ABSTRACT
Linear accelerators from the same vendor in a radiation therapy center are usually beam-matched following Vendor's acceptance criteria. This protocol is limited to check the difference at particular points on the ionization curve for depth dose or beam profiles. This article describes different tests done after commissioning to evaluate the level of agreement between matched beams of two ONCOR Impression plus linear accelerators from Siemens. Total scatter factors, collimator scatter factors, wedge transmission factors were measured in water for 6-MV photon. All these factors for ONCOR2 were within ±1% of those values for ONCOR1. Along with these point dose measurements we have essentially used γ-index to compare the planar dose distribution from two beam-matched accelerators. For this study a set of ready packed EDR2 films was exposed on both accelerators. The set consisted of films for percentage depth dose, beam profiles, a pyramid shape, multileaf collimator's positional and dose delivery accuracy, and a film to compare head scatter at tray level. To include treatment planning system calculations, a film kept in axial plane was exposed to 3DCRT and IMRT plans with actual gantry angles and monitor units. These films were analyzed for γ in OmniPro IMRT software using different combinations of Δdose and - Δdistances. All these films have shown good agreement for - Δdistance of 3 mm and Δdose of 3 %.
ABSTRACT
PURPOSE: This article aims to introduce a novel algorithm for fast beam angle selection in intensity modulated radiotherapy (IMRT). METHODS: The algorithm models the optimization problem as a beam angle ranking problem and chooses suitable beam angles according to their rank. A new parameter called "beam intensity profile perturbation score (BIPPS)" is used for ranking the beam angles. The BIPPS-based beam angle ranking implicitly accounts for the dose-volume effects of the involved structures. A simulated phantom case with obvious optimal beam angles is used to verify the validity of the presented technique. In addition, the efficiency of the algorithm was examined in three clinical cases (prostate, pancreas, and head and neck) in terms of DVH and dose distribution. In all cases, the judgment of the algorithm's efficiency was based on the comparison between plans with equidistant beams (equal-angle-plan) and plans with beams obtained using the algorithm (suitable-angle-plan). RESULTS: It is observed from the study that the beam angle ranking function over BIPPS instantly picks up a suitable set of beam angles for a specific case. It takes only about 15 min for choosing the suitable beam angles even for the most complicated cases. The DVHs and dose distributions confirm that the proposed algorithm can efficiently reduce the mean or maximum dose to OARs, while guaranteeing the target coverage and dose uniformity. On the average, about 17% reduction in the mean dose to critical organs, such as rectum, bladder, kidneys and parotids, is observed. Also, about 12% (averaged) reduction in the maximum dose to critical organs (spinal cord) is observed in the clinical cases presented in this study. CONCLUSIONS: This study demonstrates that the algorithm can be effectively applied to IMRT scenarios to get fast and case specific beam angle configurations.
