3. Dosimetric comparison of flattened and unflattened beams for stereotactic of brain metastasis

Introduction The aim of this study was to investigate the dosimetric impact of flattening filter free (FFF) beams with a nominal energy of 6 MV for brain metastasis treatments in stereotactic conditions. Methods This study was performed in 10 patients with one brain metastasis. A comparison of two different volumetric modulated arc therapy plans were prepared using a 6 MV flattened photon beam (X6) and a unflattened beam with a nominal energy of 6 MV (X6FFF). The plans were optimised on the Pinnacle TPS v9.10 (Philips). Optimization constraints were set to produce dose distributions that meet the criteria of the STIC1 and ANOCEF referential 2. A dose of 33 Gy in 3 fractions was prescribed to the isocenter. 99% of PTV should be covered by 70% of the prescribed dose. The dose rate used for the X6 was 600 UM/min and 1200 UM/min for the X6FFF. The treatments were performed on a Truebeam Novalis STX (Varian). Results Obtained dose distributions were conformal (Conformity index >0.77). Volumes of 20 Gy isodose (V20Gy) for X6 and X6FFF were 11.64 cm 3  ± 5.8 cm 3 and 10.67 ± 5.71 cm 3 , respectively. The volumes of the 10 Gy isodose (V10Gy) for X6 and X6FFF were 41.59 ± 17.2 cm 3 and 38.28 ± 14.63 cm 3 , respectively. The volume of the 5 Gy isodose (V5Gy) for X6 and X6FFF were 142.3 ± 61.3 cm 3 and 130.3 cm 3  ± 43.4 cm 3 , respectively. The gradient indices in X6 and X6FFF were respectively 4 ± 0.88 and 3.77 ± 0.73. The number of monitor units (UM) was 1700.8 ± 270.1 UM for the X6 and 1712.3 ± 351.5 UM for the X6FFF. The delivery times were 178.4 ± 28.3 s for the X6 and 179.6 ± 36.9 s for the X6FFF. Conclusions V20Gy, V10Gy and V5Gy are the same for both plans. X6FFF beam does not improve dose conformity index or decrease delivery time. Dose rate in X6FFF is more variable than X6 beams.