Collimator Rotation Research Articles

Dosimetric effect of limited aperture multileaf collimator on VMAT plan quality: A study of prostate and head-and-neck cancers.

The aim of study was to evaluate the dosimetric effect of collimator-rotation on VMAT plan quality, when using limited aperture multileaf collimator of Elekta Beam Modulator™ providing a maximum aperture of 21cm×16cm. The increased use of VMAT technique to deliver IMRT from conventional to very specialized treatments present a challenge in plan optimization. In this study VMAT plans were optimized for prostate and head and neck cancers using Elekta Beam-ModulatorTM, whereas previous studies were reported for conventional Linac aperture. VMAT plans for nine of each prostate and head-and-neck cancer patients were produced using the 6MV photon beam for Elekta-SynergyS® Linac using Pinnacle3 treatment planning system. Single arc, dual arc and two combined independent-single arcs were optimized for collimator angles (C) 0°, 90° and 0°-90° (0°-90°; i.e. the first-arc was assigned C0° and second-arc was assigned C90°). A treatment plan comparison was performed among C0°, C90° and C(0°-90°) for single-arc dual-arc and two independent-single-arcs VMAT techniques to evaluate the influence of extreme collimator rotations (C0° and 90°) on VMAT plan quality. Plan evaluation criteria included the target coverage, conformity index, homogeneity index and doses to organs at risk. A 'two-sided student t-test' (p≤0.05) was used to determine if there was a significant difference in dose volume indices of plans. For both prostate and head-and-neck, plan quality at collimator angles C0° and C(0°-90°) was clinically acceptable for all VMAT-techniques, except SA for head-and-neck. Poorer target coverage, higher normal tissue doses and significant p-values were observed for collimator angle 90° when compared with C0° and C(0°-90°). A collimator rotation of 0° provided significantly better target coverage and sparing of organs-at-risk than a collimator rotation of 90° for all VMAT techniques.

Sensitivity study of an automated system for daily patient QA using EPID exit dose images.

The dosimetric consequences of errors in patient setup or beam delivery and anatomical changes are not readily known. A new product, PerFRACTION (Sun Nuclear Corporation), is designed to identify these errors by comparing the exit dose image measured on an electronic portal imaging device (EPID) from each field of each fraction to those from baseline fraction images. This work investigates the sensitivity of PerFRACTION to detect the deviation caused by these errors in a variety of realistic scenarios. Integrated EPID images were acquired in clinical mode and saved in ARIA. PerFRACTION automatically pulled the images into its database and performed the user‐defined comparison. We induced errors of 1 mm and greater in jaw, multileaf collimator (MLC), and couch position, 1° and greater in collimation rotation (patient yaw), 0.5–1.5% in machine output, rail position, and setup errors of 1–2 mm shifts and 0.5–1° roll rotation. The planning techniques included static, intensity modulated radiation therapy (IMRT) and VMAT fields. Rectangular solid water phantom or anthropomorphic head phantom were used in the beam path in the delivery of some fields. PerFRACTION detected position errors of the jaws, MLC, and couch with an accuracy of better than 0.4 mm, and 0.5° for collimator rotation error and detected the machine output error within 0.2%. The rail position error resulted in PerFRACTION detected dose deviations up to 8% and 3% in open field and VMAT field delivery, respectively. PerFRACTION detected induced errors in IMRT fields within 2.2% of the gamma passing rate using an independent conventional analysis. Using an anthropomorphic phantom, setup errors as small as 1 mm and 0.5° were detected. Our work demonstrates that PerFRACTION, using integrated EPID image, is sensitive enough to identify positional, angular, and dosimetric errors.

Variations in dosimetric distribution and plan complexity with collimator angles in hypofractionated volumetric arc radiotherapy for treating prostate cancer.

PurposeHypofractionated radiotherapy can reduce treatment durations and produce effects identical to those of conventionally fractionated radiotherapy for treating prostate cancer. Volumetric arc radiotherapy (VMAT) can decrease the treatment machine monitor units (MUs). Previous studies have shown that VMAT with multileaf collimator (MLC) rotation exhibits better target dose distribution. Thus, VMAT with MLC rotation warrants further investigation.Methods and materialsTen patients with prostate cancer were included in this study. The prostate gland and seminal vesicle received 68.75 and 55 Gy, respectively, in 25 fractions. A dual‐arc VMAT plan with a collimator angle of 0° was generated and the same constraints were used to reoptimize VMAT plans with different collimator angles. The conformity index (CI), homogeneity index (HI), gradient index (GI), normalized dose contrast (NDC), MU, and modulation complexity score (MCSV) of the target were analyzed. The dose–volume histogram of the adjacent organs was analyzed. A Wilcoxon signed‐rank test was used to compare different collimator angles.ResultsOptimum values of CI, HI, and MCSV were obtained with a collimator angle of 45°. The optimum values of GI, and NDC were observed with a collimator angle of 0°. In the rectum, the highest values of maximum dose and volume receiving 60 Gy (V60 Gy) were obtained with a collimator angle of 0°, and the lowest value of mean dose (Dmean) was obtained with a collimator angle of 45°. In the bladder, high values of Dmean were obtained with collimator angles of 75° and 90°. In the rectum and bladder, the values of V60 Gy obtained with the other tested angles were not significantly higher than those obtained with an angle of 0°.ConclusionThis study found that MLC rotation affects VMAT plan complexity and dosimetric distribution. A collimator angle of 45° exhibited the optimal values of CI, HI, and MCSv among all the tested collimator angles. Late side effects of the rectum and bladder are associated with high‐dose volumes by previous studies. MLC rotation did not have statistically significantly higher values of V60 Gy in the rectum and bladder than did the 0° angle. We thought a collimator angle of 45° was an optimal angle for the prostate VMAT treatment plan. The findings can serve as a guide for collimator angle selection in prostate hypofractionated VMAT planning.

40. Validation of a transit EPID device for a clinical use: application to iViewDose (Elekta)

Introduction IMRT/VMAT and stereotactic treatments require patient specific quality assurance before and during treatment. In that context, Electronic Portal Imaging Device (EPID) can be used for pre-treatment and in vivo verification. The aim of this study was to evaluate a new in vivo portal dosimetry tool iViewDose (Elekta) for a clinical use. Methods The study was performed on a VersaHD LINAC equipped with Agility collimator and EPID iView v.3.4.1 (Elekta). iViewDose v.1.0.1 (Elekta) software allows users to reconstruct delivered dose by the LINAC in 3D on the planning CT scan using EPID images. Treatment plans were calculated with Pinnacle v.9.10 (Philips) for VMAT technique. Agreement between calculation and measurement was evaluated with gamma index analysis in 3D ( γ 3D, criteria: 3% global/3 mm, threshold 50%). Detector validation was realized in two steps: (i) detector intrinsic characteristics evaluation (repeatability, reproducibility, linearity, dose rate dependence) and (ii) evaluation of error detectability for two treatment plan complexities (prostate and Head & Neck) delivered on an anthropomorphic phantom with: leaves shift from 1 to 5 mm, collimator rotation from 1 to 5° and MU increase from 1% to 5%. The in vivo agreement between calculation and measurement was investigated for nine patients (H&N, liver, esophagus, iliac bone) corresponding in a total of 86 fractions. According to the obtained results, a parallel analysis with CBCT images was carried out. Results Dose response of the detector was linear (R2 = 1). A x ¯ / σ of 0.06% was obtained for measurements performed with a dose rate varying from 70 to 500 MU/min. For a same irradiation, reconstructed dose was repeatable ( x ¯ / σ = 0.07%) and reproducible ( x ¯ / σ = 0.2%). For H&N treatment plan, iViewDose could detect a 1 mm bank shift ( γ 3D percentage decreased from 91.7% to 67.6%), a 2° collimator rotation ( γ 3D percentage decreased to 76.8%) and an increase of 1% MU ( γ 3D percentage decreased to 84.6%). A two bank shift in the same direction was not detected. Similar results were obtained for prostate treatment plans. In vivo calculation/measurement coherence for five patients (liver, esophagus, iliac bone) showed on average a γ 3D percentage of 90.9% (30 fractions). Those measurements showed punctually a wrong matching for a liver treatment case near diaphragm ( Fig. 1 b). For H&N treatment plans, in vivo calculation/measurement coherence is deteriorated during treatment: 84.4% of points have a gamma Fig. 1 a). Conclusions This study shows the iViewDose capability to detect LINAC and patient potential errors. With that kind of detector, it becomes necessary to correlate those results to a clinical impact and to define action thresholds justifying treatment re-planning. It could be very useful in an adaptive radiotherapy and would be more accurate with dose calculation on CBCT images of the day.

A generic multimodule phantom for testing geometry of a linac c-arm as a part of quality control in radiotherapy.

To develop an assumption-free methodology for testing geometry of linacs. The problem of projecting a fiducial positioned in a predefined point in a 3D space and attached rigidly to a treatment table of a radiotherapeutic device onto an imaging plane with unknown characteristics from a source with unknown coordinates is formulated. The problem of determining these unknowns is formulated as an optimization problem. The problem of determining the gantry/the collimator rotation axis and angle from projection of additional fiducials is also formulated and solved. Analytical methodology is developed for determining isocenter position and an error of estimating isocenter position. The developed methodology is tested in simulations. Very good agreement between preset and calculated values of quantities of interest was found in the simulations. In all cases, the proposed schemes enabled determination of the geometric characteristics of a radiotherapeutic device with accuracy better than one hundredth of a millimeter and one hundredth of a degree. A concept of a multimodule multifiducial phantom has been introduced. Analytical framework has been developed to extract geometric characteristics of radiotherapy devices from projection images of a phantom. The phantom design and the methodology developed have been tested in simulations.

Beam focal spot position: The forgotten linac QA parameter. An EPID-based phantomless method for routine Stereotactic linac QA.

Modern day Stereotactic treatments require high geometric accuracy of the delivered treatment. To achieve the required accuracy the IGRT imaging isocenter needs to closely coincide with the treatment beam isocenter. An influence on this isocenter coincidence and on the spatial positioning of the beam itself is the alignment of the treatment beam focal spot with collimator rotation axis. The positioning of the focal spot is dependent on the linac beam steering and on the stability of the monitor chamber and beam steering servo system. As such, there is the potential for focal spot misalignment and this should be checked on a regular basis. Traditional methods for measuring focal spot position are either indirect, inaccurate, or time consuming and hence impractical for routine use. In this study a novel, phantomless method has been developed using the EPID (Electronic Portal Imaging Device) that utilizes the different heights of the MLC and jaws. The method has been performed on four linear accelerators and benchmarked against an alternate ion chamber‐based method. The method has been found to be reproducible to within ±0.012 mm (1 SD) and in agreement with the ion chamber‐based method to within 0.001 ± 0.015 mm (1 SD). The method could easily be incorporated into a departmental routine linac QA (Quality Assurance) program.

On-site audits to investigate the quality of radiation physics of radiation therapy institutions in the Republic of Korea

PurposeTo investigate and improve the domestic standard of radiation therapy in the Republic of Korea. MethodsOn-site audits were performed for 13 institutions in the Republic of Korea. Six items were investigated by on-site visits of each radiation therapy institution, including collimator, gantry, and couch rotation isocenter check; coincidence between light and radiation fields; photon beam flatness and symmetry; electron beam flatness and symmetry; physical wedge transmission factors; and photon beam and electron beam outputs. ResultsThe average deviations of mechanical collimator, gantry, and couch rotation isocenter were less than 1mm. Those of radiation isocenter were also less than 1mm. The average difference between light and radiation fields was 0.9±0.6mm for the field size of 20cm×20cm. The average values of flatness and symmetry of the photon beams were 2.9%±0.6% and 1.1%±0.7%, respectively. Those of electron beams were 2.5%±0.7% and 0.6%±1.0%, respectively. Every institutions showed wedge transmission factor deviations less than 2% except one institution. The output deviations of both photon and electron beams were less than ±3% for every institution. ConclusionsThrough the on-site audit program, we could effectively detect an inappropriately operating linacs and provide some recommendations. The standard of radiation therapy in Korea is expected to improve through such on-site audits.

Verification of Radiation Isocenter on Linac Beam 6 MV using Computed Radiography

Radiation isocenter is more important part of quality assurance for the linear accelerator (Linac) due to radiation isocenter is a main location in irradiation radiotherapy, isocenter can shift when the gantry and collimator rotation. In general, the radiation isocenter verification using a special film. This research was conducted radiation isocenter verification using computed radiography with digital image processing techniques. Image acquisition was done using the modalities of Linac 6 MV with star shot method is star-shaped beam due to rotation of the collimator, gantry and couch. Then do the delineation on each beam to determine the centroid and beam diameter. By the results of verification of radiation isocenter performed on collimator and the couch, it shows that the size diameter for rotational collimator is 0.632 mm and 0.458 mm for the couch. Based on AAPM report 40 about the size of the Linac radiation isocenter diameter used in this study is still in good condition and worth to be operated because the value of the radiation isocenter diameter is below 2 mm.

Rapid acceptance testing of modern linac using on-board MV and kV imaging systems.

The purpose of this study was to develop a novel process for using on-board MV and kV Electronic Portal Imaging Devices (EPIDs) to perform linac acceptance testing (AT) for two reasons: (a) to standardize the assessment of new equipment performance, and (b) to reduce the time to clinical use while reducing physicist workload. In this study, Varian TrueBeam linacs equipped with amorphous silicon-based EPID (aS1000) were used. The conventional set of AT tests and tolerances were used as a baseline guide. A novel methodology was developed or adopted from published literature to perform as many tests as possible using the MV and kV EPIDs. The developer mode on Varian TrueBeam linacs was used to automate the process. In the EPID-based approach, most of mechanical tests were conducted by acquiring images through a custom phantom and software tools were developed for quantitative analysis to extract different performance parameters. The embedded steel-spheres in a custom phantom provided both visual and radiographic guidance for beam geometry testing. For photon beams, open field EPID images were used to extract inline/crossline profiles to verify the beam energy, flatness and symmetry. EPID images through a double wedge phantom were used for evaluating electron beam properties via diagonal profile. Testing was augmented with a commercial automated application (Machine Performance Check) which was used to perform several geometric accuracy tests such as gantry, collimator rotations, and couch rotations/translations. The developed process demonstrated that the tests, which required customer demonstration, were efficiently performed using EPIDs. The AT tests that were performed using EPIDs were fully automated using the developer mode on the Varian TrueBeam system, while some tests, such as the light field versus radiation field congruence, and collision interlock checks required user interaction. On-board imagers are quite suitable for both geometric and dosimetric testing of linac system involved in AT. Electronic format of the acquired data lends itself to benchmarking, transparency, as well as longitudinal use of AT data. While the tests were performed on a specific model of a linear accelerator, the proposed approach can be extended to other linacs.

Smartphone application for mechanical quality assurance of medical linear accelerators

Mechanical quality assurance (QA) of medical linear accelerators consists of time-consuming and human-error-prone procedures. We developed a smartphone application system for mechanical QA. The system consists of two smartphones: one attached to a gantry for obtaining real-time information on the mechanical parameters of the medical linear accelerator, and another displaying real-time information via a Bluetooth connection with the former. Motion sensors embedded in the smartphone were used to measure gantry and collimator rotations. Images taken by the smartphone’s high-resolution camera were processed to evaluate accuracies of jaw-positioning, crosshair centering and source-to-surface distance (SSD). The application was developed using Android software development kit and OpenCV library. The accuracy and precision of the system was validated against an optical rotation stage and digital calipers, prior to routine QA measurements of five medical linear accelerators. The system accuracy and precision in measuring angles and lengths were determined to be 0.05 ± 0.05° and 0.25 ± 0.14 mm, respectively. The mean absolute errors (MAEs) in QA measurements of gantry and collimator rotation were 0.05 ± 0.04° and 0.05 ± 0.04°, respectively. The MAE in QA measurements of light field was 0.39 ± 0.36 mm. The MAEs in QA measurements of crosshair centering and SSD were 0.40 ± 0.35 mm and 0.41 ± 0.32 mm, respectively. In conclusion, most routine mechanical QA procedures could be performed using the smartphone application system with improved precision and within a shorter time-frame, while eliminating potential human errors.

Investigation of X-ray focal spot alignment using a jig of novel design.

The X-ray focal spot and the centre of the flattening filter should be placed as close as possible to the central axis (CAX) on a linear accelerator (linac) to produce a radially symmetric beam. The aim of this study is to devise a method to easily optimise the focal spot position of Varian linac photon beams generated with a flattening filter. A simple and robust jig was designed and built to be inserted into the largest electron applicator. Accessory and jaw position interlocks were overridden to enable photon beam operation. The jig was made from aluminium and consists of a plate permanently fixed inside a low melting point alloy (LMPA) insert frame and a block machined to suspend an ion chamber below the plate, such that the axis of the chamber is at the linac isocentre. The jig was used to optimise the position of the X-ray beam focal spot with respect to the device central axis (CAX). This was achieved by minimising the percentage change in ionisation chamber signal between collimator rotations from 90° to 270° as position steering was changed. As part of the investigation, in-plane (radial) and cross-plane (transverse) profiles obtained from water phantom scans were used to quantify how large the percentage ionisation change can be before profiles are distorted in the region of the CAX.

Analysis of Influence of Errors in Angular Settings of Couch and Collimator on the Dosimetric and Radiobiological Parameters in VMAT Plans

ObjectiveTo evaluate the impact of couch and collimator angular variations on dose volume histogram (DVH), tumour control probability (TCP), and normal tissue complication probability (NTCP) of the volumetric-modulated arc therapy (VMAT) plans. MethodsStereotactic radiosurgery and stereotactic body radiation therapy VMAT plans were generated for three different hypothetical planning target volumes (PTVs) that mimic brain metastases, single brain lesion, and single spine lesion. Thirty routine VMAT plans (10 prostate, 10 head and neck, and 10 brain cases) treated in our clinic were also selected for this study. The plans were generated using an Eclipse Treatment Planning System and delivered using a Clinac iX linear accelerator equipped with a Millennium 120 multileaf collimator. All the plans were generated using two complementary full arcs (with gantry angle from 179° to 181° and collimator rotation of 30° and 330°) except the brain tumour cases, which used single full arc with collimator rotation of 30°. In all the cases, the couch angle was zero. Impact of the angular variations in the collimator and couch was studied by varying the collimator and couch angular settings by 1°, 2°, and 3°, and creating six erroneous plans corresponding to the original plans. The variation due to these errors on different DVH and radiobiological parameters (TCP, equivalent uniform dose (EUD), and NTCP) of the PTVs and organs-at-risk (OARs) were observed. The relative percentage of difference in these parameters (ΔD, ΔTCP, ΔEUD, and ΔNTCP) were analysed, and statistical significance was tested. ResultsThe variation due to collimator misplacement was observed to be larger than the couch misplacement. Furthermore, in both cases, the variation increased as the degree of error increased. Among the DVH parameters, D98%, D95%, and V95Gy were affected more by the errors than D2%, D5%, and D50%, in both hypothetical and clinical PTVs. In the clinical PTVs, the TCP showed the most variation among all parameters. The ΔNTCP of the bladder and brain OARs were zero, whereas for head and neck OARs, it was high. ConclusionsThe couch and collimator angular variation has different effects on different planning situations and different parameters. The outcome produced by the errors is specific to the treatment sites.

Validation of the Mobius system for patient-specific quality assurance using introduced intentional errors.

Mobius3D and MobiusFX are model-based verification tools for treatment plan dose calculation and treatment delivery. The software facilitates patient-specific quality assurance by extracting data from linear accelerator treatment log files and performing a 3D dose calculation on the original patient CT dataset using an independent collapsed cone algorithm. In this study, we evaluate the ability of the Mobius system to detect linear accelerator-related errors compared with existing measurement-based systems, namely the ArcCHECK® and 3DVH® systems. Three original treatment plans and 47 plans with introduced delivery errors, for a total of 50 plan deliveries, were investigated. The results from this study demonstrated comparable gamma passing rates and error detectability between the Mobius and ArcCHECK® systems while the 3DVH system generally exhibited a lower sensitivity. This work also demonstrated the ability of the Mobius system to detect delivery errors of down to 2° collimator rotation, 1mm MLC bank offset and 10mm collimator jaw offset.

A method to reconstruct and apply 3D primary fluence for treatment delivery verification.

MotivationIn this study, a method is reported to perform IMRT and VMAT treatment delivery verification using 3D volumetric primary beam fluences reconstructed directly from planned beam parameters and treatment delivery records. The goals of this paper are to demonstrate that 1) 3D beam fluences can be reconstructed efficiently, 2) quality assurance (QA) based on the reconstructed 3D fluences is capable of detecting additional treatment delivery errors, particularly for VMAT plans, beyond those identifiable by other existing treatment delivery verification methods, and 3) QA results based on 3D fluence calculation (3DFC) are correlated with QA results based on physical phantom measurements and radiation dose recalculations.MethodsUsing beam parameters extracted from DICOM plan files and treatment delivery log files, 3D volumetric primary fluences are reconstructed by forward‐projecting the beam apertures, defined by the MLC leaf positions and modulated by beam MU values, at all gantry angles using first‐order ray tracing. Treatment delivery verifications are performed by comparing 3D fluences reconstructed using beam parameters in delivery log files against those reconstructed from treatment plans. Passing rates are then determined using both voxel intensity differences and a 3D gamma analysis. QA sensitivity to various sources of errors is defined as the observed differences in passing rates. Correlations between passing rates obtained from QA derived from both 3D fluence calculations and physical measurements are investigated prospectively using 20 clinical treatment plans with artificially introduced machine delivery errors.ResultsStudies with artificially introduced errors show that common treatment delivery problems including gantry angle errors, MU errors, jaw position errors, collimator rotation errors, and MLC leaf position errors were detectable at less than normal machine tolerances. The reported 3DFC QA method has greater sensitivity than measurement‐based QA methods. Statistical analysis‐based Spearman's correlations shows that the 3DFC QA passing rates are significantly correlated with passing rates of physical phantom measurement‐based QA methods.ConclusionAmong measurement‐less treatment delivery verification methods, the reported 3DFC method is less demanding than those based on full dose re‐calculations, and more comprehensive than those that solely checks beam parameters in treatment log files. With QA passing rates correlating to measurement‐based passing rates, the 3DFC QA results could be useful for complementing the physical phantom measurements, or verifying treatment deliveries when physical measurements are not available. For the past 4+ years, the reported method has been implemented at authors’ institution 1) as a complementary metric to physical phantom measurements for pretreatment, patient‐specific QA of IMRT and VMAT plans, and 2) as an important part of the log file‐based automated verification of daily patient treatment deliveries. It has been demonstrated to be useful in catching both treatment plan data transfer errors and treatment delivery problems.

P29. Proposal of technique for whole-brain radiotherapy with hippocampal sparing

Introduction Irradiation of the hippocampi is responsible for neurocognitive deficits, whereas metastasis incidence in this part of brain is low [1] . In this study, the objective is to develop a treatment technique with acceptable dosimetric coverage of the brain while minimizing the dose to hippocampi. Materials and methods Treatment planning of 10 patients was performed. Volumes were delineated according to the RTOG 0933 protocol. The prescribed dose was 30 Gy in 10 fractions and plans were normalized to the average of the PTV, minus the hippocampi’s PRV. For each dosimetry, goal was minimize the value of D40% and the maximum dose at hippocampi without unacceptable degradation of PTV’s coverage. VMAT technique is proposed with 5 non coplanar arcs. Isocenter is positioned at the gravity center of the two hippocampi. The first pair of arcs consist of two whole arcs with 10° couch rotation and 30° collimator rotation. The second pair of arc is identical but with an opposite rotation of table and collimator. Each arc of one pair has an asymmetric collimation so that one of the jaws, having a direction of movement identical to MLC leaves, is closed about 1 cm from the axis of the beam. The sum of two arcs of one pair covers the PTV. The fifth arc with table rotation and collimator rotation at 90° is an half-arc. It is adjusted so that the edges of jaws with MLC intersect the hippocampal volumes. For each patient, this technique was compared with an usually VMAT technique, based on 2 coplanar wholes arcs in clockwise and counterclockwise directions and collimator rotation at 30° and 330° respectively [2] . For each optimization, we used a dummy structure, between hippocampi, to control the dose. Results The average gain on the D40% of the hippocampus is 1.5 Gy and 3.2 Gy on the maximum dose. There is also an increase on the D98% of the PTV of 0.7 Gy and a diminution of 0.35 Gy to the D2%. The V95% is improved by 2%. Conclusions This technique with 5 arcs, although time-consuming during the treatment, seems to show a superiority in the hippocampal avoidance, but also in the covering of whole brain.

37. Hippocampal sparing whole brain radiotherapy with volumetric modulated arc therapy

Introduction Hippocampal Avoidance whole brain radiotherapy (HA-WBRT) is a strategy that aims to mitigate the neuro-cognitive side effects of whole brain radiotherapy treatment. The purpose of this study was to investigate the feasibility and efficiency of using volumetric modulated arc therapy (VMAT) to spare the hippocampi while delivering the prescribed dose to the rest of the brain. Methods A total of four patients were replanned for hippocampal sparing for a prescription dose of 30 Gy in 10 fractions. A cranial mask system with a 25° angle, inclined board was utilized for patient positioning. The hippocampi were contoured and hippocampal avoidance regions were created using a 5 mm volumetric expansion around the hippocampi. The whole brain planning target volume was defined as the whole brain tissue minus hippocampal avoidance region. The VMAT plans were generated in Eclipse treatment planning system version 11 for a Varian True Beam STx linear accelerator equipped with a high definition multileaf collimator. Different beam arrangements were studied.The treatment plans were evaluated on target coverage, homogeneity of the PTV, and minimum, maximum, mean dose, D40% to the hippocampi and maximum and mean dose to the lens. The goal was to generate treatment plans that met the RTOG 0933 [1] criteria for HAWBRT treatment. Results The choice of beam arrangements consisted in six arcs with two non-coplanar arcs and different collimator rotation. All the VMAT plans achieved target coverage of the brain while maintaining hippocampal doses conform to the RTOG 0933 protocol. Concerning the PTV, the mean D98%, D95% and D2% were 27.3 ± 0.3 Gy, 29.0 ± 0.1 Gy and 30.8 ± 0.1 Gy, respectively. The mean values of homogeneity index and conformity index were 0.11 ± 0.01 and 0.94 ± 0.01, respectively. The D100%, mean and maximum doses to hippocampi were 7.9 ± 0.4 Gy, 10.1 ± 0.8 Gy and 14.6 ± 1.0 Gy, on average, respectively. On average, the mean and maximum doses to lenses were 4.5 ± 0.4 Gy and 5.2 ± 0.2 Gy respectively. Conclusion An inclined head board at 25° combined to six VMAT arcs with two non-coplanar arcs provided highly conformal plans that spared hippocampi and met the RTOG 0933 criteria. In the future, we plan to evaluate dose escalation on multiple brain metastases and assess the minimum distance from the metastases to the hippocampi to satisfy hippocampal avoidance.

Volumetric modulated arc therapy with dynamic collimator rotation for improved multileaf collimator tracking of the prostate

PurposeTo improve MLC tracking of prostate VMAT plans by dynamic rotation of the collimator to align the MLC leaves with the dominant prostate motion direction. MethodsFor 22 prostate cancer patients, two dual arc VMAT plans were made with (1) fixed collimators (45° and 315°) and (2) a rotating collimator that aligned the MLC leaves with the dominant prostate motion direction (population-based first principal component). The fixed and rotating collimator plan quality was compared using selected dose–volume indices. Next, MLC tracking treatments were simulated with 695 patient-measured prostate traces. The MLC exposure error (under- and overexposed MLC area in beam’s eye view) was calculated as a surrogate for the MLC tracking error. Finally, motion including dose reconstruction was performed for 35 motion traces for one patient, and the root-mean-square dose error was compared with the MLC exposure error. ResultsRotating collimator VMAT plans were of similar quality as the fixed collimator plans, but significantly improved MLC tracking with 33% lower MLC exposure errors (p≪0.0001). The reductions in MLC exposure error correlated significantly with dose error reductions. ConclusionProstate VMAT plans with rotating collimator were of similar quality as fixed collimator plans, but more suitable for MLC tracking with significantly better agreement between planned and delivered dose distributions. MLC tracking for prostate cancer patients can therefore be improved without the requirement of additional efforts or hardware changes.

Simplified low-dose-rate infant total body irradiation

PurposeLow-dose-rate total body irradiation (LDR TBI) in infants possesses unique challenges because of the setup limitations imposed by anesthesia. We present an LDR TBI method with an anteroposterior/posteroanterior arrangement and dose rate < 10 cGy/minute for infants (thickness <14 cm) without the use of a beam attenuator. Materials and methodsThe delivery used a 6 MV TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA) with adjustable meterset rates below 100 MU/minute. A platform was constructed to support the patient 6 cm above the floor (~2 m from the source). A 40 × 40 cm2 field at isocenter along with a 45° collimator rotation was used to achieve a maximum field width of approximately 113 cm at 2 m. The patient was positioned supine with head turned toward the patient’s right side for the anteroposterior beam and prone with the head continuing to turn towards the patient’s right side for the posteroanterior beam. A scattering plate with custom organ blocks was placed approximately 20 cm from the patient’s skin surface. Output, depth-dose, surface-dose, block transmission, and off-axis measurements were taken using solid water slabs, ion chambers, and film. ResultsThe TrueBeam provides several options for this treatment geometry to deliver LDR TBI at less than 10 cGy/min by lowering the linear accelerator meterset rate below 100 MU/minute. This allowed for a compromise between dose rate and treatment time to be selected by the radiation oncology team without the use of a beam attenuator, thus improving patient safety. Depth-dose and off-axis measurements confirmed a dose distribution within ±5% of the central-axis dose. A scattering plate distance of ~20 cm from the patient increases surface dose while allowing easy access to the patient during anesthesia. ConclusionsThe method described is useful to pediatric radiation oncologists seeking to deliver LDR TBI to an infant patient.

Bladder radiotherapy treatment: A retrospective comparison of 3-dimensional conformal radiotherapy, intensity-modulated radiation therapy, and volumetric-modulated arc therapy plans

To examine tumor׳s and organ׳s response when different radiotherapy plan techniques are used. Ten patients with confirmed bladder tumors were first treated using 3-dimensional conformal radiotherapy (3DCRT) and subsequently the original plans were re-optimized using the intensity-modulated radiation treatment (IMRT) and volumetric-modulated arc therapy (VMAT)-techniques. Targets coverage in terms of conformity and homogeneity index, TCP, and organs׳ dose limits, including integral dose analysis were evaluated. In addition, MUs and treatment delivery times were compared. Better minimum target coverage (1.3%) was observed in VMAT plans when compared to 3DCRT and IMRT ones confirmed by a statistically significant conformity index (CI) results. Large differences were observed among techniques in integral dose results of the femoral heads. Even if no statistically significant differences were reported in rectum and tissue, a large amount of energy deposition was observed in 3DCRT plans. In any case, VMAT plans provided better organs and tissue sparing confirmed also by the normal tissue complication probability (NTCP) analysis as well as a better tumor control probability (TCP) result. Our analysis showed better overall results in planning using VMAT techniques. Furthermore, a total time reduction in treatment observed among techniques including gantry and collimator rotation could encourage using the more recent one, reducing target movements and patient discomfort.

Re-examining TG-142 recommendations in light of modern techniques for linear accelerator based radiosurgery.

The recent development of multifocal stereotactic radiosurgery (SRS) using a single isocenter volumetric modulated arc theory (VMAT) technique warrants a re-examination of the quality assurance (QA) tolerances for routine mechanical QA recommended by the American Association of Physicists in Medicine Task Group Report Number 142. Multifocal SRS can result in targets with small volumes being at a large off-axis distance from the treatment isocenter. Consequently, angular errors in the collimator, patient support assembly (PSA), or gantry could have an increased impact on target coverage. The authors performed a retrospective analysis of dose deviations caused by systematic errors in PSA, collimator, and gantry angle at the tolerance level for routine linear accelerator QA as recommended by TG-142. Dosimetric deviations from multifocal SRS plans (N = 10) were compared to traditional single target SRS using dynamic conformal arcs (N = 10). The chief dosimetric quantities used in determining clinical impact were V100% and D99% of the individual planning target volumes and V12Gy of the healthy brain. Induced errors at tolerance levels showed the greatest change in multifocal SRS target coverage for collimator rotations (±1.0°) with the average changes to V100% and D99% being 5% and 6%, respectively, with maximum changes of 33% and 20%. A reduction in the induced error to half the TG-142 tolerance (±0.5°) demonstrated similar changes in coverage loss to traditional single target SRS assessed at the recommended tolerance level. The observed change in coverage for multifocal SRS was reduced for gantry errors (±1.0°) at 2% and 4.5% for V100% and D99%, respectively, with maximum changes of 18% and 12%. Minimal change in coverage was noted for errors in PSA rotation. This study indicates that institutions utilizing a single isocenter VMAT technique for multifocal disease should pay careful attention to the angular mechanical tolerances in designing a robust and complete QA program.