Gantry Speed Research Articles

SU-FF-T-152: Comparison Between Fixed Gantry Angle Intensity Modulated Radiotherapy and Intensity Modulated Arc Therapy for Head-And-Neck Cancers

Purpose: To compare the dose distributions resulting form fixed gantry angle IMRT and IMAT (dynamic arc) optimization for head‐and‐neck tumors (HN). Materials and Methods: Four (HN) cases were retrospectively investigated. Pinnacle3 (v. 8.1x) was used to generate treatment plans for an IMAT and a 9‐field IMRT delivery techniques. The targets were the planning target volume (PTV) and the lymph nodes (LN). The organs at risk (OARs) were the spinal cord (cord), the cricopharyngeus, the esophagus and the parotid glands (parotids). The IMAT and IMRT plan cross‐comparison was based on dose indices used as objectives in the optimization process. The evaluated dose indices were D95 for PTV and LN and D1 for the cord and average doses for the parotids, the cricopharyngeus and the esophagus. The standard deviations of the dose (expressed as a fraction of the prescription dose) over the PTV and the LN were also tallied. The highest priority in the optimization was given to target coverage. Results: The dose indices for the PTV and the LN differed by less than 1% and 3% respectively. A reduction in OAR dose was possible in the IMRT plans while maintaining the dose standard‐deviation below 3% (used as a dose uniformity criterion) for all targets. The IMAT plans resulted in higher dose indices/average doses to the OARs by as much as 50%. The IMAT dose standard deviation in all patients for the LN and in one patient for the PTV was greater than 3%. Conclusions: A better balance between dose uniformity and dose coverage was found in the IMRT plans. In addition the IMRT plans yielded lower doses to the OARs. We hypothesize that allowing for variable gantry speed in the IMAT optimization will result in IMAT plans which are at least as good as the fixed gantry angle IMRT plans.

SU‐FF‐T‐301: Clinical Implementation and Commission of Volumetric Modulated Arc Therapy

Purpose Volumetric modulated arc therapy (VMAT) is a rotational IMRT modality that combines dose rate modulation with gantry and MLC motion, thereby increasing the complexity of IMRT delivery. Effective implementation of this technology is still quite new to radiation therapy clinics. This study reports our clinical experience for acceptance testing, commissioning, and routine QA procedure development. Materials and Methods VMAT was implemented on a Varian 21EX™ with a Millennium120 MLC and a Novalis Tx™ with a 2.5mm High‐Definition MLC. The Eclipse™ treatment planning system was used with the associated ARIA record‐and‐verify system. Clinical implementation involves (1) measurements of linac parameters and performance such as MLC leaf positioning and motion accuracy, VMAT dosimetric accuracy, and treatment safety checks; (2) planning algorithms and parameters; (3) patient‐specific QA studies; and (4) routine QA baselines and procedures. Results The difference between planned and measured MLC leaf positions at stationary gantry positions and during VMAT delivery were < 1mm, as measured with film and EPID. The dosimetric accuracy of VMAT delivery was tested with different combinations of gantry speed, dose rate, and MLC motion. Measured variations between these deliveries were < 2%. Guidelines for safe couch positions and placement of immobilization devices were established for both CBCT and VMAT isocenters. The Anisotropic Analytical Algorithm (AAA) dose calculation algorithm was commissioned, with measured point‐dose accuracy of better than 2% at central axis and 5% off‐axis. Patient specific QA data showed that, for prostate and head‐neck cases, point dose accuracy was within 2.5%. Gamma agreements (3mm/3%) > 90% were observed for planar dose distributions measured with an ion chamber array and film. Conclusion A methodology for clinical implementation of VMAT is developed. A critical component for implementation is the design of acceptance tests and commissioning protocols which can be used as the baseline for routine QA procedures.

Development and evaluation of an efficient approach to volumetric arc therapy planning

An efficient method for volumetric intensity modulated arc therapy (VMAT) planning was developed, where a single arc (360 degrees or less) is delivered under continuous variation of multileaf collimator (MLC) segments, dose rate, and gantry speed. Plans can be generated for any current linear accelerator that supports these degrees of freedom. MLC segments are derived from fluence maps at relatively coarsely sampled angular positions. The beam segments, dose rate, and gantry speed are then optimized using direct machine parameter optimization based on dose volume objectives and leaf motion constraints to minimize arc delivery time. The method can vary both dose rate and gantry speed or alternatively determine the optimal plan at constant dose rate and gantry speed. The method was used to retrospectively generate variable dose rate VMAT plans to ten patients (head and neck, prostate, brain, lung, and tonsil). In comparison to step-and-shoot intensity modulated radiation therapy, dosimetric plan quality was comparable or improved, estimated delivery times ranged from 70 to 160 s, and monitor units were consistently reduced in nine out of the ten cases by an average of approximately 6%. Optimization and final dose calculation took between 5 and 35 min depending on plan complexity.

RapidArc volumetric modulated therapy planning for prostate cancer patients

Purpose. Recently, Varian Medical Systems have announced the introduction of a new treatment technique, RapidArc, in which dose is delivered over a single gantry rotation with dynamically variable MLC positions, dose rate and gantry speed. At Rigshospitalet, the RapidArc technique was brought into clinical practice in May 2008 for treatment of prostate cancer patients. We report here our experiences with performing treatment planning using the Eclipse RapidArc optimization software for this patient group. Material and methods. A stand-alone installation of Eclipse 8.5 with RapidArc optimization capability was performed at Rigshospitalet. Patient data for 8 prostate cancer patients were imported, most of whom were previously treated at Rigshospitalet using IMRT. Three of the patients were treated at Rigshospitalet using the RapidArc technique. Treatment plans were optimized using objectives as given by standard guidelines for clinical treatment planning. RapidArc plans were compared to the IMRT plans by which the patients were actually treated or in the three cases treated with the RapidArc technique to IMRT plans achieved using standard guidelines. Comparison was done with respect to target coverage, doses to rectum and bladder, over-all maximum dose and number of monitor units. Results. Overall, the RapidArc treatment plans gave better or equal sparing of the organs at risk than the IMRT treatment plans. The number of monitor units was lower in most cases, by up to approximately 75%. However, the target dose homogeneity was not as high as for IMRT. The low-dose bath was larger than for IMRT. Conclusion. RapidArc optimization is very promising, especially regarding the potential of reducing the number of monitor units, while providing good sparing of organs at risk. Some improvement is still warranted with respect to achieving high target dose homogeneity.

Dosimetric verification of RapidArc treatment delivery

Purpose. Recently, Varian Medical Systems have announced the introduction of a new treatment technique, in which dose is delivered over a single gantry rotation with variable MLC positions, dose rate and gantry speed. In February 2008, a preclinical installation of the RapidArc™ beam delivery approach was carried out on a Varian Clinac at Rigshospitalet in Copenhagen. The purpose of the installation was to perform measurements to verify the correctness of doses delivered with the RapidArc technique. In May 2008, the clinical release of the RapidArc application was installed at Rigshospitalet. Methods and materials. Nine treatment plans were generated in the Eclipse version 8.5 including the RapidArc optimizer for H&N and prostate cases. The plans were delivered to the Scandidos Delta4® cylindrical diode array phantom. First, the measured dose distributions were compared with the calculated doses. All plans were then delivered several times to verify consistency of the delivery. Gamma analysis was used to verify the correspondence between dose distributions. The temporal resolution of the delivery was analysed by investigating the arc segments between control points separately. Results. Overall, good agreement was observed between measured and calculated doses in most cases with gamma values above 1 in >95% of measured points. The reproducibility of delivery was also very high. Gamma analysis between two consecutive runs of the same delivery plan generally showed gamma values above 1 in none of the measured points, and dose deviation less than 1%. Temporal analysis showed small discrepancies between doses delivered between control points (∼2 degrees of the rotation) in consecutive runs of a plan, however these were cancelled out in the accumulated dose. Conclusion. The delivery of RapidArc beam delivery has been verified to correspond well with calculated dose distributions for a number of different cases. The delivery was very reproducible, and was carried out with high stability of the accelerator performance.

State-Of-The-Art Cardiac CT At The Methodist Hospital

The last decade has witnessed dramatic improvements in computed tomography (CT) technology. Developments have included increasing gantry speeds, addition of multiple thin detector arrays, and more sophisticated software processing for informative image displays. The results have been improved temporal resolution, near isotropic voxels for submillimeter spatial resolution, reduced image acquisition times, and lower radiation exposures with fast scan times.1 As a result, cardiac CT has evolved into a robust technology used to noninvasively assess cardiac structure and function, determine the presence and extent of coronary artery calcification (CAC) and noncalcified plaque, hunt for coronary anomalies, and provide a noninvasive coronary angiogram for diagnosing significant coronary artery disease (CAD} in native vessels or coronary artery bypass grafts (Figures 1-3).Accompanying research literature has evolved at an equally dizzying pace. Whether experimenting with newer CT technologies, comparing them to other cardiac imaging modalities, learning to better characterize plaque, or changing acquisition parameters to reduce radiation exposures, CT is currently generating a level of excitement that is reminiscent of the 1970s, when echocardiography rapidly came into common clinical use.2 Cardiac computed tomography has the potential to revolutionize the current practice of cardiology for CAD detection and treatment, and it is expected that its use and development will continue to accelerate in the upcoming years.The Methodist DeBakey Heart & Vascular Center (MDHVC) is actively committed to the research, clinical activity, and teaching of cardiac CT. Under the leadership of the center's medical director of nuclear cardiology John J. Mahmarian, M.D., two board-certified nuclear-CT cardiologists have come on board and two state-of-the-art multidetector CT (MDCI) systems dedicated to cardiac imaging are now place. Also, under Mahmarian, the clinical volumes have rapidly expanded to more than 400 cardiac cases per year. Every effort is being made to design, coordinate, and participate in research trials to advance this promising field while at the same time showcasing this robust tool to clinicians.

Quality assurance of volumetric modulated arc therapy using Elekta Synergy

Purpose. Recently, Elekta has supplied volumetric modulated arc therapy (VMAT) in which multi-leaf collimator (MLC) shape, jaw position, collimator angle, and gantry speed vary continuously during gantry rotation. A quality assurance procedure for VMAT delivery is described. Methods and materials. A single-arc VMAT plan with 73 control points (CPs) and 5-degree gantry angle spacing for a prostate cancer patient has been created by ERGO + + treatment planning system (TPS), where MLC shapes are given by anatomic relationship between a target and organs at risk and the monitor unit for each CP is optimized based on given dose prescriptions. Actual leaf and jaw positions, gantry angles and dose rates during prostate VMAT delivery were recorded in every 0.25 seconds, and the errors between planned and actual values were evaluated. The dose re-calculation using these recorded data has been performed and compared with the original TPS plan using the gamma index. Results. Typical peak errors of gantry angles, leaf positions, and jaw positions were 3 degrees, 0.6 mm, and 1 mm, respectively. The dose distribution obtained by the TPS plan and the recalculated one agreed well under 2%-2 mm gamma index criteria. Conclusions. Quality assurance for prostate VMAT delivery has been performed with a satisfied result.

Quality and accuracy of cone beam computed tomography gated by active breathing control.

The purpose of this study was to evaluate the quality and accuracy of cone beam computed tomography (CBCT) gated by active breathing control (ABC), which may be useful for image guidance in the presence of respiration. Comparisons were made between conventional ABC-CBCT (stop and go), fast ABC-CBCT (a method to speed up the acquisition by slowing the gantry instead of stopping during free breathing), and free breathing respiration correlated CBCT. Image quality was assessed in phantom. Accuracy of reconstructed voxel intensity, uniformity, and root mean square error were evaluated. Registration accuracy (bony and soft tissue) was quantified with both an anthropomorphic and a quality assurance phantom. Gantry angle accuracy was measured with respect to gantry speed modulation. Conventional ABC-CBCT scan time ranged from 2.3 to 5.8 min. Fast ABC-CBCT scan time ranged from 1.4 to 1.8 min, and respiratory correlated CBCT scans took 2.1 min to complete. Voxel intensity value for ABC gated scans was accurate relative to a normal clinical scan with all projections. Uniformity and root mean square error performance degraded as the number of projections used in the reconstruction of the fast ABC-CBCT scans decreased (shortest breath hold, longest free breathing segment). Registration accuracy for small, large, and rotational corrections was within 1 mm and 1 degrees. Gantry angle accuracy was within 1 degrees for all scans. For high-contrast targets, performance for image-guidance purposes was similar for fast and conventional ABC-CBCT scans and respiration correlated CBCT.

Commissioning and Quality Assurance of RapidArc Radiotherapy Delivery System

The Varian RapidArc is a system for intensity-modulated radiotherapy (IMRT) treatment planning and delivery. RapidArc incorporates capabilities such as variable dose-rate, variable gantry speed, and accurate and fast dynamic multileaf collimators (DMLC), to optimize dose conformality, delivery efficiency, accuracy and reliability. We developed RapidArc system commissioning and quality assurance (QA) procedures. Tests have been designed that evaluate RapidArc performance in a stepwise manner. First, the accuracy of DMLC position during gantry rotation is examined. Second, the ability to vary and control the dose-rate and gantry speed is evaluated. Third, the combined use of variable DMLC speed and dose-rate is studied. Adapting the picket fence test for RapidArc, we compared the patterns obtained with stationary gantry and in RapidArc mode, and showed that the effect of gantry rotation on leaf accuracy was minimal (< or =0.2 mm). We then combine different dose-rates (111-600 MU/min), gantry speeds (5.5-4.3 degrees /s), and gantry range (Deltatheta = 90-12.9 degrees ) to give the same dose to seven parts of a film. When normalized to a corresponding open field (to account for flatness and asymmetry), the dose of the seven portions show good agreement, with a mean deviation of 0.7%. In assessing DMLC speed (0.46, 0.92, 1.84, and 2.76 cm/s) during RapidArc, the analysis of designed radiation pattern indicates good agreement, with a mean deviation of 0.4%. The results of these tests provide strong evidence that DMLC movement, variable dose-rates and gantry speeds can be precisely controlled during RapidArc.

The GLAaS algorithm for portal dosimetry and quality assurance of RapidArc, an intensity modulated rotational therapy

BackgroundTo expand and test the dosimetric procedure, known as GLAaS, for amorphous silicon detectors to the RapidArc intensity modulated arc delivery with Varian infrastructures and to test the RapidArc dosimetric reliability between calculation and delivery.MethodsThe GLAaS algorithm was applied and tested on a set of RapidArc fields at both low (6 MV) and high (18 MV) beam energies with a PV-aS1000 detector. Pilot tests for short arcs were performed on a 6 MV beam associated to a PV-aS500. RapidArc is a novel planning and delivery method in the category of intensity modulated arc therapies aiming to deliver highly modulated plans with variable MLC shapes, dose rate and gantry speed during rotation. Tests were repeated for entire (360 degrees) gantry rotations on composite dose plans and for short partial arcs (of ~6 or 12 degrees) to assess GLAaS and RapidArc mutual relationships on global and fine delivery scales. The gamma index concept of Low and the Modulation Index concept of Webb were applied to compare quantitatively TPS dose matrices and dose converted PV images.ResultsThe Gamma Agreement Index computed for a Distance to Agreement of 3 mm and a Dose Difference (ΔD) of 3% was, as mean ± 1 SD, 96.7 ± 1.2% at 6 MV and 94.9 ± 1.3% at 18 MV, over the field area. These findings deteriorated slightly is ΔD was reduced to 2% (93.4 ± 3.2% and 90.1 ± 3.1%, respectively) and improved with ΔD = 4% (98.3 ± 0.8% and 97.3 ± 0.9%, respectively). For all tests a grid of 1 mm and the AAA photon dose calculation algorithm were applied. The spatial resolution of the PV-aS1000 is 0.392 mm/pxl. The Modulation Index for calculations resulted 17.0 ± 3.2 at 6 MV and 15.3 ± 2.7 at 18 MV while the corresponding data for measurements were: 18.5 ± 3.7 and 17.5 ± 3.7. Partial arcs findings were (for ΔD = 3%): GAI = 96.7 ± 0.9% for 6° rotations and 98.0 ± 1.1% for 12° rotations.ConclusionThe GLAaS method can be considered as a valid Quality Assurance tool for the verification of RapidArc fields. The two implementations (composite rotation or short arcs) allow the verification of either the entire delivery or of short partial segments to possibly identify local discrepancies between delivery and calculations. RapidArc, according to the findings, appears to be a safe delivery method in terms of dosimetric accuracy allowing its clinical application.

RapidArc Verification Measurements using Three Different Dosimetric Systems

Recently, Varian Medical Systems have announced the introduction of a new treatment technique, in which dose is delivered over a single gantry rotation with variable MLC positions, dose rate and gantry speed. In February 2008, a preclinical installation of this RapidArc beam delivery approach was carried out at Rigshospitalet in Copenhagen. The purpose of the installation was to perform measurements to verify the correctness of doses delivered with the RapidArc technique.

Potential Impact of Volumetric Modulated Arc Therapy on the Planning and Delivery of Radiation Therapy

Volumetric modulated arc therapy (VMAT) is a technique in which radiation therapy can be delivered in single or multiple gantry rotations with dynamic MLC motion and dose rate and gantry speed modulation of a linear accelerator. The aim of this work was to evaluate the implications of using VMAT in place of IMRT or conformal plans on plan quality, delivery efficiency, and dosimetric accuracy.

TH‐C‐350‐04: Characterization of a Delivery System for Volumetric Modulated Arc Therapy

Purpose: To evaluate the capabilities and characteristics of a delivery system for volumetric modulated arc therapy (VMAT). Method and Materials: A linear accelerator control system capable of dynamic MLC motion, variable gantry speed, and dose rate modulation during arc delivery was evaluated. The speed and stability of MLC and gantry motion, and the stability of the dose rate was analyzed through a series of test arcs. The transition efficiency between variable dose rates and gantry speeds, reproducibility, and delivery performance of a clinical prostate VMAT treatment were also analyzed. Delivery characteristics and mechanical reproducibility were evaluated by analyzing dynamic delivery log file output and dosimetric agreement was evaluated using a cylindrical phantom with two orthogonal diode arrays. Results: The VMAT control system was capable of maintaining a constant dose rate within 3–5% after a 40° stabilization distance during constant MU/degree arcs from 0.21 to 3.33 MU/deg. Dose rate fluctuations were accounted for by gantry speed regulation during delivery. Leaf motion was stable with maximum positional errors of &lt; 1 mm as the leaves traveled to their maximum extent and changed direction. Leaf errors were independent of gantry angle. Transitions between different MU/deg during delivery were smooth, with stabilization at the new dose rate in &lt; 5 degrees. Dosimetric analysis of a prostate VMAT delivery had a gamma index (3%/3mm) of &lt; 1 for 98.4% of diodes receiving &gt; 4% of the maximum dose. Conclusion: The linear accelerator control system was capable of delivering VMAT plans with constant and variable MU/deg. Small dose rate fluctuations are inherent in the system and are compensated for by dynamic changes of the gantry speed. Dynamic MLC motion is stable and not dependent on the gantry angle. Evaluation of the system on additional clinical plans is warranted.

SU‐GG‐T‐533: Rotational Intensity‐Modulated Therapies: Can Conventional Arcs Match Tomotherapy?

Purpose: To investigate the potential of dynamic arc therapy delivered by a conventional linac to match tomotherapy both in terms of plan quality and efficiency. Method and Materials: For a series of different clinical cases, four treatment plans each were generated: a conventional fixed‐beam IMRT plan, an intensity‐modulated arc plan (IMAT), an IMAT plan restricted to one arc (AT) and a tomotherapy plan for the Hi‐Art system. All treatment plans were generated by the same optimization algorithm using EUD‐based cost functions and Monte‐Carlo dose calculation for each modality. The IMRT and IMAT plans were obtained by optimizing MLC segment shapes directly. Results: In general, the intensity‐modulated techniques were all capable of producing comparable plan qualities, sometimes at the price of a large number of beam directions or a reasonably large number of arcs (up to 8). The single arc technique could never match these results fully if normal structures became dose limiting. In general, the trade‐off between target dose homogeneity and organ sparing was worst with AT. In terms of efficiency, IMAT plans were limited by gantry speed, while IMRT plans were limited by MLC constraints. Conclusion: Intensity modulation cannot be replaced fully by rotational delivery, but multiple modulated arcs are more efficient in terms of delivery than fixed beam IMRT when MLC constraints require frequent beam interrupts.

SU‐GG‐J‐154: Quality and Accuracy of Cone Beam Computed Tomography Gated by Active Breathing Control.

Purpose: To evaluate the quality and accuracy of cone beam computed tomography (CBCT) gated by active breathing control (ABC) for image guidance. Method and Materials: Comparisons were made between conventional ABC‐CBCT (stop and go), modified ABC‐CBCT (a method to speed up the acquisition by slowing the gantry instead of stopping during free breathing), and free breathing respiration correlated CBCT. All CBCT images were acquired with an Elekta LINAC equipped with an on board KV imager with two gantry speed settings. Speed changes for the modified ABC‐CBCT were triggered by the ABC. Breath hold and free breathing patterns used were: (breath hold seconds / free breathing seconds) 3 / 15, 5 / 10, and 10 / 15. Image quality was assessed with a CATPHAN with high contrast, low contrast, and uniformity inserts. CT number linearity was evaluated. Registration assessment (bony and soft tissue) was quantified with both an anthropomorphic and a quality assurance phantom. A calibration phantom was used to assess gantry angle accuracy with respect to gantry speed modulation. Results: Conventional ABC‐CBCT scan time for the breath hold patterns assessed ranged from 2.3 to 5.8 minutes. Modified ABC‐CBCT scan time ranged from 1.4 to 1.8 minutes, and respiratory correlated CBCT scans took 2.1 minutes to complete. CT number linearity for ABC gated scans was comparable to a normal clinical scan with all projections. Registration accuracy for small, large, and rotational corrections was within 1 mm and 1. Conclusion: Modified ABC‐CBCT (slow gantry during breath hold) scans can potentially improve the efficiency of conventional ABC‐CBCT (stop and go), or replace respiration correlated CBCT, without degradation of performance for image guidance purposes. Conflict of Interest: Research sponsored by Elekta, Inc.

MO-D-351-02: An Efficient Approach To Volumetric Modulated Arc Therapy Optimization and Sequencing

Purpose: To develop and evaluate an efficient algorithm for inverse treatment planning of volumetric arc therapy (VMAT). Method and Materials: VMAT plans were generated by first applying a direct machine parameter optimization algorithm from a research version of a commercial treatment planning system (Pinnacle 8.1v) to 18 beams spaced equidistantly at 20 degrees. At each beam angle, the number of segments was variable (typically 3 to 8) to match the intensity modulation fluence map. Low weight or small area segments were eliminated during optimization. The resulting segments were redistributed around the arc such that each individual segment was at a unique arc angle. Segments were sorted according to shape similarity in order to reduce leaf travel in between arc angle positions. To ensure accuracy of the final dose calculation, additional interpolated segments were generated, followed by a segment weight optimization to identify beam‐off segments that would otherwise conflict with plan objectives. The approach was evaluated for six cases (three head‐and‐necks, two prostate, and one brain) with respect to treatment delivery time, and overall plan quality in comparison to step‐and‐shoot IMRT using a conventional number of static beams and identical objectives and constraints as in the VMAT case. Results: The dynamic arc plans consistently demonstrated target coverage and critical structure sparing comparable to step‐and‐shoot plans. For the head‐and‐neck cases, sparing of the brainstem and the spinal cord could be improved substantially. Treatment parameter optimization and dose calculation required 15 to 20 minutes on standard hardware. Estimated delivery times were between 1.5 and 4 minutes, which is mainly determined by the leaf trajectories, gantry speed and dose rate. Conclusion: A method for inverse planning for VMAT delivery was developed. The method produces treatment plans with high dosimetric quality and efficient delivery time with clinically feasible user time and effort.

SU‐GG‐T‐545: Dosimetric Verification of RapidArc Treatment Delivery Using the Delta4 Phantom

Purpose: Recently, Varian Medical Systems have introduced a new treatment technique, in which dose is delivered over one gantry rotation with variable MLC positions, dose rate and gantry speed, RapidArc. A preclinical RapidArc installation was carried out on a Varian Clinac at our institution. We report dosimetric verification of reproducibility and consistency of RapidArc delivery using the Delta4 phantom (Scandidos, SE). Method and materials: 9 treatment plans generated in the Eclipse preclinical version of the RapidArc optimizer (H&amp;N, prostate and lung) were delivered using a Clinac equipped with RadidArc delivery capability. The delivered dose was measured using the Scandidos Delta4 phantom. The measured dose distributions were compared with doses calculated in Eclipse. All plans were delivered three times consecutively, and the first run was used as reference for constancy checks. The temporal resolution of the delivery was analysed by investigating the arc segments between control points separately (177 control points per rotation). Gamma analysis (3%/3mmDTA) was used to quantify correspondence between dose distributions. Results: Overall, good agreement was observed between measured and calculated doses, generally with gamma values &lt;1 in &gt;95% of measured points. Comparison of the accumulated doses for two consecutive runs of a plan showed gamma values &lt;1 in all measured points. The segment‐by‐segment analysis showed discrepancies between two consecutive runs, with &gt;90% gamma values &lt;1 in around 50–60% of the segments (gamma dose criterium 3%; &lt;1–2 mGy for single segments). Segments with large dose deviations were typically counter‐balanced by following segments with large deviations of the opposite sign. The segment discrepancies cancelled out in the accumulated dose. Conclusion: The delivery of RapidArc beam delivery corresponded well with calculated dose distributions for different cases. The delivery was reproducible, and was carried out with high stability of the accelerator performance.Research sponsored by Varian Medical Systems, Inc.

Respiratory motion effects on whole breast helical tomotherapy

The effects of intrafraction respiratory motion on nonhelical intensity-modulated radiotherapy have been well addressed in the literature, both theoretically and experimentally. However, the consequences of respiratory motion on helical tomotherapy, for patient-specific treatment plans, are less well known. Parameters specific to this treatment modality such as pitch, gantry speed, and degree of modulation may play prominent roles in radiation delivery with respect to intrafraction respiratory motion. This phantom-based study specifically addressed the effects of intrafraction respiratory motion on whole breast helical tomotherapy. A device capable of driving an acrylic phantom with reproducible, one-dimensional, anterior-posterior motion resembling a sinusoid of 4.6 mm crest-trough amplitude was developed. A plan to irradiate the corner of an acrylic phantom using parameters typical of a whole breast helical tomotherapy technique was developed using the TomoTherapy Hi-Art-II System. The treatment was delivered to the phantom, with Kodak EDR2 film in the axial plane, for each of the following conditions: (i) phantom at 270 degrees initial sinusoidal phase and 12 cycles/min motion, (ii) phantom at 270 degrees initial sinusoidal phase and 18 cycles/min motion, and (iii)-(v) phantom at 18 cycles/min motion with 0 degrees, 90 degrees, and 180 degrees initial sinusoidal phases. A measure of technique reproducibility was also performed for several irradiations with the phantom static at 270 degrees initial sinusoidal phase. Films were processed using a Kodak MIN-R mammography film processor, scanned with a Vidar NXR-16 Dosimetry Pro scanner and analyzed with RIT113 v.4.2 software. Films were compared to a reference film irradiated under the conditions of no motion and 270 degrees sinusoidal phase. For all comparisons, 5% dose difference threshold, 3% dose difference and 2 mm distance-to-agreement gamma analysis, and isodose plots were generated. The results of this study show a small area of greater than 5% decrease in dose at the phantom's anterior surface and a 1.5-3 mm posterior-medial shift of isodose lines in the penumbral and apex regions of the PTV. Frequency and phase effects are apparent within the PTV where dose varies with high spatial frequency. As the reference film was produced by delivering the treatment plan to the phantom static and in the position corresponding to maximum expiration, results are representative of extreme deviations between planned and delivered dose with respect to sinusoidal motion of clinically relevant magnitudes and frequencies.

Treatment of lung cancer using volumetric modulated arc therapy and image guidance: A case study

Background. Volumetric modulated arc therapy (VMAT) is a radiotherapy technique in which the gantry rotates while the beam is on. Gantry speed, multileaf collimator (MLC) leaf position and dose rate vary continuously during the irradiation. For optimum results, this type of treatment should be subject to image guidance. The application of VMAT and image guidance to the treatment of a lung cancer patient is described. Material and methods. In-house software AutoBeam was developed to facilitate treatment planning for VMAT beams. The algorithm consisted of a fluence optimisation using the iterative least-squares technique, a segmentation and then a direct-aperture optimisation. A dose of 50 Gy in 25 fractions was planned, using a single arc with 35 control points at 10° intervals. The resulting plan was transferred to a commercial treatment planning system for final calculation. The plan was verified using a 0.6cm3 ionisation chamber and film in a rectangular phantom. The patient was treated supine on a customised lung board and imaged daily with cone-beam CT for the first three days then weekly thereafter. Results. The VMAT plan provided slightly improved coverage of the planning target volume (PTV) and slightly lower volume of lung irradiated to 20 Gy (V20) than a three-field conformal plan (PTV minimum dose 85.0 Gy vs. 81.8 Gy and lung V20 31.5% vs. 34.8%). The difference between the measured and planned dose was −1.1% (measured dose lower) and 97.6% of the film passed a gamma test of 3% and 3mm. The VMAT treatment required 90s for delivery of a single fraction of 2 Gy instead of 180s total treatment time for the conformal plan. Conclusion. VMAT provides a quality dose distribution with a short treatment time as shown in an example of a lung tumour. The technique should allow for more efficient delivery of high dose treatments, such as used for hypofractionated radiotherapy of small volume lung tumours, and the technique may also be used in conjunction with Active Breathing Control, where fewer breath holds will be required.

Breathing-Synchronized Delivery: A Potential Four-Dimensional Tomotherapy Treatment Technique

To introduce a four-dimensional (4D) tomotherapy treatment technique with improved motion control and patient tolerance. Computed tomographic images at 10 breathing phases were acquired for treatment planning. The full exhalation phase was chosen as the planning phase, and the CT images at this phase were used as treatment-planning images. Region of interest delineation was the same as in traditional treatment planning, except that no breathing motion margin was used in clinical target volume-planning target volume expansion. The correlation between delivery and breathing phases was set assuming a constant gantry speed and a fixed breathing period. Deformable image registration yielded the deformation fields at each phase relative to the planning phase. With the delivery/breathing phase correlation and voxel displacements at each breathing phase, a 4D tomotherapy plan was obtained by incorporating the motion into inverse treatment plan optimization. A combined laser/spirometer breathing tracking system has been developed to monitor patient breathing. This system is able to produce stable and reproducible breathing signals representing tidal volume. We compared the 4D tomotherapy treatment planning method with conventional tomotherapy on a static target. The results showed that 4D tomotherapy can achieve dose distributions on a moving target similar to those obtained with conventional delivery on a stationary target. Regular breathing motion is fully compensated by motion-incorporated breathing-synchronized delivery planning. Four-dimensional tomotherapy also has close to 100% duty cycle and does not prolong treatment time. Breathing-synchronized delivery is a feasible 4D tomotherapy treatment technique with improved motion control and patient tolerance.