Kilovoltage X-ray Source Research Articles

Comparison of entrance surface air kerma measurement with MTS-N (LiF: Mg, Ti) chips with a kilovoltage X-ray source

Objective: Radiation detectors are key components that ensure the accuracy and performances of dosimetry equipment. The study is aimed to compare the mean entrance surface air kerma (ESAK) between a DCT-10mm ionization chamber (IC) and MTS-N (LiF: Mg, Ti) chips when both detectors are exposed to ≤ 5mGy with a 10 by 10 field size, with an X-ray source and to determine the accuracy of the Thermoluminescent (TL) chips. Also, the dose will be compared to similar studies. Materials and Methods: A functional, Digital Radiography (DR) X-ray System was used. A DCT-10mm ionization chamber (IC) and an XR Multidetector was positioned at a Source to Image Distance (SID) of 100cm on polystyrene, about 20cm thick. An X-ray spectrum generated at a Practical Peak Voltage (PPV) of 60-107kV with Half Value Layer (HVL) of 2.4-4.3mmAl and filtration > 3mmAl was used. The same setup was used for the MTS-N chips. Results: The mean doses for 1-5 mGy with the MTS-N chips were 1.07±0.07, 1.60±0.13, 2.23±0.11, 2.58±0.07 and 3.45±0.10 mGy respectively, with accuracies of 7, 20, 26, 36 and 31%. Dose accuracy at 1and 2mGy was within 25% respectively. Dose accuracies at 3, 4 and 5mGy was within >25%. The correction factor for 1-5mGy was 0.94, 1.25, 1.35, 1.55 and 1.45 respectively. Conclusion: Validation of the MTS-N chips with the reference ionization chamber to this study was within 36%. The Radiation and Nuclear Safety Authority (STUK) recommends that ESAK be within 25% for entrance surface dose. ESAK accuracy mostly increased with dose as observed in this study.

Feasibility of polymer-based composite materials as radiation shield

Researchers have tested the potential of various polymer composites for use in radiation shielding. They are light and non-toxic compared to Pb and have potential as shielding in diagnostic radiology where low-energy photons are predominantly used. This study documents the production of four composites using the combination of a base, i.e. high-density polyethylene (HDPE) polymer, and four other elements/compounds, i.e. molybdenum, molybdenum carbide, tungsten, or tungsten carbide. The resulting composites were synthesized in the form of 20 disks, each with a thickness of 2 mm. The disks were irradiated by a kilovoltage X-ray source (1.5 cm2 field size). The mass attenuation coefficient (μm), half value layer (HVL), mean free path (MFP), and equivalent atomic number of the disks were measured. We found that supplementing HDPE with additives enhanced the attenuation of beams; the μm values of the composites were higher than that of the pure HDPE polymer. The μm values of the measurement and calculation results were in good agreement with each other, with an average error of 5.2%. Among the three additive concentrations that were tested, i.e., 5, 10, and 15%, the 15% concentration in HDPE yielded the best shielding efficiency. Since they possess smaller values of HVL and MFP, the polymer composites containing W and WC performed the best at attenuating the radiation beams.

Feasibility of external beam radiation therapy to deep-seated targets with kilovoltage x-rays.

Radiation therapy to deep-seated targets is typically delivered with megavoltage x-ray beams generated by medical linear accelerators or 60 Co sources. Here, we used computer simulations to design and optimize a lower energy kilovoltage x-ray source generating acceptable dose distributions to a deep-seated target. The kilovoltage arc therapy (KVAT) x-ray source was designed to treat a 4-cm diameter target located at a 10-cm depth in a 40-cm diameter homogeneous cylindrical phantom. These parameters were chosen as an example of a clinical scenario for testing the performance of the kilovoltage source. A Monte Carlo (MC) model of the source was built in the EGSnrc/BEAMnrc code and source parameters, such as beam energy, tungsten anode thickness, beam filtration, number of collimator holes, collimator hole size and thickness, and source extent were varied. Dose to the phantom was calculated in the EGSnrc/DOSXYZnrc code for varying treatment parameters, such as the source-to-axis distance and the treatment arc angle. The quality of dose distributions was quantified by means of target-to-skin ratio and dose output expressed in D50 (50% isodose line) for a 30-min irradiation in the homogeneous phantom as well as a lung phantom. Additionally, a patient KVAT dose distribution to a left pararenal lesion (~1.6 cm in diameter) was calculated and compared to a 15 MV volumetric modulated arc therapy (VMAT) plan. In the design of the KVAT x-ray source, the beam energy, beam filtration, collimator hole size, source-to-isocenter distance, and treatment arc had the largest effect on the source output and the quality of dose distributions. For the 4-cm target at 10-cm depth, the optimized KVAT dose distribution generated a conformal plan with target-to-skin ratio of 5.1 and D50 in 30 min of 24.1 Gy in the homogeneous phantom. In the lung phantom, a target-to-skin ratio of 7.5 and D50 in 30 min of 25.3 Gy were achieved. High dose conformity of the 200 kV KVAT left pararenal plan was comparable to the 15 MV VMAT plan. The volume irradiated to at least 10% (<240 cGy) of the prescription dose was 2.2 × larger in the 200 kV KVAT plan than in the 15 MV VMAT plan, but considered clinically insignificant. This study demonstrated that conformal treatments of deep-seated targets were achievable with kilovoltage x-rays with dose distributions comparable to MV beams. However, due to the larger volumes irradiated to clinically tolerated low doses, KVAT x-ray source usage for deep-seated lesions will be further evaluated to determine optimal target size.

A national dosimetry audit of intraoperative radiotherapy.

National dosimetry audits are a fundamental part of quality assurance in radiotherapy, especially for new techniques. Intraoperative radiotherapy with a compact mobile kilovoltage X-ray source is a novel approach for the treatment of breast and other cancers. All seven current clinical sites in the UK were audited by a single visiting group and set of measurement equipment. Measurements of output, isotropy and depth doses were performed using an ion chamber in solid water, thermoluminescent dosemeters and radiochromic film, respectively. The mean difference between measured and planned dose across all centres was -3.2±2.7%. Measured isotropy was within ±3% around the lateral plane of the X-ray source and +11±4% in the forward direction compared with the lateral plane. Measured depth doses were agreed within 5±2% of manufacturer-provided calibration values or a mean gamma index of 97% at a tolerance of 7%/0.5 mm. Agreement within measurement uncertainties was found for all three parameters except forward anisotropy, which is unlikely to be clinically significant. Steep dose gradients increase the sensitivity to small variations in positioning, but these tests are practical for use in interdepartmental audits and local baseline comparison. The first UK interdepartmental audit of intraoperative radiotherapy builds confidence in the delivery of this treatment.

Monte Carlo modeling of converging small-field contrast-enhanced radiotherapy of prostate

Radiation therapy using a kilovoltage X-ray source to irradiate a target previously loaded with a radiological contrast agent, contrast-enhanced radiotherapy (CERT), has been shown both theoretically and in a preliminary experimental study to represent a potential alternative to high-energy treatments. It has also been shown, however, to produce an integral dose that can be up to twice that resulting from a conventional megavoltage treatment. In this work, using a realistic patient model and Monte Carlo simulation, a CERT prostate treatment plan is designed that makes use of a plurality of small circular beams aimed at the target in such a way as to minimize the radiological trajectory to the target volume. Gold nanoparticles are assumed to be the contrast agent. Two cases are examined, one with a concentration level in the target of 10 mg-Au per gram of tissue and the second with a concentration of 3 mg-Au per gram of tissue in the target. A background concentration of 1 mg of contrast agent per gram of tissue was assumed everywhere else in both cases. The Cimmino feasibility algorithm was then used to find each beam weight in order to obtain the prescribed target dose, set at 72 Gy to 100% of the tumor volume. It is shown that the approach using the small circular fields, a radiosurgery treatment, produces treatment plans with excellent absorbed dose distributions while at the same time it reduces by up to 60% the non-tumor integral dose imparted to the irradiated subject. A brief discussion on the technology necessary to clinically implement this treatment modality is also presented.

Evaluation of Different External Radiation Therapy Techniques for the Treatment of Malignant Pleural Mesothelioma After Extrapleural Pleuropneumonectomy

ratio and beam output. Finally, a hypothetical treatment of a 2-cm diameter target located at a depth of 11 cm and near the pancreas was simulated on patient CT images using an optimized kV x-ray source and compared to a dose distribution generated with a 6 MV beam. Results: MC simulated depth-dose curves of the existing large-area x-ray source agreed with film measurements to within 5%. Beam energy and filtration, target thickness, collimator hole geometry, and treatment SSD had the largest effect on the quality of dose distributions in terms of both target-to-skin ratio and beam output. An optimized kV x-ray source design for the 4-cm diameter target resulted in target-to-skin ratio of 5.2 and D50 of 19.5 Gy delivered in 30 minutes. For the 5 mm-diameter target, the target-to-skin ratio increased to 11.2 and D50 in 30 minutes exceeded 35 Gy. The MC models of treatment coverage for a target near the pancreas demonstrated that optimized kV and 6 MV treatment plans delivered comparable doses to critical structures. The integral dose delivered by the kV x-ray source was similar to the integral dose delivered by the 6 MV beam due to the fast fall-off of kV beams. Conclusion: A novel radiation therapy kilovoltage x-ray source has been modeled and optimized using Monte Carlo simulations. Due to its unique design, the source delivers conformal doses of radiation to deep-seated tumors with dosimetric parameters and treatment times acceptable for clinical use. A hypothetical clinical scenario showed that treatment plans with the novel cost-effective kV x-ray source have the potential to match treatment plans generated with commonly used 6 MV beams. Author Disclosure: M. Bazalova: None. E. Graves: None. B. Wilfley: None. M. Weil: P. Ownership Other; yes. Q. Leadership; yes.

Evaluation of automated image registration algorithm for image-guided radiotherapy (IGRT)

The performance of an image registration (IR) software was evaluated for automatically detecting known errors simulated through the movement of ExactCouch using an onboard imager. Twenty-seven set-up errors (11 translations, 10 rotations, 6 translation and rotation) were simulated by introducing offset up to ± 15 mm in three principal axes and 0° to ± 1° in yaw. For every simulated error, orthogonal kV radiograph and cone beam CT were acquired in half-fan (CBCT_HF) and full-fan (CBCT_FF) mode. The orthogonal radiographs and CBCTs were automatically co-registered to reference digitally reconstructed radiographs (DRRs) and planning CT using 2D-2D and 3D-3D matching software based on mutual information transformation. A total of 79 image sets (ten pairs of kV X-rays and 69 session of CBCT) were analyzed to determine the (a) reproducibility of IR outcome and (b) residual error, defined as the deviation between the known and IR software detected displacement in translation and rotation. The reproducibility of automatic IR of planning CT and repeat CBCTs taken with and without kilovoltage detector and kilovoltage X-ray source arm movement was excellent with mean SD of 0.1 mm in the translation and 0.0° in rotation. The average residual errors in translation and rotation were within ± 0.5 mm and ± 0.2°, ± 0.9 mm and ± 0.3°, and ± 0.4 mm and ± 0.2° for setup simulated only in translation, rotation, and both translation and rotation. The mean (SD) 3D vector was largest when only translational error was simulated and was 1.7 (1.1) mm for 2D-2D match of reference DRR with radiograph, 1.4 (0.6) and 1.3 (0.5) mm for 3D-3D match of reference CT and CBCT with full fan and half fan, respectively. In conclusion, the image-guided radiation therapy (IGRT) system is accurate within 1.8 mm and 0.4° and reproducible under control condition. Inherent error from any IGRT process should be taken into account while setting clinical IGRT protocol.

Tracking brachytherapy sources using emission imaging with one flat panel detector

This work proposes to use the radiation from brachytherapy sources to track their dwell positions in three-dimensional (3D) space. The prototype device uses a single flat panel detector and a BB tray. The BBs are arranged in a defined pattern. The shadow of the BBs on the flat panel is analyzed to derive the 3D coordinates of the illumination source, i.e., the dwell position of the brachytherapy source. A kilovoltage x-ray source located 3.3 m away was used to align the center BB with the center pixel on the flat panel detector. For a test plan of 11 dwell positions, with an Ir-192 high dose rate unit, one projection was taken for each dwell point, and locations of the BB shadows were manually identified on the projection images. The 3D coordinates for the 11 dwell positions were reconstructed based on two BBs. The distances between dwell points were compared with the expected values. The average difference was 0.07 cm with a standard deviation of 0.15 cm. With automated BB shadow recognition in the future, this technique possesses the potential of tracking the 3D trajectory and the dwell times of a brachytherapy source in real time, enabling real time source position verification.

Imaging in radiotherapy treatment planning and delivery

Recent years have seen major developments in radiotherapy techniques and equipment, focusing on conformal external beam techniques with the emphasis on intensity-modulated radiotherapy (IMRT). The progress of IMRT implementation has been summarized in a series of review articles in the British Journal of Radiology during 2003 and 2004. One of the fundamental prerequisites for conformal radiotherapy is the localization of the target, starting with the gross tumour volume (GTV) and moving outwards to the planning target volume (PTV). The judgement of how to define these volumes is crucial. The future value of IMRT and related techniques may depend on the underpinning images of these volumes and how they are interpreted. Fortunately, in parallel with the developments in radiotherapy delivery techniques has come an equally impressive and timely development in associated imaging. Although CT remains the gold standard for radiotherapy planning, MRI has much to offer. Superior soft tissue characterization and new developments in dynamic contrast enhanced, diffusion-weighted and diffusion tensor imaging are making their mark in target delineation. Magnetic resonance spectroscopy (MRS) promises insights into tissue metabolism and, although currently limited by large voxel sizes, may provide information on tumour response and recurrence to complement the structural information of MRI and CT. The traditional role of the simulator is now complemented by the use of the CT scanner as a virtual simulator, thus making 3D image datasets available. Verification on a fraction-by-fraction basis is now possible by several means. CT scanners within the treatment room have been used to compare images at treatment time with planning CT scans and also to measure and correct for interfraction variations in anatomy or positioning. Portal imaging has seen equally impressive developments as a result of the introduction of flat panel imaging systems. As well as their improved image quality for traditional radiographic projection imaging, when used in conjunction with cone beam reconstruction algorithms, they have made megavoltage CT a reality. Alternatively, if used with a kilovoltage X-ray source and cone beam CT algorithm, the CT reconstructions can harness the improved tissue contrast associated with the lower X-ray energies. The development of PET/CT imaging for radiotherapy planning provides, like MRS, another example of the imaging of metabolic function, but in this case the functional information is displayed within the same coordinate system as the familiar structure provided by CT. This is an unusual luxury, however, and the problems of extracting and overlaying data from different imaging studies and modalities has prompted several advances in image registration and fusion algorithms. So these are exciting times in radiotherapy imaging and this special issue reflects both the current practice and future opportunities for exploiting advances in several imaging modalities in order to increase the confidence with which modern conformal radiotherapy treatments may be planned and delivered. The British Journal of Radiology, 79 (2006), S1

SU-FF-T-212: Quality Assurance (QA) Procedure of On-Board Imager (OBI) for Portal Imaging

Purpose: On-board imager (OBI) has been implemented to improve the in-room patient positioning. It is therefore important to establish quality assurance (QA) procedure to ensure proper functionality and operational accuracy. This report presents monthly OBI QA procedure, which includes checking collision interlocks, isocenter, digital measurements, and image qualities. Method and Materials: OBI system consists of mega-voltage detector (MVD), kilo-voltage x-ray source (KVS) and detector (KVD), which are linked to the workstation. The workstation provides a platform for acquisition/analysis of images and auto-couch motion for repositioning. For collision interlock test, possible collisions are made to check sound alarm and motion interlocks of the system itself, radiation machine (LINAC), treatment couch and OBI console. The isocenters of MVD, KVS and KVD should coincide with the isocenter of the LINAC. The vertical distance from the isocenter to each device and longitudinal/lateral shift of the isocenter should be measured. The isocenter reproducibility is tested after vertical movement of each device. The digital measurement tool accuracy should be tested as a part of mechanical tests. This digital measurement test confirms magnification and accuracy of digital read for MVD and KVD. For image quality tests, the Las Vegas phantom and the Leeds TOR 18FG phantom are used for MVD and KVD respectively. The visible disks in raw and column in the Vegas image determines the contrast and resolution of MVD. The number of disks visible in the Leeds image determines the contrast and the visible line-pair per mm determines the resolution of KVD. Results: The procedure has been performed for monthly OBI QA. Only small variations within the acceptable range have been found. Conclusion: The suggested QA tests assure reliable portal images for verification of daily patient positioning using OBI. It is very important to set up a standard procedure to provide acceptable clinical service.

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Soft tissue imaging in the treatment room is one of the main challenges faced today in high-precision radiation therapy. Megavoltage cone beam CT (MV-CBCT) is a promising imaging technique for image-guided radiotherapy due to its simplicity and its potentially higher accuracy than kV-CBCT (i.e., cone beam CT with a kilovoltage x-ray source mounted on a Linac machine). MV-CBCT uses an electronic portal imaging device (EPID) attached to a Linac to acquire CT images by rotating the MV x-ray source (emitting a cone beam) and the EPID around the patient.

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Towards optimization of megavoltage cone-beam CT for image-guided radiotherapy

Kilovision: thermal modeling of a kilovoltage x-ray source integrated into a medical linear accelerator.

The thermal and thermo-mechanical (fatigue) properties of a stationary-anode kilovoltage x-ray source that can be integrated into the head of a medical linear accelerator have been modeled. A finite element program has been used to model two new target designs. The first design makes minor modifications to the existing target assembly of a Varian medical linear accelerator, while the second design adds an additional cooling tube, changes the target angle, and uses a tungsten-rhenium alloy rather than tungsten as the kilovoltage target material. The thermal calculations have been used to generate cyclic stress/strain values from which estimates of fatigue in the target designs have been made. Both kilovoltage and megavoltage operation have been studied. Analysis of the megavoltage operation shows that there are only small differences in the thermal and fatigue characteristics after the target assembly is modified to include a kilovoltage target. Thus, megavoltage operation should not be compromised. The first kilovoltage target design can handle a 900 W heat load (e.g., 120 kVp, 7.5 mA, 2 x 2 mm2 source size); the heat load being limited by the temperature at the surface of the cooling tubes and mechanical fatigue at the surface of the target. The second design can handle a 1250 W heat load (e.g., 120 kVp, approximately 10.4 mA, 2 x 2 mm2 source size). Our calculations show that installation of a kilovoltage x-ray target is practical from thermal and thermo-mechanical perspectives.