Leaf Open Time Research Articles

SU‐F‐T‐456: Tomotherapy MLC Synchronization and Leaf Open Time Measurements Using ArcCheck

Purpose:To develop a practical method for routine QA of the MLC of a Tomotherapy unit using ArcCheck.Methods:Two standard test plans were used in this study. One was a helical test, in which the central leaves No. 32 and 33 opened simultaneously for 277.8ms at projections centered at 0°, 120° and 240° gantry angles. The other test plan was a static test with the gantry angle set at 0°, 45°, 90° and 135° respectively and leaves No. 32 and 33 opened sequentially for total 20s which was further divided into eleven or ten segments at each beam angle. The ArcCheck was isocentrically set up and adjusted for couch sag. Movie files which took a snapshot exposure every 50ms were recorded. The start/stop time of leaf open was decided by the ramp‐up/ramp‐down of the detectors.Results:The percentage differences between measured and planned leaf open time were calculated to be within 0.5% in all the tests. In static test, if leaves are synchronized perfectly, the sum of the two detectors’ signals after normalization should equal one when the leaves are in transition. Our results showed mean values of 0.982, 0.983, 0.978 and 0.995 at static gantry angle 0°, 45°, 90° and 135° respectively.Conclusion:A method for estimating the Tomotherapy binary MLC leaf open time using ArcCheck is proposed and proved to be precise enough to verify the planned leaf open time as small as 277.8ms. This method also makes the observation and quantification of the synchronization of leaves possible.

Effects of changing modulation and pitch parameters on tomotherapy delivery quality assurance plans.

This study was aimed at investigating delivery quality assurance (DQA) discrepancies observed for helical tomotherapy plans. A selection of tomotherapy plans that initially failed the DQA process was chosen for this investigation. These plans failed the fluence analysis as assessed using gamma criteria (3%, 3 mm) with radiographic film. Each of these plans was modified (keeping the planning constraints the same), beamlets rebatched and reoptimized. By increasing and decreasing the modulation factor, the fluence in a circumferential plane as measured with a diode array was assessed. A subset of these plans was investigated using varied pitch values. Metrics for each plan that were examined were point doses, fluences, leaf opening times, planned leaf sinograms, and uniformity indices. In order to ensure that the treatment constraints remained the same, the dose‐volume histograms (DVHs) of all the modulated plans were compared to the original plan. It was observed that a large increase in the modulation factor did not significantly improve DVH uniformity, but reduced the gamma analysis pass rate. This also increased the treatment delivery time by slowing down the gantry rotation speed which then increases the maximum to mean non‐zero leaf open time ratio. Increasing and decreasing the pitch value did not substantially change treatment time, but the delivery accuracy was adversely affected. This may be due to many other factors, such as the complexity of the treatment plan and site. Patient sites included in this study were head and neck, right breast, prostate, abdomen, adrenal, and brain. The impact of leaf timing inaccuracies on plans was greater with higher modulation factors. Point‐dose measurements were seen to be less susceptible to changes in pitch and modulation factors. The initial modulation factor used by the optimizer, such that the TPS generated ‘actual’ modulation factor within the range of 1.4 to 2.5, resulted in an improved deliverable plan.PACS number: 87.55.‐x, 87.55.Qr, 87.55.D‐

An efficient procedure for tomotherapy treatment plan verification using the on-board detector

In this work, a fast and simple procedure for tomotherapy treatment plan verification using the on-board detector (OBD) has been developed. This procedure allows verification of plans with static and dynamic jaws (TomoEDGE). A convolution-based calculation model has been derived in order to link the leaf control sinogram from the treatment planning system to the data acquired by the OBD during a static couch procedure. The convolution kernel has been optimized using simple plans calculated in the Tomotherapy Cheese phantom. The optimal kernel has been found to be a lorentzian function, whose parameter Γ is 0.186 for the 1 cm jaw opening, 0.232 for the 2.5 cm jaw opening and 0.373 for the 5 cm jaw opening. The evaluation has been performed with a γ-index analysis. The dose criterion was 3% of the 95th percentile of the dose distribution and the distance-to-agreement criterion is 2 mm. In order to validate the procedure, it has been applied to around 50 clinical treatment plans, which had already been validated by the Delta4 phantom (Scandidos, Sweden). 96% of the tested plans have passed the criteria. Concerning the other 4%, significant discrepancies between the leaf pattern in the leaf control sinogram and the OBD data have been shown, which might be due to differences in the leaf open time. This corresponds also to a higher sensitivity of this method over the Delta4, adding the possibility of better monitoring the treatment delivery.

The sensitivity of ArcCHECK-based gamma analysis to manufactured errors in helical tomotherapy radiation delivery.

Three‐dimensional measurement arrays are an efficient means of acquiring a distribution of data for patient plan delivery QA. However, the tie between plan integrity and traditional gamma‐based analysis of these data are not clear. This study explores the sensitivity of such analysis by creating errors in Helical Tomotherapy delivery and measuring the passing rates with an ArcCHECK cylindrical diode array. Errors were introduced in each of the couch speed, leaf open time, and gantry starting position in increasing magnitude while the resulting gamma passing rates were tabulated. The error size required to degrade the gamma passing rate to 90% or below was on average a 3% change in couch speed, 5° in gantry synchronization, or a 5 ms in leaf closing speed for a 3%/3 mm Van Dyk gamma analysis. This varied with plan type, with prostate plans exhibiting less sensitivity than head and neck plans and with gamma analysis criteria, but in all cases the error magnitudes were large compared to actual machine tolerances. These findings suggest that the sensitivity of ArcCHECK‐based gamma analysis to single‐mode errors in tomotherapy plans is dependent upon plan and analysis type and at traditional passing thresholds unable to detect small defects in the plan.PACS number: 87.55.Qr

A virtual source model for Monte Carlo simulation of helical tomotherapy.

The purpose of this study was to present a Monte Carlo (MC) simulation method based on a virtual source, jaw, and MLC model to calculate dose in patient for helical tomotherapy without the need of calculating phase‐space files (PSFs). Current studies on the tomotherapy MC simulation adopt a full MC model, which includes extensive modeling of radiation source, primary and secondary jaws, and multileaf collimator (MLC). In the full MC model, PSFs need to be created at different scoring planes to facilitate the patient dose calculations. In the present work, the virtual source model (VSM) we established was based on the gold standard beam data of a tomotherapy unit, which can be exported from the treatment planning station (TPS). The TPS‐generated sinograms were extracted from the archived patient XML (eXtensible Markup Language) files. The fluence map for the MC sampling was created by incorporating the percentage leaf open time (LOT) with leaf filter, jaw penumbra, and leaf latency contained from sinogram files. The VSM was validated for various geometry setups and clinical situations involving heterogeneous media and delivery quality assurance (DQA) cases. An agreement of <1% was obtained between the measured and simulated results for percent depth doses (PDDs) and open beam profiles for all three jaw settings in the VSM commissioning. The accuracy of the VSM leaf filter model was verified in comparing the measured and simulated results for a Picket Fence pattern. An agreement of <2% was achieved between the presented VSM and a published full MC model for heterogeneous phantoms. For complex clinical head and neck (HN) cases, the VSM‐based MC simulation of DQA plans agreed with the film measurement with 98% of planar dose pixels passing on the 2%/2 mm gamma criteria. For patient treatment plans, results showed comparable dose‐volume histograms (DVHs) for planning target volumes (PTVs) and organs at risk (OARs). Deviations observed in this study were consistent with literature. The VSM‐based MC simulation approach can be feasibly built from the gold standard beam model of a tomotherapy unit. The accuracy of the VSM was validated against measurements in homogeneous media, as well as published full MC model in heterogeneous media.PACS numbers: 87.53.‐j, 87.55.K‐

Phantomless patient-specific TomoTherapy QA via delivery performance monitoring and a secondary Monte Carlo dose calculation.

To describe the validation and implementation of a novel quality assurance (QA) system for TomoTherapy using a Monte Carlo (MC)-based secondary dose calculation and CT detector-based multileaf collimator (MLC) leaf opening time measurement QA verification. This system is capable of detecting plan transfer and delivery errors and evaluating the dosimetric impact of those errors. The authors' QA process, MCLogQA, utilizes an independent pretreatment MC secondary dose calculation and postdelivery TomoTherapy exit detector-based MLC sinogram comparison and log file examination to confirm accurate dose calculation, accurate dose delivery, and to verify machine performance. MC radiation transport simulations are performed to estimate patient dose utilizing prestored treatment machine-specific phase-space information, the patient's planning CT, and MLC sinogram data. Sinogram data are generated from both the treatment planning system (MC_TPS) and from beam delivery log files (MC_Log). TomoTherapy treatment planning dose (DTPS) is compared with DMC_TPS and DMC_Log via dose-volume metrics and mean region of interest dose statistics. For validation, in-phantom ionization chamber dose measurements (DIC) for ten sample patient plans are compared with the computed values. Dose comparisons to in-phantom ion chamber measurements validate the capability of the MCLogQA method to detect delivery errors. DMC_Log agreed with DIC within 1%, while DTPS values varied by 2%-5% compared to DIC. The authors demonstrated that TomoTherapy treatments can be vulnerable to MLC deviations and interfraction output variations during treatment delivery. Interfractional Linac output variations for each patient were approximately 2% and average output was 1%-1.5% below the gold standard. While average MLC leaf opening time error from patient to patient varied from -0.6% to 1.6%, the MLC leaf errors varied little between fractions for the same patient plan, excluding one patient. MCLogQA is a new TomoTherapy QA process that validates the planned dose before delivery and analyzes the delivered dose using the treatment exit detector and log file data. The MCLogQA procedure is an effective and efficient alternative to traditional phantom-based TomoTherapy plan-specific QA because it allows for comprehensive 3D dose verification, accounts for tissue heterogeneity, uses patient CT density tables, reduces total QA time, and provides for a comprehensive QA methodology for each treatment fraction.

SU‐E‐T‐475: An Accurate Linear Model of Tomotherapy MLC‐Detector System for Patient Specific Delivery QA

Purpose:An accurate leaf fluence model can be used in applications such as patient specific delivery QA and in‐vivo dosimetry for TomoTherapy systems. It is known that the total fluence is not a linear combination of individual leaf fluence due to leakage‐transmission, tongue‐and‐groove, and source occlusion effect. Here we propose a method to model the nonlinear effects as linear terms thus making the MLC‐detector system a linear system.Methods:A leaf pattern basis (LPB) consisting of no‐leaf‐open, single‐leaf‐open, double‐leaf‐open and triple‐leaf‐open patterns are chosen to represent linear and major nonlinear effects of leaf fluence as a linear system. An arbitrary leaf pattern can be expressed as (or decomposed to) a linear combination of the LPB either pulse by pulse or weighted by dwelling time. The exit detector responses to the LPB are obtained by processing returned detector signals resulting from the predefined leaf patterns for each jaw setting. Through forward transformation, detector signal can be predicted given a delivery plan. An equivalent leaf open time (LOT) sinogram containing output variation information can also be inversely calculated from the measured detector signals. Twelve patient plans were delivered in air. The equivalent LOT sinograms were compared with their planned sinograms.Results:The whole calibration process was done in 20 minutes. For two randomly generated leaf patterns, 98.5% of the active channels showed differences within 0.5% of the local maximum between the predicted and measured signals. Averaged over the twelve plans, 90% of LOT errors were within +/−10 ms. The LOT systematic error increases and shows an oscillating pattern when LOT is shorter than 50 ms.Conclusion:The LPB method models the MLC‐detector response accurately, which improves patient specific delivery QA and in‐vivo dosimetry for TomoTherapy systems. It is sensitive enough to detect systematic LOT errors as small as 10 ms.

SU-E-T-374: Sensitivity of ArcCHECK to Tomotherapy Delivery Errors: Dependence On Analysis Technique

Purpose: ArcCHECK (Sun Nuclear) is a cylindrical diode array detector allowing three-dimensional sampling of dose, particularly useful in treatment delivery QA of helical tomotherapy. Gamma passing rate is a common method of analyzing results from diode arrays, but is less intuitive in 3D with complex measured dose distributions. This study explores the sensitivity of gamma passing rate to choice of analysis technique in the context of its ability to detect errors introduced into the treatment delivery. Methods: Nine treatment plans were altered to introduce errors in: couch speed, gantry/sonogram synchronization, and leaf open time. Each plan was then delivered to ArcCHECK in each of the following arrangements: “offset,” when the high dose area of the plan is delivered to the side of the phantom so that some diode measurements will be on the order of the prescription dose, and “centered,” when the high dose is in the center of the phantom where an ion chamber measurement may be acquired, but the diode measurements are in the mid to low-dose region at the periphery of the plan. Gamma analysis was performed at 3%/3mm tolerance and both global and local gamma criteria. The threshold of detectability for each error type was calculatedmore » as the magnitude at which the gamma passing rate drops below 90%. Results: Global gamma criteria reduced the sensitivity in the offset arrangement (from 2.3% to 4.5%, 8° to 21°, and 3ms to 8ms for couch-speed decrease, gantry-error, and leaf-opening increase, respectively). The centered arrangement detected changes at 3.3%, 5°, and 4ms with smaller variation. Conclusion: Each arrangement has advantages; offsetting allows more sampling of the higher dose region, while centering allows an ion chamber measurement and potentially better use of tools such as 3DVH, at the cost of positioning more of the diodes in the sometimes noisy mid-dose region.« less

Stereotactic body radiation therapy in lung and hepatic cancer (SBRT) with helical tomotherapy (HT): Description of our method

Introduction We chose to implement the technique of SBRT in lung and liver lesions with Helical Tomotherapy (HT) for its high accuracy and dosimetric quality obtained, so as to deliver at least 1 Gy per minute for PTV less than 5 cm in optimal conditions for the machine (Gantry rotation, couch motion, leaf open time). Materials and methods From June 2012 until March 2013, 6 patients with 10 lung lesions and 2 patients with liver lesions were treated with HT-SBRT. Internal tumor volume (ITV) is defined by the sum of the GTV from 4 sets of kV-CT (The dose calculation using a density averaged ITV for lung lesions) and a series PET SCAN in free-breathing, patient immobilized with diaphragmatic compression Body ProLock commercialized by Civco ® . Planned treatment volume (PTV) consisted of 5-mm radial and a 10-mm craniocaudal expansion of ITV. The prescribed doses were: 3 × 20Gy, 3 × 18Gy, 8 × 7,5 Gy. Dose constraints to normal tissue were defined accorded to RTOG 0236 and 0438 protocols. The characteristics of the treatment plan (Pitch = 0.86/ n (0.123), modulation factor (1.5), field size (2.5 or 1 cm)) were chosen to achieve a compromise dosimetric quality (homogeneity index HI and conformity index CI)/ processing time. Results The average homogeneity and conformity index that we obtained are 1.04 and 0.96. Accurate positioning of the patient and control intra and post fraction of it through MVCT scan and adaptive planning tool allows us to assess for central pulmonary and hepatics lesions displacements in x, y and z Conclusions HT-SBRT is feasible, accurate and well tolerated for lung and liver lesions. The improvement of our methodology will be to introduce the 4D PET to define a more precise ITV and to make the calculation on a weighted averaged density scan.

Measurement and correction of leaf open times in helical tomotherapy

The binary multileaf collimator (MLC) is one of the most important components in helical tomotherapy (HT), as it modulates the dose delivered to the patient. However, methods to ensure MLC quality in HT treatments are lacking. The authors obtained data on the performance of the MLC in treatments administered in their department in order to assess possible delivery errors due to the MLC. Correction methods based on their data are proposed. Twenty sinograms from treatments delivered using both of the authors HT systems were measured and analyzed by recording the fluence collected by the imaging detector. Planned and actual sinograms were compared using distributions of leaf open time (LOT) errors, as well as differences in fluence reconstructed at each of the 51 projections into which the treatment planning system divides each rotation for optimization purposes. They proposed and applied a method based on individual leaf error correction and the increase in projection time to prevent latency effects when LOT is close to projection time. In order to analyze the dosimetric impact of the corrections, inphantom measurements were made for four corrected treatments. The LOTs measured were consistent with those planned. Most of the mean errors in LOT distributions were within 1 ms with standard deviations of over 4 ms. Reconstructed fluences showed good results, with over 90% of points passing the 3% criterion, except in treatments with a short mean LOT, where the percentage of passing points was as low as 66%. Individual leaf errors were as long as 4 ms in some cases. Corrected sinograms improved error distribution, with standard deviations of over 3 ms and increased percentages of points passing 3% in the fluence per angle analysis, especially in treatments with a short mean LOT and those that were more subject to latency effects. The minimum percentage of points within 3% increased to 86%. In-phantom measurements of the corrected treatments showed that, while treatments affected by latency effects were improved, those affected by individual leaf errors were not. Measurement of MLC performance in real treatments provides the authors with a valuable tool for ensuring the quality of HT delivery. The LOTs of MLC are very accurate in most cases. Sources of error were found and correction methods proposed and applied. The corrections decreased the amount of LOT errors. The dosimetric impact of these corrections should be evaluated more thoroughly using 3D dose distribution analysis.

SU-E-T-68: Helical Tomotherapy DQA with ArcCHECK: Sensitivity to Possible Delivery Errors

Purpose: To test the ability of the SunNuclear ArcCHECK QA device to detect errors artificially introduced in a helical Tomotherapy plan. Methods: Delivery sinograms from clinically-used plans were extracted from the Tomotherapy treatment planning system (TPS), and modified to reflect several delivery errors: leaf pair incorrectly always open, leaf pair always closed, leaves slow to close once open, and desynchronization between leaf pattern and projection number. QA plans were created using the Tomotherapy TPS with the high dose area aligned to the annular region containing the diodes, and delivered to the ArcCHECK device. The position of the device remained constant between deliveries of the altered sinograms. Measured data were compared to calculated dose using the ArcCHECK dose analysis tools with a 3%/3mm γ tolerance. Results: ArcCHECK was able to detect delivery differences except for peripheral leaves remaining open for the entirety of the procedure (the % agreement for measurement points increased in this case). Altering the sinogram by one projection (∼7°) or increasing leaf open time by 50ms resulted in <80% agreement. Increasing leaf open time by 250ms, fully opening or closing the central two leaves, or creating a ∼100° gantry starting position error yielded passing rates below 30% and were clearly detectable. Conclusions: For many years the gamma passing rate has been used as the standard for comparing measured and calculated dose distributions. However, arbitrary shifting of the dose distribution may mask delivery errors, especially in a less intuitive measurement pattern. Having the ability to simulate possible errors should provide the medical physicists with tools to evaluate the sensitivity and response of a particular QA device. Investigation into the types and magnitudes of detectable errors may lead to more robust QA.

Investigation of the feasibility of a simple method for verifying the motion of a binary multileaf collimator synchronized with the rotation of the gantry for helical tomotherapy

In this paper, we suggest a new method for verifying the motion of a binary multileaf collimator (MLC) in helical tomotherapy. For this we used a combination of a cylindrical scintillator and a general‐purpose camcorder. The camcorder records the light from the scintillator following photon irradiation, which we use to track the motion of the binary MLC. The purpose of this study is to demonstrate the feasibility of this method as a binary MLC quality assurance (QA) tool. First, the verification was performed using a simple binary MLC pattern with a constant leaf open time; secondly, verification using the binary MLC pattern used in a clinical setting was also performed. Sinograms of simple binary MLC patterns, in which leaves that were open were detected as “open” from the measured light, define the sensitivity which, in this case, was 1.000. On the other hand, the specificity, which gives the fraction of closed leaves detected as “closed”, was 0.919. The leaf open error identified by our method was −1.3±7.5%. The 68.6% of observed leaves were performed within ± 3% relative error. The leaf open error was expressed by the relative errors calculated on the sinogram. In the clinical binary MLC pattern, the sensitivity and specificity were 0.994 and 0.997, respectively. The measurement could be performed with −3.4±8.0% leaf open error. The 77.5% of observed leaves were performed within ± 3% relative error. With this method, we can easily verify the motion of the binary MLC, and the measurement unit developed was found to be an effective QA tool.PACS numbers: 87.56.Fc, 87.56.nk

TomoTherapy MLC verification using exit detector data

Treatment delivery verification (DV) is important in the field of intensity modulated radiation therapy (IMRT). While IMRT and image guided radiation therapy (IGRT), allow us to create more conformal plans and enables the use of tighter margins, an erroneously executed plan can have detrimental effects on the treatment outcome. The purpose of this study is to develop a DV technique to verify TomoTherapy's multileaf collimator (MLC) using the onboard mega-voltage CT detectors. The proposed DV method uses temporal changes in the MVCT detector signal to predict actual leaf open times delivered on the treatment machine. Penumbra and scattered radiation effects may produce confounding results when determining leaf open times from the raw detector data. To reduce the impact of the effects, an iterative, Richardson-Lucy (R-L) deconvolution algorithm is applied. Optical sensors installed on each MLC leaf are used to verify the accuracy of the DV technique. The robustness of the DV technique is examined by introducing different attenuation materials in the beam. Additionally, the DV technique has been used to investigate several clinical plans which failed to pass delivery quality assurance (DQA) and was successful in identifying MLC timing discrepancies as the root cause. The leaf open time extracted from the exit detector showed good agreement with the optical sensors under a variety of conditions. Detector-measured leaf open times agreed with optical sensor data to within 0.2 ms, and 99% of the results agreed within 8.5 ms. These results changed little when attenuation was added in the beam. For the clinical plans failing DQA, the dose calculated from reconstructed leaf open times played an instrumental role in discovering the root-cause of the problem. Throughout the retrospective study, it is found that the reconstructed dose always agrees with measured doses to within 1%. The exit detectors in the TomoTherapy treatment systems can provide valuable information about MLC behavior during delivery. A technique to estimate the TomoTherapy binary MLC leaf open time from exit detector signals is described. This technique is shown to be both robust and accurate for delivery verification.

SU‐E‐J‐72: In Vivo Quality Assurance of Head and Neck Cancer IMRT Treatments Using TomoTherapyˈˈs Exit Detector Data

Purpose: The primary objective of this study was to explore the feasibility of utilizing the exit‐detector‐data (EDD) for in vivo quality assurance of head and neck (H&amp;N) cancer patient treatments. Methods: Using TomoTherapyˈs Delivery‐Verification (DVS) software we retrospectively evaluated treatments (53 fractions) of ten H&amp;N cancer patients for dose delivery deviations. MV‐CT scans covering the entire length of treatment volumes and respective EDD files were used for DVS reconstruction and subsequent dose computation on a GPU platform. Differences between planned and reconstructed sinograms were analyzed to evaluate the leaf‐opening time (LOT) differences. Reconstructed dose distributions of fractionated‐deliveries were scaled, convolved to estimate composite dose distributions, and compared against planned dose distributions. The dose summation was strictly based on rigid registrations of the fractionated doses and it did not account for soft‐tissue/dose deformations. Computed DVHs were strictly based on the plannned contours and they did not include anatomical changes, if any. Results: Typical average LOT differences were −3msec‐eq with a standard‐deviation of 8msec‐eq, indicating the integrity of delivered treatments. Although, significant differences were noted for individual reconstructed fraction‐doses, the differences in cumulative reconstructed dose‐distributions were found to be small. Cumulative reconstructed plans show that the minimal dose (D99%) to the PTV was decreased by 1.6±1.4Gy. Average mean doses of the left and right parotids decreased by 1.0±0.6Gy and 0.8±0.7Gy, respectively. The mean dose and volume receiving &gt;60Gy for the upper‐esophagus decreased by 0.7±1.0Gy and 0.6±2.3%, respectively. Overall all the reconstructed doses are lower than the original plans by ∼1.0% and this is consistent with our antropomorphic phantom measurements. Conclusions: The results of study suggest that EDDs are useful for DVS reconstruction and subsequent dose computation and can be a great tool for in vivo quality assurance of clinical H&amp;N cancer treatments. Further studies are needed to account for deformation effects.

SU‐D‐BRA‐06: In Vivo Quality Assurance of Breast Cancer IMRT Treatments Using TomoTherapyˈˈs Exit Detector Data

Purpose: The primary objective of this study was to explore the possibility of using the exit detector data (EDD) for in vivo quality assurance of our clinical breast cancer patient (BC) treatments. Methods: Eleven clinical BC patients were evaluated for dose delivery accuracy using TomoTherapyˈs Delivery‐Verification software on a GPU platform. Evaluations consisted of reconstructed delivery verification sinogram (DVS) and subsequent dose computation on MV‐CT datasets. The DVS reconstructions and dose computations were calculated utilizing MV‐CT scans covering the complete length of the treatment volume and the EDD files collected on the same day. Leaf‐opening‐time (LOT) differences were analyzed to investigate the frequency and magnitude of the sinogram differences between planned and reconstructed. Reconstructed dose distributions of fraction‐deliveries were scaled and convolved to give an approximation of the composite dose distributions. Composite dose distributions were compared against the original plans. Summed doses were based on rigid‐registrations of the fraction‐doses and do not account for soft‐tissue and dose deformations. Reconstructed DVHs are based solely on planning structures.Results: Characteristic average LOT differences were ∼5msec‐eq. It can be noted that large differences between individual fractions are small for composite dose distribution. For the reconstructed summed fractions, both minimal dose (D99%) and dose inhomogeneity (D5%–D95%) of the PTV were found to be within ±0.8Gy of the planned. Ipsi‐lateral lung volume receiving &gt;20Gy and contra‐lateral lung volume receiving &gt;5Gy were decreased by 0.5Gy%±0.4% and 3.2%Gy±1.9%, respectively. The heart mean dose and volume receiving &gt;35Gy were decreased by 0.3Gy±0.4 and 0.3%Gy±0.3%, respectively. Conclusion: The results of study indicate that the EDDs are useful for DVS reconstruction and dose computation which can translate into being a great tool for in vivo quality assurance of BC treatments. Currently further studies are being investigated to account for the deformation of soft‐tissue and dose distributions.

Independent quality assurance of a helical tomotherapy machine using the dose magnifying glass

Helical tomotherapy is a complex delivery technique, integrating CT image guidance and intensity modulated radiotherapy in a single system. The integration of the CT detector ring on the gantry not only allows patient position verification but is also often used to perform various QA procedures. This convenience lacks the rigor of a machine-independent QA process. In this article, a Si strip detector, known as the Dose Magnifying Glass (DMG), was used to perform machine-independent QA measurements of the multileaf collimator alignment, leaf open time threshold, and leaf fluence output factor (LFOF). The DMG measurements showed good agreements with EDR2 film for the MLC alignment test while the CT detector agrees well with DMG measurements for leaf open time threshold and LFOF measurements. The leaf open time threshold was found to be approximately 20 ms. The LFOF measured with the DMG agreed within error with the CT detector measured LFOF. The DMG with its 0.2 mm spatial resolution coupled to TERA ASIC allowed real-time high temporal resolution measurements of the tomotherapy leaf movement. In conclusion, DMG was shown to be a suitable tool for machine-independent QA of a tomotherapy unit.

3D Dose Verification Using Tomotherapy CT Detector Array

To evaluate a three-dimensional dose verification method based on the exit dose using the onboard detector of tomotherapy. The study included 347 treatment fractions from 24 patients, including 10 prostate, 5 head and neck (HN), and 9 spinal stereotactic body radiation therapy (SBRT) cases. Detector sonograms were retrieved and back-projected to calculate entrance fluence, which was then forward-projected on the CT images to calculate the verification dose, which was compared with ion chamber and film measurement in the QA plans and with the planning dose in patient plans. Root mean square (RMS) errors of 2.0%, 2.2%, and 2.0% were observed comparing the dose verification (DV) and the ion chamber measured point dose in the phantom plans for HN, prostate, and spinal SBRT patients, respectively. When cumulative dose in the entire treatment is considered, for HN patients, the error of the mean dose to the planning target volume (PTV) varied from 1.47% to 5.62% with a RMS error of 3.55%. For prostate patients, the error of the mean dose to the prostate target volume varied from -5.11% to 3.29%, with a RMS error of 2.49%. The RMS error of maximum doses to the bladder and the rectum were 2.34% (-4.17% to 2.61%) and 2.64% (-4.54% to 3.94%), respectively. For the nine spinal SBRT patients, the RMS error of the minimum dose to the PTV was 2.43% (-5.39% to 2.48%). The RMS error of maximum dose to the spinal cord was 1.05% (-2.86% to 0.89%). An excellent agreement was observed between the measurement and the verification dose. In the patient treatments, the agreement in doses to the majority of PTVs and organs at risk is within 5% for the cumulative treatment course doses. The dosimetric error strongly depends on the error in multileaf collimator leaf opening time with a sensitivity correlating to the gantry rotation period.

Treatment Planning for Pulsed Reduced Dose-Rate Radiotherapy in Helical Tomotherapy

Pulsed reduced dose-rate radiotherapy (PRDR) is a valuable method of reirradiation because of its potential to reduce late normal tissue toxicity while still yielding significant tumoricidal effect. A typical method using a conventional linear accelerator (linac) is to deliver a series of 20-cGy pulses separated by 3-min intervals to give an effective dose-rate of just under 7 cGy/min. Such a strategy is fraught with difficulties when attempted on a helical tomotherapy unit. We investigated various means to overcome this limitation. Phantom and patient cases were studied. Plans were generated with varying combinations of field width (FW), pitch, and modulation factor (MF) to administer 200 cGy per fraction to the planning target in eight subfractions, thereby mimicking the technique used on conventional linacs. Plans were compared using dose-volume histograms, homogeneity indices, conformation numbers, and treatment time. Plan delivery quality assurance was performed to assess deliverability. It was observed that for helical tomotherapy, intrinsic limitations in leaf open time in the multileaf collimator deteriorate plan quality and deliverability substantially when attempting to deliver very low doses such as 20-40 cGy. The various permutations evaluated revealed that the combination of small FW (1.0 cm), small MF (1.3-1.5), and large pitch (∼0.86), along with the half-gantry-angle-blocked scheme, can generate clinically acceptable plans with acceptable delivery accuracy (±3%). Pulsed reduced dose-rate radiotherapy can be accurately delivered using helical tomotherapy for tumor reirradiation when the appropriate combination of FW, MF, and pitch is used.

Real-time motion-adaptive-optimization (MAO) in TomoTherapy

IMRT delivery follows a planned leaf sequence, which is optimized before treatment delivery. However, it is hard to model real-time variations, such as respiration, in the planning procedure. In this paper, we propose a negative feedback system of IMRT delivery that incorporates real-time optimization to account for intra-fraction motion. Specifically, we developed a feasible workflow of real-time motion-adaptive-optimization (MAO) for TomoTherapy delivery. TomoTherapy delivery is characterized by thousands of projections with a fast projection rate and ultra-fast binary leaf motion. The technique of MAO-guided delivery calculates (i) the motion-encoded dose that has been delivered up to any given projection during the delivery and (ii) the future dose that will be delivered based on the estimated motion probability and future fluence map. These two pieces of information are then used to optimize the leaf open time of the upcoming projection right before its delivery. It consists of several real-time procedures, including ‘motion detection and prediction’, ‘delivered dose accumulation’, ‘future dose estimation’ and ‘projection optimization’. Real-time MAO requires that all procedures are executed in time less than the duration of a projection. We implemented and tested this technique using a TomoTherapy® research system. The MAO calculation took about 100 ms per projection. We calculated and compared MAO-guided delivery with two other types of delivery, motion-without-compensation delivery (MD) and static delivery (SD), using simulated 1D cases, real TomoTherapy plans and the motion traces from clinical lung and prostate patients. The results showed that the proposed technique effectively compensated for motion errors of all test cases. Dose distributions and DVHs of MAO-guided delivery approached those of SD, for regular and irregular respiration with a peak-to-peak amplitude of 3 cm, and for medium and large prostate motions. The results conceptually proved that the proposed method is applicable for real-time motion compensation in TomoTherapy delivery. Extension of the method to real-time adaptive radiation therapy (ART) that compensates for all kinds of delivery errors was proposed. Further validation and clinical implementation is underway.For more information on this article see medicalphysicsweb.org

Dosimetric effects of rotational output variation and x‐ray target degradation on helical tomotherapy plans

In this study, two potential sources of IMRT delivery error have been identified for helical tomotherapy delivery using the HiART system (TomoTherapy, Inc., Madison, WI): Rotational output variation and target degradation. The HiArt system is known to have output variation, typically about +/- 2%, due to the absence of a dose servo system. On the HiArt system, x-ray target replacement is required approximately every 10-12 months due to target degradation. Near the end of target life, the target thins and causes a decrease in the beam energy and a softening of the beam profile at the lateral edges of the beam. The purpose of this study is to evaluate the dosimetric effects of rotational output variation and target degradation by modeling their effects and incorporating them into recalculated treatment plans for three clinical scenarios: Head and neck, partial breast, and prostate. Models were created to emulate both potential sources of error. For output variation, a model was created using a sine function to match the amplitude (+/- 2%), frequency, and phase of the measured rotational output variation data. A second model with a hypothetical variation of +/- 7% was also created to represent the largest variation that could exist without violating the allowable dose window in the delivery system. A measured beam profile near the end of target life was used to create a modified beam profile model for the target degradation. These models were then incorporated into the treatment plan by modifying the leaf opening times in the delivery sinogram. A new beam model was also created to mimic the change in beam energy seen near the end of target life. The plans were then calculated using a research version of the PLANNED ADAPTIVE treatment planning software from TomoTherapy, Inc. Three plans were evaluated in this study: Head and neck, partial breast, and prostate. The D50 of organs at risk, the D95 for planning target volumes (PTVs), and the local dose difference were used to evaluate the changes in the modified treatment plans. Dosimetric effects from rotational variation were found to be low (less than 1%) for a typical variation of +/- 2%. Even using a variation of +/- 7%, DVH values and dose distributions were altered by less than 2% for all scenarios. The dosimetric effects of target degradation were found to be slightly more significant. For a model using data taken just before target failure, dosimetric differences of 2%-4% were observed in the recalculated plans when compared to the original plans. The largest effects (up to 4.5%) were observed for PTVs that were located at deeper depths as seen in the prostate plan. Overall, the recalculated plans show that the dosimetric effects of rotational variation and target degradation are on the order of 1%-4% for helical tomotherapy on the HiART system and do not pose a risk for significant deviations from the original treatment plan.