real-time patient tracking during the delivery. For both arms, dose constraints to the organs at risk were the following: brain stem: V