Automated algorithm for CBCT-based dose calculations of prostate radiotherapy with bilateral hip prostheses

ABSTRACTOBJECTIVE:Cone beam CT (CBCT) images contain more scatter than a conventional CT image and therefore provide inaccurate Hounsfield units (HUs). Consequently, CBCT images cannot be used directly for radiotherapy dose calculation. The aim of this study is to enable dose calculations to be performed with the use of CBCT images taken during radiotherapy and evaluate the necessity of replanning.METHODS:A patient with prostate cancer with bilateral metallic prosthetic hip replacements was imaged using both CT and CBCT. The multilevel threshold (MLT) algorithm was used to categorize pixel values in the CBCT images into segments of homogeneous HU. The variation in HU with position in the CBCT images was taken into consideration. This segmentation method relies on the operator dividing the CBCT data into a set of volumes where the variation in the relationship between pixel values and HUs is small. An automated MLT algorithm was developed to reduce the operator time associated with the process. An intensity-modulated radiation therapy plan was generated from CT images of the patient. The plan was then copied to the segmented CBCT (sCBCT) data sets with identical settings, and the doses were recalculated and compared.RESULTS:Gamma evaluation showed that the percentage of points in the rectum with γ < 1 (3%/3 mm) were 98.7% and 97.7% in the sCBCT using MLT and the automated MLT algorithms, respectively. Compared with the planning CT (pCT) plan, the MLT algorithm showed −0.46% dose difference with 8 h operator time while the automated MLT algorithm showed −1.3%, which are both considered to be clinically acceptable, when using collapsed cone algorithm.CONCLUSION:The segmentation of CBCT images using the method in this study can be used for dose calculation. For a patient with prostate cancer with bilateral hip prostheses and the associated issues with CT imaging, the MLT algorithms achieved a sufficient dose calculation accuracy that is clinically acceptable. The automated MLT algorithm reduced the operator time associated with implementing the MLT algorithm to achieve clinically acceptable accuracy. This saved time makes the automated MLT algorithm superior and easier to implement in the clinical setting.ADVANCES IN KNOWLEDGE:The MLT algorithm has been extended to the complex example of a patient with bilateral hip prostheses, which with the introduction of automation is feasible for use in adaptive radiotherapy, as an alternative to obtaining a new pCT and reoutlining the structures

Biological effects of exposure to magnetic resonance imaging: an overview

The literature on biological effects of magnetic and electromagnetic fields commonly utilized in magnetic resonance imaging systems is surveyed here. After an introduction on the basic principles of magnetic resonance imaging and the electric and magnetic properties of biological tissues, the basic phenomena to understand the bio-effects are described in classical terms. Values of field strengths and frequencies commonly utilized in these diagnostic systems are reported in order to allow the integration of the specific literature on the bio-effects produced by magnetic resonance systems with the vast literature concerning the bio-effects produced by electromagnetic fields. This work gives an overview of the findings about the safety concerns of exposure to static magnetic fields, radio-frequency fields, and time varying magnetic field gradients, focusing primarily on the physics of the interactions between these electromagnetic fields and biological matter. The scientific literature is summarized, integrated, and critically analyzed with the help of authoritative reviews by recognized experts, international safety guidelines are also cited

Optimization of biphasic waveforms for human nonthoracotomy defibrillation.

Background. Biphasic waveforms reduce defibrillation threshold (DFT) in a wide variety of models. Although there are several human studies of long- duration, high-tilt biphasic waveform defibrillation, the specific biphasic waveform shape required to achieve optimal DFT reduction is unknown. Methods and Results. This study tested the effect of single capacitor biphasic waveform tilt modification on DFT using a paired study design in 18 patients undergoing nonthoracotomy defibrillator implantation. Baseline DFT was obtained using a 65% tilt, simultaneous pulse, bidirectional monophasic shock from a right ventricular cathode to a coronary sinus or superior vena cava lead and a subscapular patch. The single-capacitor biphasic waveform shocks, delivered over the same pathways, consisted of either both phases at 65% tilt (65/65 biphasic waveform) to produce an overall tilt of 88% and a delivered energy 11% greater than monophasic shock or both phases at 42% tilt (42/42 biphasic waveform) to produce an overall tilt of 66% and delivered energy equal to monophasic shock. The 65/65 biphasic waveform reduced stored energy DFT 25%, from 16.2±4.4 J with monophasic shock to 12.1±5.3 J (P\u3c.02); however, it did not significantly reduce the delivered energy DFT. In contrast, the 42/42 biphasic waveform required 49% less stored energy (16.2±4.4 J, monophasic shock, vs 8.3±3.3 J, biphasic waveform; P\u3c.001) and 49% less delivered energy (14.2±3.8 J, monophasic shock, vs 7.3±2.9 J, biphasic waveform; P\u3c.001) than monophasic shock for successful defibrillation. The 42/42 biphasic waveform delivered energy DFT was 4.6±5.2 J (39%) less than 65/65 biphasic waveform DFT (P\u3c.002). Conclusions. DFT reduction is an inherent electrophysiological property of biphasic waveforms that is independent of delivered energy. Overall biphasic waveform tilt and the relative amplitudes of the waveform phases are important factors in defibrillation efficacy. Defibrillation with a 42/42 biphasic waveform is more efficacious than 65/65 biphasic waveform defibrillation; however, the optimal biphasic waveform remains unknown