Advances in Intensity-Modulated Radiotherapy Delivery
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Keywords:
Tomotherapy
Cyberknife
Multileaf collimator
Image-guided radiation therapy
A preliminary goal of radiation therapy is to deliver radiation doses that conform to targets, while minimizing dose to surrounding normal tissue. Intensity-modulated radiation therapy (IMRT) delivered with a multileaf collimator (MLC) is a promising resource to achieve conformal therapy. A medical linear accelerator (linac) equipped with an IMRT system can increase dose to the target and decrease dose to normal tissue. These capabilities may be significantly enhanced with dose optimization techniques. IMRT integrates rapidly evolving imaging, computer, optimization, and treatment delivery technologies. Resulting systems fundamentally distinguish themselves from prior technology. Thus, new quality assurance (QA) procedures are needed.
Multileaf collimator
Image-guided radiation therapy
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A fully integrated system for treatment planning, application, and verification for automated multileaf collimator (MLC) based, intensity‐modulated, image‐guided, and adaptive radiation therapy (IMRT, IGRT and ART, respectively) is proposed. Patient comfort, which was the major development goal, will be achieved through a new unit design and short treatment times. Our device for photon beam therapy will consist of a new dual energy linac with five fixed treatment heads positioned evenly along one plane but one electron beam generator only. A minimum of moving parts increases technical reliability and reduces motion times to a minimum. Motion is allowed solely for the MLCs, the robotic patient table, and the small angle gantry rotation of . Besides sophisticated electron beam guidance, this compact setup can be built using existing modules. The flattening‐filter‐free treatment heads are characterized by reduced beam‐on time and contain apertures restricted in one dimension to the area of maximum primary fluence output. In the case of longer targets, this leads to a topographic intensity modulation, thanks to the combination of “step and shoot” MLC delivery and discrete patient couch motion. Owing to the limited number of beam directions, this multislice cone beam serial tomotherapy is referred to as “multibeam tomotherapy.” Every patient slice is irradiated by one treatment head at any given moment but for one subfield only. The electron beam is then guided to the next head ready for delivery, while the other heads are preparing their leaves for the next segment. The “Multifocal MLC‐positioning” algorithm was programmed to enable treatment planning and optimize treatment time. We developed an overlap strategy for the longitudinally adjacent fields of every beam direction, in doing so minimizing the field match problem and the effects of possible table step errors. Clinical case studies show for the same or better planning target volume coverage, better organ‐at‐risk sparing, and comparable mean integral dose to the normal tissue a reduction in treatment time by more than 50% to only a few minutes in comparison to high‐quality 3‐D conformal and IMRT treatments. As a result, it will be possible to incorporate features for better patient positioning and image guidance, while sustaining reasonable overall treatment times at the same time. The virtual multibeam tomotherapy design study TOM’5‐CT contains a dedicated electron beam CT (TOM'AGE) and an objective optical topometric patient positioning system (TOPOS ® ). Thanks to the wide gantry bore of and slim gantry depths of , patients can be treated very comfortably, in all cases tumor‐isocentrically, as well as with noncoplanar beam arrangements as in stereotactic radiosurgery with a couch rotation of up to . The TOM’5 treatment unit on which this theoretical concept is based has a stand‐alone depth of and an outer diameter of ; the focus‐isocenter distance of the heads is with a field size of and leaves, which operate perpendicular to the axis of table motion.
Tomotherapy
Multileaf collimator
Image-guided radiation therapy
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Collimator
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Pre-treatment cone beam CT image guidance combined with linear accelerator based intensity-modulated radiotherapy (IMRT) is compared to and contrasted with tomotherapy an alternative commercially available IMRT planning and delivery system. The underlying rationale of the two different IMRT approaches are discussed and their methods pre-treatment image guidance and modes of treatment delivery are described. Differences between dose-distributions delivered using tomotherapy and conventional linear accelerators are outlined. Because conventional linear accelerator design has been refined over many decades, innovative design enhancement of one aspect of system performance often limits another facet of system capability. Consequently the two IGRT/IMRT delivery systems may prove optimal for different types of treatment, proving advantageous for certain disease sites while being of almost equal utility in others. Educational Objectives: 1. Understand the differences and similarities in pre-treatment image guidance for the two systems discussed. 2. Understand the differences and similarities in treatment delivery for the two systems discussed.
Tomotherapy
Image-guided radiation therapy
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Delivery system
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Quality assurance (QA) of the multileaf collimator (MLC) is a critical step for the delivery of intensity modulated radiation therapy treatment plan. While QA procedures for motor‐driven MLC have been published extensively, those for binary MLCs such as the one used for helical tomotherapy have not been presented in the literature, as this is still a fairly new technology. In this study, seven test patterns for the MLC QA of a helical tomotherapy unit have been designed and implemented. The seven test patterns check the MLC alignment, MLC leakage, MLC timing and MLC leaf position error in detail. Those patterns can be easily implemented in any center with a helical tomotherapy unit as part of the routine QA. The QA procedures can be performed using existing QA resources such as solid water phantom and EDR2 film. A software toolkit called “Tomo MLC QA” has been developed to assist in generating the QA procedures and analyzing the results. Our results showed that the helical tomotherapy MLC is very robust, exhibiting interleaf leakage of . Several issues with the MLC have been found and discussed. The QA results also illustrate the utilization and usefulness of the proposed QA procedures.
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Multileaf collimator
Collimator
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Tomotherapy
Cyberknife
Multileaf collimator
Image-guided radiation therapy
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Purpose: To investigate the dosimetric accuracy of the sliding window gated IMRT compared with the static treatment, using varying dose rates.Materials and methods: This study measured changes in output and diode array response with changing dose rate, verified the precision of the motion table, and measured changes in dose distribution accuracy with film and diodes at two depths with changing dose rate.During 4DCT (4 Dimensional Computed Tomography), the patient's respiratory signals and target motion were recorded and imported to the XY4D simulation table of SUN NUCLEAR Corporation to simulate the patient's respiration and tumour motion.A single field of each sliding window IMRT plan with 30º wedge and one for lung cancer were used in this study.Three irradiating conditions, static and moving target with and without gating, were applied to both plans.Results: The standard deviations of output, with the dose rates changing from 300-600 MU/min, were 0.065 cGy and 0.169 cGy for the ionisation chamber and diode, respectively.The verification of the motion table shows very good precision with 9.98 ± 0.02 cm (true value = 10.0 cm).The measurements by MapCheck show the gamma index of the planned absolute dose distribution in static and moving targets with gating, resulting in more than 96% passing for all dose rates.The absolute dose distribution measured by film for the static target was agreeable with the value of moving target with gating. Conclusion:The sliding window gated IMRT technique is able to deliver an accurate dose to a moving target with the dose rate of 300-600 MU/min that is suitable for clinical treatment.
Tomotherapy
Multileaf collimator
Image-guided radiation therapy
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Tomotherapy
Image-guided radiation therapy
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A helical tomotherapy machine combines a straight 6 MV linear accelerator mounted on a ring gantry with CT technology for image-guided intensity-modulated radiation therapy (IMRT) treatment. A fan beam created by the collimator and jaws produces a maximum of 40 Χ 5 cm 2 field size at the isocenter. The gantry and hence the fan beam rotates at a constant speed while the couch moves linearly into the gantry bore, thus producing a helical delivery. The beam is modulated by a 64-leaf binary multileaf collimator (MLC), which enables IMRT treatment. The linac can be operated at a lower voltage (3.5 MV) and dose rate to produce megavoltage CT images, which are used for image-guided patient setup. We have installed two such units since 2004 and treated more than 2000 patients. The machine comes precommissioned from the manufacturer, and the beam characteristics and IMRT plans on phantom are measured and compared with manufacturer's data after acceptance tests are performed on site. Our experience with commissioning the machines and periodic quality assurance with tolerance limits for optimal performance are described.
Tomotherapy
Multileaf collimator
Isocenter
Collimator
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Image-guided radiation therapy
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Stereotactic Radiosurgery (SRS) has evolved as a unique discipline that combines aspects of both surgery and radiation oncology. Technological developments in the past few decades have provided a wide array of treatment techniques, including (i) the Gamma Knife TM ; (ii) Linac-based stereotactic techniques using circular collimators or using micro multileaf collimators (mMLCs); (iii) the Cyber Knife TM , using an x-band linac mounted on a robotic arm; and (iv) serial and spiral tomotherapy. This paper provides a review of the treatment planning methods for stereotactic radiosurgery. Because of the differences in planning strategies used for each SRS technique, this paper will provide both a general review of the pre-requisites and common features of SRS treatment planning and the planning techniques specific to each of the SRS techniques.
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Cyberknife
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Helical tomotherapy has been developed at the University of Wisconsin, and 'Hi-Art II' clinical machines are now commercially manufactured. At the core of each machine lies a ring-gantry-mounted short linear accelerator which generates x-rays that are collimated into a fan beam of intensity-modulated radiation by a binary multileaf, the modulation being variable with gantry angle. Patients are treated lying on a couch which is translated continuously through the bore of the machine as the gantry rotates. Highly conformal dose-distributions can be delivered using this technique, which is the therapy equivalent of spiral computed tomography. The approach requires synchrony of gantry rotation, couch translation, accelerator pulsing and the opening and closing of the leaves of the binary multileaf collimator used to modulate the radiation beam. In the course of clinically implementing helical tomotherapy, we have developed a quality assurance (QA) system for our machine. The system is analogous to that recommended for conventional clinical linear accelerator QA by AAPM Task Group 40 but contains some novel components, reflecting differences between the Hi-Art devices and conventional clinical accelerators. Here the design and dosimetric characteristics of Hi-Art machines are summarized and the QA system is set out along with experimental details of its implementation. Connections between this machine-based QA work, pre-treatment patient-specific delivery QA and fraction-by-fraction dose verification are discussed.
Tomotherapy
Multileaf collimator
Collimated light
Collimator
Image-guided radiation therapy
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