Beam characterization for step-and-shoot IMRT

Abstract

Step-and-shoot IMRT usually involves the delivery of beam segments of small size and few monitor units. These conditions differ from the ones used during machine commissioning and quality control procedures. Several papers emphasize the importance of characterizing linear accelerator beam performance prior to the implementation of IMRT techniques. The purpose of this work was to characterize the performance of a Siemens Primus dual energy linear accelerator, producing 6 and 18 MV x-rays and equipped with a 82 leaves multi-leaf collimator (MLC), for step-and-shoot IMRT treatment delivery. The measurements were done using a slab solid water phantom with a Farmer type ionization chambe and a 2D array. The measurements were made for both energies on normal and IMRT mode. Linearity of MU was studied in the range of 1-100 MU, using a 10x10 cm2 field. Beam flatness and symmetry were evaluated for a 10x10 cm2 field size, for a range of 1-10, 12, 15, 20, 50 and 100 MU, using the 2D Array. The results are according to other published studies and show that the linear accelerator is suited for step-and-shot IMRT using segments with more than 4 MU. © 2009 Springer-Verlag.