Patient-specific Quality Assurance Procedures Research Articles

Evaluation of HyperArc™ using film and portal dosimetry quality assurance.

HyperArc™ is a stereotactic radiotherapy modality designed for targeting multiple brain metastases using a single isocenter with multiple non-coplanar arcs. This study aimed to assess the efficacy of two patient-specific quality assurance methods, film and the Varian Portal Dosimetry System with Varian's HyperArc™ technique and raise important considerations in the customisation of patient-specific quality assurance to accommodate HyperArc™ delivery. Assessment criteria included gamma analysis and mean dose at full width half maximum. The minimum metastasis size, maximum off-axis distance and suitable energy were identified and validated. Patient-specific quality assurance procedures were applied to a range of clinically relevant brain metastasis plans. Initial investigation into energy selection showed no significant differences in gamma pass rates using 6MV, 6MV FFF, or 10MV FFF for metastasis sizes greater than 15mm diameter at the isocenter. Gamma pass rates (2%/2mm) for 15mm metastases at the isocenter for all energies were greater than 96.0% for portal dosimetry and greater than 98.7% for film. Fields of size 15mm placed at various distances (10-70mm) from the isocenter resulted in a maximum mean dose difference of 1.5% between film and planned. Clinically relevant plans resulted in a maximum mean dose difference for selected metastases of 1.0% between film and plan and a maximum point dose difference of 2.9% between portal dose and plan. Portal dose image prediction was a quick and convenient quality assurance tool for metastases larger than 15mm near the isocenter but provided diminished geometrical relevance for off-axis metastases. Film QA required exacting procedures but offered the ability to assess the accuracy of geometrical targeting for off-axis metastases and provided dosimetric accuracy for metastases to well below 15mm diameter.

Patient Specific Quality Assurance Automation with Scripting on HID Devices

<h3>Purpose/Objective(s)</h3> Many quality assurance procedures in radiotherapy are tedious and time-consuming but can be automated to improve the efficient and minimum human-errors. In this study we focused on the patient specific quality assurance (PSQA) procedure. The purpose was to develop a robust and quick software interface for Human Interactive Device (HID) operations and significantly accelerate the workflow. <h3>Materials/Methods</h3> The clinical PSQA system of our clinics consists of s treatment planning system (TPS), an oncology information system (OIS) and a measurement and analysis software. The complete PSQA process includes three main parts: (1) QA verification plan creation, (2) treatment plan loading to treatment machine and (3) measurement, data analysis and report. A homemade software tool, AUTOFLOW was developed as a general interface and installed on multiple workstations to automate the operations of local HID devices. The workflow of HID operations in these components was modularized for maximum efficiency and reliability. The required patient information for each computer was stored in a QA task file created from patient database and synchronized by the AUTOFLOW interface in individual workstations. All human operations on HID devices and communication between multiple computers were automated by the AUTOFLOW interface sequentially or parallelly during the entire PSQA process. <h3>Results</h3> We tested the developed automation workflow on 20 clinical QA plans in our current clinical environments. Overall, around 175 (±12) human HID operations on various workstations were removed from the PSQA procedure. On average, 8(±0.8) minutes was needed to complete a PSQA case with the AUTOFLOW software tool, compared to 13(±3.1) minutes for complete manual operations. These results represent a 38% saving in time. No errors were found for all operations performed with this interface. <h3>Conclusion</h3> The AUTOFLOW software interface was successfully developed and used to perform PSQA in our current clinical environment. This interface has substantially reduced the time, and eliminated user stress and input errors for our PSQA procedure. It can potentially work with other TPS, QA, R&V systems of our department to improve the efficiency and reduce human errors for other QA procedures. For future works we plan to improve the system flexibility/stability and implement this application to other QA procedures

Overall Accuracy of Single-Isocenter Multiple Metastases SRS Treatments over Time: Comparison of Two Commercially Available Methods

<h3>Purpose/Objective(s)</h3> Single isocenter stereotactic radiosurgery (SI-SRS) is an efficient approach for dose delivery to multiple brain metastases since the treatment duration is considerably reduced, compared to the standard method of the isocenter placement at each tumor center. However, the complexity of this technique results in increased sensitivity to geometric and dosimetric uncertainties during plan delivery, thus patient-specific quality assurance procedures of high accuracy are mandatory. This study presents the investigation into the consistency of this technique, using the a 4D dose verification system and 3D printed head phantoms. <h3>Materials/Methods</h3> A treatment plan with two brain metastases was delivered four times in the period of one month on a Linac, to evaluate the reproducibility of the SI-SRS technique. The targets had a concave (2.4 cc) and a spherical (1.5 cc) shape, placed at a distance of 5 cm. Dose prescription was set to 8 Gy for both targets and a treatment plan of a single fraction using four Hyperarc beams, one full coplanar arc and three half noncoplanar arcs, arranged with a single isocenter automatically located on the center of the distance between the two targets was created. Before plan delivery, Winston-Lutz (WL) tests were performed to determine the diameter of the sphere containing the radiation isocenter for various positions of the collimator, gantry and couch. The dose distribution was recalculated on 4D geometry and measured with the 1000 SRS ion chamber array. The head phantom accommodated a film insert and the irradiation procedure consisted of positioning, imaging and plan delivery as if it was the real patient. Gamma analysis was performed for 3%/1mm acceptance criteria using a dose threshold of 1 Gy for films and 10% of maximum dose for 4D. <h3>Results</h3> The Root Mean Square (RMS) variation of the isocenter was 0.71±0.15 mm for gantry, 0.66±0.14 mm for collimator and 0.87±0.22 mm for couch, validating that the isocenter diameter was within the tolerance of 1 mm before each measurement. Gamma analysis revealed a mean passing rate (GPR) of 93.2%±0.07% for 4D measurements, compared to 91.2%±1.8% for the film ones. The reduced GPR in the case of the head phantom is expected, since it is an End-to-End QA procedure, incorporating not only the plan delivery uncertainties, but also the ones related to image registration and couch rotation. <h3>Conclusion</h3> Both QA methods showed a good agreement with each other and the calculated dose distribution having GPRs above 90% for the 3%/1mm criteria, rendering them suitable for verification of SRS plans' dosimetric and spatial accuracy. The standard deviation of the GPRs revealed that the plan delivery was consistent between the measurements, demonstrating high reproducibility and accuracy of the SI-SRS technique.

14: Knowledge-Based Planning to Improve and Automate Patient-Specific Quality Assurance Procedures in Clinical Trials - Updated Secondary Analysis of CCTG HN6

14: Knowledge-Based Planning to Improve and Automate Patient-Specific Quality Assurance Procedures in Clinical Trials - Updated Secondary Analysis of CCTG HN6

A method for in vivo treatment verification of IMRT and VMAT based on electronic portal imaging device

BackgroundIntensity-modulated radiation therapy (IMRT) and volume-modulated arc therapy (VMAT) are rather complex treatment techniques and require patient-specific quality assurance procedures. Electronic portal imaging devices (EPID) are increasingly used in the verification of radiation therapy (RT). This work aims to develop a novel model to predict the EPID transmission image (TI) with fluence maps from the RT plan. The predicted TI is compared with the measured TI for in vivo treatment verification.MethodsThe fluence map was extracted from the RT plan and corrections of penumbra, response, global field output, attenuation, and scatter were applied before the TI was calculated. The parameters used in the model were calculated separately for central axis and off-axis points using a series of EPID measurement data. Our model was evaluated using a CIRS thorax phantom and 20 clinical plans (10 IMRT and 10 VMAT) optimized for head and neck, breast, and rectum treatments.ResultsComparisons of the predicted and measured images were carried out using a global gamma analysis of 3%/2 mm (10% threshold) to validate the accuracy of the model. The gamma pass rates for IMRT and VMAT were greater than 97.2% and 94.5% at 3%/2 mm, respectively.ConclusionWe have developed an accurate and straightforward EPID-based quality assurance model that can potentially be used for in vivo treatment verification of the IMRT and VMAT delivery.

Clinical experiment on quality control comparison of complex treatment plans of the VMAT technique using a diode-based cylindrical phantom (ArcCHECK) and an amorphous silicon-based planar detector (A-Si1000)

BackgroundWith increasing application of Volumetric Modulated Arc Therapy (VMAT) and the complexity of treatment plans, necessity of patient-specific quality assurance procedures is raised. Therefore, this study aimed to perform VMAT plan control using the ArcCHECK® (Sun Nuclear) and the EPID (Electronic Portal Imaging Device) A-Si1000 and make a comparison. MethodsThe study was carried out at AL KAWTAR Oncology Center (Fez-Meknes/Morocco) with the TrueBeamTM accelerator and EclipseTM Treatment Planning System (TPS). GeoGebra 6.0 and Origin 2019 software were applied to statistical analysis and to reconstruct the dose distribution maps for ArcCHECK® Phantom, respectively. Gamma index test with the 3%/3 and 2%/2 mm criteria applied to compare two pretreatment quality control instrumentation. ResultsAlthough the EPID A-Si1000 and the ArcCHECK® phantom both were capable of performing VMAT plan control, results of gamma index analysis revealed some differences in the quality of the reviewed plans with the two devices. Conclusionbased on the results, a single device to implement patient QA for complex VMAT plans may not be reliable. Therefore, to ensure patient protection against machine or human errors, applying the EPID A-Si1000 and the ArcCHECK® phantom both are proposed to evaluate the quality of complex VMAT plans for different cancer types.

The GATE-RTion/IDEAL Independent Dose Calculation System for Light Ion Beam Therapy

Patient specific quality assurance can be improved using an independent dose calculation system. In addition, the implementation of such a system may support light ion beam therapy facilities in reducing the needs for beam time, by substituting some of the experimental patient-specific quality assurance procedures by independent dose calculation. The GATE-RTion-based IDEAL system for light ion beam therapy was developed for this purpose. It was built in a DICOM-in, DICOM-out fashion, for easy integration into a state-of-the-art technology-based workflow for scanned ion beam therapy. This article describes the IDEAL system, followed by its clinical implementation at MedAustron for proton and carbon ion beams. Medical physics acceptance and commissioning steps are presented together with key results: for 3D proton and carbon ion reference boxes, 97% of the points agreed within 5% from the measurements. Experimental validation of stopping powers using real pig samples were between 1.8% and 3.8% for soft tissues. Finally, five clinical cases are described, i.e. two proton and three carbon ion treatments. Dosimetric benchmarking against TPS calculations are presented and discussed in details. As expected, the IDEAL software evidenced limitations arising from the pencil beam algorithm available in the TPS for carbon ions, especially in the presence of air cavities. The IDEAL system was found to satisfy the clinical requirements for independent dose calculation of scanned ion beam delivery systems and is being clinically implemented at MedAustron. The open-source code as well as the documentation was released on the OpenGATE collaboration website, thus allowing for long term maintenance and future upgrades based on a more widespread utilization.

Applications of machine and deep learning to patient-specific IMRT/VMAT quality assurance.

In order to deliver accurate and safe treatment to cancer patients in radiation therapy using advanced techniques such as intensity modulated radiation therapy (IMRT) and volumetric‐arc radiation therapy (VMAT), patient specific quality assurance (QA) should be performed before treatment. IMRT/VMAT dose measurements in a phantom using various devices have been clinically adopted as standard method for QA. This approach allows the verification of the accuracy of the dose calculation, data transfer, and the delivery system. However, patient‐specific QA procedures are expensive and require significant time and effort by the physicists. Over the past 5 years, machine learning (ML) and deep learning (DL) algorithms for predictions of IMRT/VMAT QA outcome have been investigated. Various ML and DL models have shown promising prediction accuracy and a high potential as time‐efficient virtual QA tool. In this paper, we review the ML and DL based models that were developed for patient specific IMRT and VMAT QA outcome predictions from algorithmic and clinical applicability perspectives. We focus on comparing the algorithms, the dataset sizes, the input parameters and features, the QA outcome prediction approaches, the validation, the performance, the clinical applicability, and the potential clinical impact. In addition, we discuss the present challenges as well as the future directions in the implementation of these models. To the best of our knowledge, this is the first review on the application of ML and DL based models in IMRT/VMAT QA predictions.

P281] Choice of reference dose value for dose plan comparison procedures using gafchromictm EBT3 and film QA software

Purpose In VMAT patient-specific quality assurance procedures using Gafchromic™ EBT3 film dosimetry and FilmQA software, to eliminate scan-to-scan variability and uncertainty therefrom, EBT3 films exposed to each VMAT arc are coupled (and digitalized in a single scan) to two reference films: one film exposed to a known dose and one unexposed film. The aim of this work is give indications on how to choose reference dose value based on some experimental evidence. Methods 6MV, 10MV and 10FFF photon beams were used to obtain calibration dose curves from 0 to 25 Gy and to study energy dependence on Gafchromic™ EBT3 film dosimetry digitized with Epson 10000 XL and analyzed with FILM QA software. Triple-channel method was used to improve film dosimetry accuracy. 16 VMAT treatment plans (8 conventionally fractionated and 8 SBRT) calculated with Eclipse™ TPS and delivered with TrueBeam 2.5 were imported as DICOM files in FILM QA software. This data file include all plan data including dose range and isocenter. The density distribution on the plane delivered for each individual arc was calculated. The choice of the reference know dose value was made on the basis of such density distribution. This allows to obtain a linear dose scaling on the calibration curve and therefore a more accurate dose map. Pre-treatment verifications were quantified using the global gamma index approach with 3%/3 mm criteria and 95% passing rate. Results The selection criteria identified were two: (1) the reference dose value is within the range 70% −90% of the cumulative distribution density; (2) the value of reference dose is closely near of the maximum distribution density value. For conventionally fractionated plans the average passing rate was 98.8% and for SBRT plans the average was 97.6%. Conclusion This approach allowed to establish a single criterion of choice for reference know dose value to eliminate scan-to-scan variability and uncertainty therefrom also for different types of treatment: conventionally fractionated and SBRT. Under the experimental conditions described above, the use of EBT3 film dosimetry analyzed with FilmQA software shows to be an accurate method for VMAT conventionally fractionated and SBRT treatment plans verifications.

OC-0534: Establishment of patient-specific quality assurance procedure for Dynamic WaveArc delivery technique

OC-0534: Establishment of patient-specific quality assurance procedure for Dynamic WaveArc delivery technique

Dosimetric comparison of single-beam multi-arc and 2-beam multi-arc VMAT optimization in the Monaco treatment planning system

The purpose of this study was to evaluate the dosimetric and practical effects of the Monaco treatment planning system “max arcs-per-beam” optimization parameter in pelvic radiotherapy treatments. We selected for this study a total of 17 previously treated patients with a range of pelvic disease sites including prostate (9), bladder (1), uterus (3), rectum (3), and cervix (1). For each patient, 2 plans were generated, one using an arc-per-beam setting of “1” and another with an arc-per-beam setting of “2” using the volumes and constraints established from the initial clinical treatments. All constraints and dose coverage objects were kept the same between plans, and all plans were normalized to 99.7% to ensure 100% of the planning target volume (PTV) received 95% of the prescription dose. Plans were evaluated for PTV conformity, homogeneity, number of monitor units, number of control points, and overall plan acceptability. Treatment delivery time, patient-specific quality assurance procedures, and the impact on clinical workflow were also assessed. We found that for complex-shaped target volumes (small central volumes with extending arms to cover nodal regions), the use of 2 arc-per-beam (2APB) parameter setting achieved significantly lower average dose-volume histogram values for the rectum V20 (p = 0.0012) and bladder V30 (p = 0.0036) while meeting the high dose target constraints. For simple PTV shapes, we found reduced monitor units (13.47%, p = 0.0009) and control points (8.77%, p = 0.0004) using 2APB planning. In addition, we found a beam delivery time reduction of approximately 25%. In summary, the dosimetric benefit, although moderate, was improved over a 1APB setting for complex PTV, and equivalent in other cases. The overall reduced delivery time suggests that the use of mulitple arcs per beam could lead to reduced patient-on-table time, increased clinical throughput, and reduced medical physics quality assurance effort.

Parallelized patient-specific quality assurance for high-dose-rate image-guided brachytherapy in an integrated computed tomography-on-rails brachytherapy suite.

To describe a parallelized patient-specific quality assurance (QA) program designed to ensure safety and quality in image-guided high-dose-rate brachytherapy in an integrated computed tomography (CT)-on-rails brachytherapy suite. A patient-specific QA program has been modified for the image-guided brachytherapy (IGBT) program in an integrated CT-on-rails brachytherapy suite. In the modification of the QA procedures of Task Group-59, the additional patient-specific QA procedures are included to improve rapid IGBT workflow with applicator placement, imaging, planning, treatment, and applicator removal taking place in one room. The IGBT workflow is partitioned into two groups of tasks that can be performed in parallel by two or more staff members. One of the unique components of our implemented workflow is that groups work together to perform QA steps in parallel and in series during treatment planning and contouring. Coordinating efforts in this systematic way enable rapid and safe brachytherapy treatment while incorporating 3-dimensional anatomic variations between treatment days. Implementation of these patient-specific QA procedures in an integrated CT-on-rails brachytherapy suite ensures confidence that a rapid workflow IGBT program can be implemented without sacrificing patient safety or quality and deliver highly-conformal dose to target volumes. These patient-specific QA components may be adapted to other IGBT environments that seek to provide rapid workflow while ensuring quality.

Patient Specific QA for Rapid Work Flow Image Guided Brachytherapy

Patient Specific QA for Rapid Work Flow Image Guided Brachytherapy

MO‐F‐16A‐05: Spot‐Scanning Proton Therapy Patient‐Specific Quality Assurance: Our Methodology and Results From 295 Treatment Plans

Purpose:To report on the patient‐specific quality assurance (PSQA) results for 295 spot‐scanning proton therapy treatment plans from the MD Anderson PTC‐Houston. We show how the results differed by treatment site and how they were affected by the treatment plan optimization method and by a range shifter in the treatment field. We also discuss some causes of PSQA problems.Methods:The PSQA procedure, which is designed to verify both the accuracy of the treatment planning system's (Eclipse™ v8.9) dose calculations and the dose delivery of the Hitachi PROBEAT synchrotron, consists of (1) an end‐to‐end test in which the beam is delivered and measured at the prescribed gantry angle, and (2) additional dose plane measurements made from gantry angle 270°. HPlusQA™ software automatically performs the gamma analysis with criteria 3% (dose tolerance), 3 mm (distance‐to‐agreement, DTA) and 2%, 2 mm. Passing is defined as at least 90% of the pixels having a gamma score less than 1.Results:The PSQA gamma passing rate was 96.2% for 3%, 3 mm, and 85.3% for 2%, 2 mm. The rate depended on the treatment site. For example, the 3%, 3 mm passing rate was 95% for head and neck plans, vs 100% for prostate plans. The passing rates of multi‐ vs. single‐field optimization plans did not significantly differ. However, the rate for fields with range shifters was 94.8±0.6%, vs 99.0±0.6% for those without (p = 0.002). Longitudinal dose gradients caused most of the low scores. Overestimation of the calculated dose proximal to the spread‐out Bragg peak (SOBP) caused many of the others.Conclusion:The planned and delivered doses consistently agreed within tolerance levels. Minor dose modeling deficiencies remain proximal to the SOBP. The 3% dose tolerance, 3 mm DTA, with 90% pixel passing rate is a reasonable action level for 2D gamma comparisons.

Dosimetric accuracy assessment of a treatment plan verification system for scanned proton beam radiotherapy: one-year experimental results and Monte Carlo analysis of the involved uncertainties

During one year of clinical activity at the Italian National Center for Oncological Hadron Therapy 31 patients were treated with actively scanned proton beams. Results of patient-specific quality assurance procedures are presented here which assess the accuracy of a three-dimensional dose verification technique with the simultaneous use of multiple small-volume ionization chambers. To investigate critical cases of major deviations between treatment planning system (TPS) calculated and measured data points, a Monte Carlo (MC) simulation tool was implemented for plan verification in water. Starting from MC results, the impact of dose calculation, dose delivery and measurement set-up uncertainties on plan verification results was analyzed. All resulting patient-specific quality checks were within the acceptance threshold, which was set at 5% for both mean deviation between measured and calculated doses and standard deviation. The mean deviation between TPS dose calculation and measurement was less than ±3% in 86% of the cases. When all three sources of uncertainty were accounted for, simulated data sets showed a high level of agreement, with mean and maximum absolute deviation lower than 2.5% and 5%, respectively.

American College of Radiology (ACR) and American Society for Radiation Oncology (ASTRO) Practice Guideline for Intensity-modulated Radiation Therapy (IMRT)

Intensity-modulated radiation therapy (IMRT) is a complex technique for the delivery of radiation therapy preferentially to target structures while minimizing doses to adjacent normal critical structures. It is widely utilized in the treatment of a variety of clinical indications in radiation oncology, including tumors of the central nervous system, head and neck, breast, prostate, gastrointestinal tract, and gynecologic organs, as well as in situations where previous radiation therapy has been delivered, and has allowed for significant therapeutic advances in many clinical areas. IMRT treatment planning and delivery is a complex process. Safe and reliable delivery of IMRT requires appropriate process design and adherence to quality assurance (QA) standards. A collaborative effort of the American College of Radiology and American Society for Therapeutic Radiation Oncology has produced a practice guideline for IMRT. The guideline defines the qualifications and responsibilities of all the involved personnel, including the radiation oncologist, physicist, dosimetrist, and radiation therapist. Factors with respect to the QA of the treatment planning system, treatment-planning process, and treatment-delivery process are discussed, as are issues related to the utilization of volumetric modulated arc therapy. Patient-specific QA procedures are presented. Successful IMRT programs involve integration of many processes: patient selection, patient positioning/immobilization, target definition, treatment plan development, and accurate treatment delivery. Appropriate QA procedures, including patient-specific QA procedures, are essential to ensure quality in an IMRT program and to assure patient safety.

RapidArc quality assurance through MapCHECK

The purpose is to devise a patient‐specific quality assurance procedure for RapidArc radiotherapy using the MapCHECK detector array. We use our existing MapCHECK system and a Solid Water phantom with an embedded ion chamber to develop a quality assurance procedure for RapidArc treatment after commissioning. The ion chamber used to measure the absolute dose is surrounded by 6 cm layers of solid water on the anterior and posterior sides. Partial arcs derived from the treatment planning system were used with MapCHECK to determine the actual shape of the dose and correct for the angular dependence. The ion chamber measurements were within 1% of the absolute doses predicted by the Eclipse treatment system. When using a partial arc from 60° to 300° on the MapCHECK array (gamma index <1: 3%, 3 mm, 10% threshold), we obtain a 97.52% average passing rate. A combination of ion chamber phantoms, partial arcs and the MapCHECK system can be used for quality assurance of RapidArc therapies.PACS number: 3.6.96.0

Ionization chamber array for patient specific VMAT, Tomotherapy and IMRT QA

The evaluation between measured and calculated dose is essential in the patient specific quality assurance procedures for intensity modulated radiation therapy. The high complexity of volumetric arc radiotherapy, Tomotherpay and intensity modulated radiation therapy deliveries attributed to the dynamic and synchronization requirements of such techniques require new methods and potentially new tools for the quality assurance of such techniques. Studies evaluating the dosimetric performance of EDR2 film and a 2D ionization chamber array quality assurance device have been performed in our institution. Our results showed that differences between the detector systems are small. The respective gamma index histograms showed that when 3% dose difference and 3mm distance to agreement are used, more than 90% of the evaluated points were within the tolerance criteria

SU-GG-T-40: Estimation of Dosimetric Uncertainties for a Patient Specific Treatment Plan in Brachytherapy

Purpose: To develop a patient specific quality assurance procedure for interstitial brachytherapy by estimating the dosimetric uncertainties of a treatment plan. Method and Materials: Three different sources of errors in brachytherapytreatment planning are considered in this study: Errors due to the limited CT slice thickness, errors due to limited accuracy of the spatial reconstruction of catheters and the corresponding dwell positions, and errors due to inter‐fraction motion of the catheters. The corresponding probability distributions of these errors are investigated and are used to estimate their impact on the dose distribution for a specific treatment plan. The quality assurance procedure consists of the quantitative comparison between the original dose distribution and the dose distribution calculated using the error probability distributions.Results:Dosimetric uncertainties of academic and clinical treatment plans indicate that the limited accuracy of the catheter reconstruction is most critical while the limited CT slice thickness mainly affects the dose distribution at locations near the tip or the connector end. The dosimetric uncertainties are depending on the magnitude of the errors and the relative weights of different sources of errors. As an example, the dosimetric impact of a 1 mm error in catheter reconstruction for a head and neck tumor is in the order of 5% for the PTV. Conclusion: In this work, a method for the estimation of dosimetric uncertainties for a brachytherapytreatment plan was developed. On a patient specific basis, different sources of errors and their particular dosimetric impact are analyzed. Consequently, by identifying and quantifying the sources of errors for an individual treatment plan, the method can be applied for highly efficient patient specific quality assurance procedures for interstitial brachytherapy.

Quantitative dosimetric verification of an IMRT planning and delivery system

Background and purpose: The accuracy of dose calculation and delivery of a commercial serial tomotherapy treatment planning and delivery system (Peacock, NOMOS Corporation) was experimentally determined. Materials and methods: External beam fluence distributions were optimized and delivered to test treatment plan target volumes, including three with cylindrical targets with diameters ranging from 2.0 to 6.2 cm and lengths of 0.9 through 4.8 cm, one using three cylindrical targets and two using C-shaped targets surrounding a critical structure, each with different dose distribution optimization criteria. Computer overlays of film-measured and calculated planar dose distributions were used to assess the dose calculation and delivery spatial accuracy. A 0.125 cm 3 ionization chamber was used to conduct absolute point dosimetry verification. Thermoluminescent dosimetry chips, a small-volume ionization chamber and radiochromic film were used as independent checks of the ion chamber measurements. Results: Spatial localization accuracy was found to be better than ±2.0 mm in the transverse axes (with one exception of 3.0 mm) and ±1.5 mm in the longitudinal axis. Dosimetric verification using single slice delivery versions of the plans showed that the relative dose distribution was accurate to ±2% within and outside the target volumes (in high dose and low dose gradient regions) with a mean and standard deviation for all points of −0.05% and 1.1%, respectively. The absolute dose per monitor unit was found to vary by ±3.5% of the mean value due to the lack of consideration for leakage radiation and the limited scattered radiation integration in the dose calculation algorithm. To deliver the prescribed dose, adjustment of the monitor units by the measured ratio would be required. Conclusions: The treatment planning and delivery system offered suitably accurate spatial registration and dose delivery of serial tomotherapy generated dose distributions. The quantitative dose comparisons were made as far as possible from abutment regions and examination of the dosimetry of these regions will also be important. Because of the variability in the dose per monitor unit and the complex nature of the calculation and delivery of serial tomotherapy, patient-specific quality assurance procedures will include a measurement of the delivered target dose.