Takao Ichida

Direct impact analysis of multi-leaf collimator leaf position errors on dose distributions in volumetric modulated arc therapy: a pass rate calculation between measured planar doses with and without the position errors*

We propose a new method for analyzing the direct impact of multi-leaf collimator (MLC) leaf position errors on dose distributions in volumetric modulated arc therapy (VMAT). The technique makes use of the following processes. Systematic leaf position errors are generated by directly changing a leaf offset in a linac controller; dose distributions are measured by a two-dimensional diode array; pass rates of the dose difference between measured planar doses with and without the position errors are calculated as a function of the leaf position error. Three different treatment planning systems (TPSs) were employed to create VMAT plans for five prostate cancer cases and the pass rates were compared between the TPSs under various leaf position errors. The impact of the leaf position errors on dose distributions depended upon the final optimization result from each TPS, which was explained by the correlation between the dose error and the average leaf gap width. The presented method determines leaf position tolerances for VMAT delivery for each TPS, which may facilitate establishing a VMAT quality assurance program in a radiotherapy facility.

Scatter Correction of Septal Penetration for 123 I-IMP Cerebral Blood Flow SPECT Adding Radioactivity from the Outside of Field of View―Comparison between Simulation-based and Multi-window Scatter Corrections

PURPOSE The N-Isopropyl-p-[123I] Iodoamphetamine (123I-IMP) SPECT imaging reduces the image quality and quantitative accuracy due to scatter and septal penetration occurred by radioactive uptake from outside of the field of view such as the lungs. We evaluated the influence of scatter and septal penetration using phantom-simulated radioactivity from outside of the field of view, and subsequently compared the effect of scatter and septal penetration corrections between the simulation-based effective scatter source estimation (ESSE) method and the multi-window method (ellipse approximation method). METHODS We used the phantom filled with 10 and 25 kBq/mL for the brain and lung parts corresponding to radioactive concentration in the clinical study. The SPECT images were acquired with and without lung phantom using low-energy high-resolution (LEHR) and cardiac high-resolution (CHR) collimators. We quantitatively evaluated a brain phantom by count analysis and coefficient of variation as reference data without lung phantom simulated the radioactivity from outside of the field of view, and compared between two scatter corrections by each collimator. RESULTS The brain count in cerebral base with the ESSE method using LEHR collimator was higher than that of the ellipse approximation method. The whole brain count with the ellipse approximation method using CHR collimator shows 28.8% lower than the ESSE method, so that it suggests that the ellipse approximation method for LEHR collimator and the ESSE method for CHR collimator was close to reference counts. The coefficient of variation of the ESSE method was lower than that of the ellipse approximation method for both two collimators. CONCLUSIONS It was possible to correct the scatter and penetration from outside the field of view with high accuracy, by using the ellipse approximation method with LEHR collimator and the ESSE method with CHR collimator.

SU‐E‐J‐12: Flex Compensation for a KV Cone Beam Computed Tomography System Integrated with Linear Accelerator: A Comparison with and without Add‐On Micro Multi‐Leaf Collimator

Purpose: Due to the weight of the accelerator head and the additional weight of the kV x‐ray tube along with its flat panel detector, flex compensation is required to match the kV cone beam CTimage center to the MV isocenter. However, the flex compensation has not been compared with and without an add‐on micro multi‐leaf collimator. The purpose of this study is to evaluate the impact of the add‐on micro multi‐leaf collimator on the flex compensation or flex map calculation. Methods: Before attaching an add‐on micro multi‐leaf collimator to Elekta Synergy, MV radiation center was determined by a ball bearing and the flex map was obtained. Subsequently, a micro multi‐leaf collimator was attached to the Synergy head and Winston‐Lutz test was performed to locate the most reasonable MV isocenter. Then a ball bearing was positioned at the MV isocenter., and another flex map was obtained and compared to the previously obtained one acquired before attaching the micro multi‐leaf collimator. Results: MV isocenter discrepancy with and without the micro multi‐leaf collimator was 0.5 mm in GT direction which is less than our tolerance limit. The flex map discrepancy with and without the micro multi‐leaf collimator was not significant, either. Conclusions: We could not detect significant flex map discrepancy with and without the micro multi‐leaf collimator. However, flex map measurement needs to be repeated after fully unifying the method for determining the MV isocenter between the two configurations.

Basic evaluation of physical characteristics and clinical study of new digital subtraction angiography system with a large-area flat-panel detector

Authors could have a clinical experience to use an angiographic system incorporating FPD. This system possesses both FPD and I.I.-CCD detectors and is capable of conducting imaging in the same geometry for both detectors. We made measurement of basic physical characteristics of this system and visual evaluation using arteriographies of lower limbs processed by IVDSA, and confirmed that FPD is an excellent detector in angiographic examination. In the foreseeable future, angiographic system will be replaced by FPD system.