Effect of ROI filtering in 3D cone-beam rotational angiography on organ dose and effective dose in cerebral investigations.

The assessment of intracranial aneurysms is increasingly performed using three-dimensional cone-beam rotational angiography (3D CBRA). To reduce the dose to the patient during 3D CBRA procedures, filtered region-of-interest imaging (FROI) is presented in literature to be an effective technique as the dose in regions of low interest is reduced, while high image quality is preserved in the ROI. The purpose of this study was to quantify the benefit of FROI imaging during a typical 3D CBRA procedure in a patient's head region. A cone-beam rotational angiography unit (Infinix) was modeled in GMctdospp, an EGSnrc-based Monte Carlo software, which calculates patient dose distributions in rotational computed tomography. Kodak Lanex, a gadolinium compound, was chosen to be the ROI filter material. The adult female ICRP reference phantom was integrated in GMctdospp to calculate organ and effective doses in simulations of FROI-CBRA examinations. During the Monte Carlo simulations, different parameters as the ROI filter thickness, the ROI opening size, the tube voltage, and the isocenter position were varied. The results showed that the reduction in dose clearly depends on these parameters. Comparing the reduction in organ dose in standard 3D CBRA and FROI-CBRA, a maximum reduction of about 60%-80% could be achieved with a small sized ROI filter and about 40%-70% of the dose could be saved using a ROI filter with a large opening. Further we could show that dose reduction strongly depends on filter thickness, the location of the organ in the radiated area, and the position of the isocenter. As a consequence, dose reduction partially differs from theoretically calculated values by a factor up to 1.6. The effective dose could be reduced to a minimum of about 40%. Due to the fact that standard 3D CBRA is only used for the assessment of aneurysms at present and, thus, most of the patient dose originates from the aneurysm treatment (with 2D techniques) itself, the dose reduction effect of ROI filtering in 3D CBRA tends to be much smaller, if the patient dose of a whole aneurysm treatment procedure is considered.

Patient specific evaluation of breathing motion induced interplay effects.

PURPOSE: In lung SABR, interplay between target motion and dynamically changing beam parameters can affect the target coverage. To identify the potential need for motion-management techniques, a comprehensive methodology for pre-treatment estimation of interplay effects has been implemented. METHODS: In conjunction with an alpha-version of VeriSoft and OCTAVIUS 4D (PTW-Freiburg, Germany), a method is presented to calculate a virtual, motion-simulated 3D dose distribution based on measurement data acquired in a stationary phantom and a subsequent correction with time-dependent target-motion patterns. In-house software has been developed to create user-defined motion patterns based on either simplistic or real patient-breathing patterns including the definition of the exact beam starting phase. The approach was validated by programmed couch and phantom motion during beam delivery. Five different breathing traces with extremely altered beam-on phases (0 % and 50 % respiratory phase) and a superior-inferior motion altitude of 25 mm were used to probe the influence of interplay effects for 14 lung SABR plans. Gamma analysis (2 %/2mm) was used for quantification. RESULTS: Validation measurements resulted in >98 % pass rates. Regarding the interplay effect evaluation, gamma pass rates of <92 % were observed for sinusoidal breathing patterns with <25 number of breaths per delivery time (NBs) and realistic patterns with <18 NBs. CONCLUSION: The potential influence of interplay effects on the target coverage is highly dependent on the patient's breathing behaviour. The presented moving-platform-free approach can be used for verification of ITV-based treatment plans to identify whether the clinical goals are achievable without explicit use of a respiratory management technique.

Operation and calibration of the novel PTW 1600SRS detector for the verification of single isocenter stereotactic radiosurgery treatments of multiple small brain metastases.

OBJECTIVES: The aim of this work was to evaluate the operation of the 1600SRS detector and to develop a calibration procedure for verifying the dose delivered by a single isocenter stereotactic radiosurgery (SRS) treatment of small multiple brain metastases (BM). METHODS: 14 clinical treatment cases were selected with the number of BM ranging from 2 to 11. The dosimetric agreement was investigated between the calculated and the measured dose by an OCTAVIUS 1600SRS array detector in an OCTAVIUS 4D phantom equipped with dedicated SRS top. The cross-calibration procedure deviated from the manufacturer's as it applied field sizes and dose rates corresponding to the volumetric modulated arc therapy segments in each plan. RESULTS: Measurements with a plan specific cross-calibration showed mean ± standard deviation (SD) agreement scores for cut-off values 50%, 80%, 95%, of 98.6 ± 1.7%, 96.5 ± 4.6%, 97.3 ± 4.4% for the 6 MV plans respectively, and 98.6 ± 1.5%, 96.6 ± 4.0% 96.4 ± 6.3%, for the 6 MV flattening filter free (FFF) plans respectively. Using the default calibration procedure instead of the plan specific calibration could lead to a combined systematic dose offset of 4.1% for our treatment plans. CONCLUSION: The 1600SRS detector array with the 4D phantom offers an accurate solution to perform routine quality assurance measurements of single isocenter SRS treatments of multiple BM. This work points out the necessity of an adapted cross-calibration procedure. ADVANCES IN KNOWLEDGE: A dedicated calibration procedure enables accurate dosimetry with the 1600SRS detector for small field single isocenter SRS treatment of multiple brain metastases for a large amount of BM.