Brain metastasis treatment plans for contrast-enhanced synchrotron radiation therapy

Introduction Contrast-enhanced stereotactic synchrotron radiotherapy (SSRT) is an innovative treatment based on localized dose enhancement effects obtained by reinforced photoelectric absorption in brain tumors. Tumors previously loaded with high-Z elements are irradiated using medium-energy monochromatic x-rays (50–100 keV). The aim of this study is to compare SSRT treatment in the presence and absence of iodine enhancement. For a single case, SSRT was compared to conventional treatments: 3D conformational radiotherapy, modulated radiation therapy (IMRT) and dynamic stereotactic arc-therapy. Material and methods Patients bearing brain metastasis received a standard helical 3D-scan without iodine. A second scan was acquired 13 min after an 80 g iodine infusion. Two SSRT treatment plans (with/without iodine) were performed for each patient. A dedicated Monte Carlo (MC) treatment planning system (TPS) based on the ISOgray TPS was developed for this purpose. Ten coplanar beams (6 * 6 cm 2 , shaped with collimator) were simulated. MC statistical error objective was less than 5% in the 50% isodose. The treatment plan validation criteria were fixed such as 100% of the prescribed dose is delivered at the beam isocentre and the 70% isodose contains the whole target volume. The comparison elements were the 70% isodose volume, the average and maximal doses delivered to organs at risk (OAR): brainstem, optical nerves, chiasma, eyes, skull bone and healthy brain parenchyma. For the single case study, the 3D conformational radiotherapy and IMRT treatment plans were established using the Eclipse ® -Varian TPS. The dynamic arc-therapy plan was achieved on the Iplan ® -Brainlab TPS. Results Iodine-enhanced SSRT exhibits a good potential for brain metastasis treatment regarding the dose distribution and OAR criteria. Iodine accumulation in metastasis may increase the dose of 20– 30% (sparing effect). Treatment without any iodine enhancement (medium-energy stereotactic radiotherapy) is not relevant with degraded HDVs (brain, parenchyma and skull bone) comparing to conventional stereotactic surgery. In the single case study, the stereotactic dynamic arc-therapy remains the best technique in terms of dose conformation. However, OAR sparing is increased with iodine-enhanced SSRT: the dose delivered to the brainstem and the brain parenchyma is significantly reduced. Conclusion This study shows the potential dosimetric advantages of contrast-enhanced SSRT for treating brain metastasis.