Optimized Varian aSi portal dosimetry: development of datasets for collective use.

Although much literature has been devoted to portal dosimetry with the Varian amorphous silicon (aSi) portal imager, the majority of the described methods are not routinely adopted because implementation procedures are cumbersome and not within easy reach of most radiotherapy centers. To make improved portal dosimetry solutions more generally available, we have investigated the possibility of converting optimized configurations into ready-to-use standardized datasets. Firstly, for all commonly used photon energies (6, 10, 15, 18, and 20 MV), basic beam data acquired on 20 aSi panels were used to assess the interpanel reproducibility. Secondly, a standardized portal dose image prediction (PDIP) algorithm configuration was created for every energy, using a three-step process to optimize the aSi dose response function and profile correction files for the dosimetric calibration of the imager panel. An approximate correction of the backscatter of the Exact arm was also incorporated. Thirdly, a set of validation fields was assembled to assess the accuracy of the standardized configuration. Variations in the basic beam data measured on different aSi panels very rarely exceeded 2% (2 mm) and are of the same order of magnitude as variations between different Clinacs when measuring in reference conditions in water. All studied aSi panels can hence be regarded as nearly identical. Standardized datasets were successfully created and implemented. The test package proved useful in highlighting possible problems and illustrating remaining limitations, but also in demonstrating the good overall results (95% pass rate for 3%,3 mm) that can be obtained. The dosimetric behavior of all tested aSi panels was found to be nearly identical for all tested energies. The approach of using standardized datasets was then successfully tested through the creation and evaluation of PDIP preconfigured datasets that can be used within the Varian portal dosimetry solution.

Implementing RapidArc into clinical routine: a comprehensive program from machine QA to TPS validation and patient QA.

PURPOSE: With the increased commercial availability of intensity modulated arc therapy (IMAT) comes the need for comprehensive QA programs, covering the different aspects of this newly available technology. This manuscript proposes such a program for the RapidArc (RA) (Varian Medical Systems, Palo Alto) IMAT solution. METHODS: The program was developed and tested out for a Millennium120 MLC on iX Clinacs and a HighDefinition MLC on a Novalis TX, using a variety of measurement equipment including Gafchromic film, 2D ion chamber arrays (Seven29 and StarCheck, PTW, Freiburg, Germany) with inclinometer and Octavius phantom, the Delta4 systam (ScandiDos, Uppsala, Sweden) and the portal imager (EPID). First, a number of complementary machine QA tests were developed to monitor the correct interplay between the accelerating/decelerating gantry, the variable dose rate and the MLC position, straining the delivery to the maximum allowed limits. Second, a systematic approach to the validation of the dose calculation for RA was adopted, starting with static gantry and RA specific static MLC shapes and gradually moving to dynamic gantry, dynamic MLC shapes. RA plans were then optimized on a series of artificial structures created within the homogeneous Octavius phantom and within a heterogeneous lung phantom. These served the double purpose of testing the behavior of the optimization algorithm (PRO) as well as the precision of the forward dose calculation. Finally, patient QA on a series of clinical cases was performed with different methods. In addition to the well established in-phantom QA, we evaluated the portal dosimetry solution within the Varian approach. RESULTS: For routine machine QA, the "Snooker Cue" test on the EPID proved to be the most sensitive to overall problem detection. It is also the most practical one. The "Twinkle" and "Sunrise" tests were useful to obtain well differentiated information on the individual treatment delivery components. The AAA8.9 dose calculations showed excellent agreement with all corresponding measurements, except in areas where the 2.5 mm fixed fluence resolution was insufficient to accurately model the tongue and groove effect or the dose through nearly closed opposing leafs. Such cases benefited from the increased fluence resolution in AAA10.0. In the clinical RA fields, these effects were smeared out spatially and the impact of the fluence resolution was considerably less pronounced. The RA plans on the artificial structure sets demonstrated some interesting characteristics of the PRO8.9 optimizer, such as a sometimes unexpected dependence on the collimator rotation and a suboptimal coverage of targets within lung tissue. Although the portal dosimetry was successfully validated, we are reluctant to use it as a sole means of patient QA as long as no gantry angle information is embedded. CONCLUSIONS: The all-in validation program allows a systematic approach in monitoring the different levels of RA treatments. With the systematic approach comes a better understanding of both the capabilities and the limits of the used solution. The program can be useful for implementation, but also for the validation of major upgrades.

A qualitative and a quantitative analysis of an auto-segmentation module for prostate cancer.

PURPOSE: This work describes the clinical validation of an automatic segmentation algorithm in CT-based radiotherapy planning for prostate cancer patients. MATERIAL AND METHODS: The validated auto-segmentation algorithm (Smart Segmentation, version 1.0.05) is a rule-based algorithm using anatomical reference points and organ-specific segmentation methods, developed by Varian Medical Systems (Varian Medical Systems iLab, Baden, Switzerland). For the qualitative analysis, 39 prostate patients are analysed by six clinicians. Clinicians are asked to rate the auto-segmented organs (prostate, bladder, rectum and femoral heads) and to indicate the number of slices to correct. For the quantitative analysis, seven radiation oncologists are asked to contour seven prostate patients. The individual clinician contour variations are compared to the automatic contours by means of surface and volume statistics, calculating the relative volume errors and both the volume and slice-by-slice degree of support, a statistical metric developed for the purposes of this validation. RESULTS: The mean time needed for the automatic module to contour the four structures is about one minute on a standard computer. The qualitative evaluation using a score with four levels ("not acceptable", "acceptable", "good" and "excellent") shows that the mean score for the automatically contoured prostate is "good"; the bladder scores between "excellent" and "good"; the rectum scores between "acceptable" and "not acceptable". Using the concept of surface and volume degree of support, the degree of support given to the automatic module is comparable to the relative agreement among the clinicians for prostate and bladder. The slice-by-slice analysis of the surface degree of support pinpointed the areas of disagreement among the clinicians as well as between the clinicians and the automatic module. CONCLUSION: The efficiency and the limits of the automatic module are investigated with both a qualitative and a quantitative analysis. In general, with efficient correction tools at hand, the use of this auto-segmentation module will lead to a time gain for the prostate and the bladder; with the present version of the algorithm, modelling of the rectum still needs improvement. For the quantitative validation, the concept of relative volume error and degree of support proved very useful.

Quality assurance in the 22991 EORTC ROG trial in localized prostate cancer: dummy run and individual case review.

INTRODUCTION: EORTC trial 22991 was designed to evaluate the addition of concomitant and adjuvant short-term hormonal treatments to curative radiotherapy in terms of disease-free survival for patients with intermediate risk localized prostate cancer. In order to assess the compliance to the 3D conformal radiotherapy protocol guidelines, all participating centres were requested to participate in a dummy run procedure. An individual case review was performed for the largest recruiting centres as well. MATERIALS AND METHODS: CT-data of an eligible prostate cancer patient were sent to 30 centres including a description of the clinical case. The investigator was requested to delineate the volumes of interest and to perform treatment planning according to the protocol. Thereafter, the investigators of the 12 most actively recruiting centres were requested to provide data on five randomly selected patients for an individual case review. RESULTS: Volume delineation varied significantly between investigators. Dose constraints for organs at risk (rectum, bladder, hips) were difficult to meet. In the individual case review, no major protocol deviations were observed, but a number of dose reporting problems were documented for centres using IMRT. CONCLUSIONS: Overall, results of this quality assurance program were satisfactory. The efficacy of the combination of a dummy run procedure with an individual case review is confirmed in this study, as none of the evaluated patient files harboured a major protocol deviation. Quality assurance remains a very important tool in radiotherapy to increase the reliability of the trial results. Special attention should be given when designing quality assurance programs for more complex irradiation techniques.

The use of magnetic sensors to monitor moderate deep inspiration breath hold during breast irradiation with dynamic MLC compensators.

BACKGROUND AND PURPOSE: To reduce the dose to the heart during left breast irradiation, a moderate deep breath hold technique (MDIBH) was introduced. Originally, verification of the MDIBH was performed with portal images acquired in movie loop during the treatment delivery. However, this verification method is not compatible with the use of dynamic MLC compensation, recently introduced because of its often superior dose distribution. Magnetic sensors were evaluated as an additional/alternative method to monitor the breath hold. MATERIAL AND METHODS: In a first phase, the reproducibility of MDIBH for breast patients was evaluated by investigating for 19 patients the set-up errors derived from portal images in cine loop acquisition during MDIBH. In a second phase, for 10 patients, the breathing curves recorded by magnetic sensors were used to monitor beam-on and beam-off while portal images were simultaneously recorded in movie loop. In a third phase, breast patients treated with dynamic MLC compensation were trained for MDIBH and monitored with magnetic sensors. RESULTS: The interfraction reproducibility of MDIBH for the initial 19 patients was recorded: the mean set-up error, the systematic and the random deviations are all smaller than 4mm in the anterior-posterior direction and in the cranio-caudal direction and smaller than 2 degrees along the rotation axis. Magnetic sensors provided a reproducible breathing curve: while the mean amplitude recorded for 10 patients varied substantially between patients, the individual standard deviation of the amplitude for each session was smaller than 3mm. For these 10 patients, the intrafraction set-up variation between the first portal image of two consecutive breath holds and the intra-breath hold set-up variation between the first and last portal image of each breath hold is smaller than 2mm in the anterior-posterior direction, smaller than 3mm in the cranio-caudal direction and smaller than 1.5 degrees along the rotation axis. CONCLUSION: Using magnetic sensors to record the breathing curve of left breast patients in MDIBH, a verification method was developed, suitable for combining MDIBH with dynamic MLC compensation.

On-line quality assurance of rotational radiotherapy treatment delivery by means of a 2D ion chamber array and the Octavius phantom.

For routine pretreatment verification of innovative treatment techniques such as (intensity modulated) dynamic arc therapy and helical TomoTherapy, an on-line and reliable method would be highly desirable. The present solution proposed by TomoTherapy, Inc. (Madison, WI) relies on film dosimetry in combination with up to two simultaneous ion chamber point dose measurements. A new method is proposed using a 2D ion chamber array (Seven29, PTW, Freiburg, Germany) inserted in a dedicated octagonal phantom, called Octavius. The octagonal shape allows easy positioning for measurements in multiple planes. The directional dependence of the response of the detector was primarily investigated on a dual energy (6 and 18 MV) Clinac 21EX (Varian Medical Systems, Palo Alto, CA) as no fixed angle incidences can be calculated in the Hi-Art TPS of TomoTherapy. The array was irradiated from different gantry angles and with different arc deliveries, and the dose distributions at the level of the detector were calculated with the AAA (Analytical Anisotropic Algorithm) photon dose calculation algorithm implemented in Eclipse (Varian). For validation on the 6 MV TomoTherapy unit, rotational treatments were generated, and dose distributions were calculated with the Hi-Art TPS. Multiple cylindrical ion chamber measurements were used to cross-check the dose calculation and dose delivery in Octavius in the absence of the 2D array. To compensate for the directional dependence of the 2D array, additional prototypes of Octavius were manufactured with built-in cylindrically symmetric compensation cavities. When using the Octavius phantom with a 2 cm compensation cavity, measurements with an accuracy comparable to that of single ion chambers can be achieved. The complete Octavius solution for quality assurance of rotational treatments consists of: The 2D array, two octagonal phantoms (with and without compensation layer), an insert for nine cylindrical ion chambers, and a set of inserts of various tissue equivalent materials of different densities. The combination of the 2D array with the Octavius phantom proved to be a fast and reliable method for pretreatment verification of rotational treatments. Quality control of TomoTherapy patients was reduced to a total of approximately 25 min per patient.

Testing of the analytical anisotropic algorithm for photon dose calculation.

The analytical anisotropic algorithm (AAA) was implemented in the Eclipse (Varian Medical Systems) treatment planning system to replace the single pencil beam (SPB) algorithm for the calculation of dose distributions for photon beams. AAA was developed to improve the dose calculation accuracy, especially in heterogeneous media. The total dose deposition is calculated as the superposition of the dose deposited by two photon sources (primary and secondary) and by an electron contamination source. The photon dose is calculated as a three-dimensional convolution of Monte-Carlo precalculated scatter kernels, scaled according to the electron density matrix. For the configuration of AAA, an optimization algorithm determines the parameters characterizing the multiple source model by optimizing the agreement between the calculated and measured depth dose curves and profiles for the basic beam data. We have combined the acceptance tests obtained in three different departments for 6, 15, and 18 MV photon beams. The accuracy of AAA was tested for different field sizes (symmetric and asymmetric) for open fields, wedged fields, and static and dynamic multileaf collimation fields. Depth dose behavior at different source-to-phantom distances was investigated. Measurements were performed on homogeneous, water equivalent phantoms, on simple phantoms containing cork inhomogeneities, and on the thorax of an anthropomorphic phantom. Comparisons were made among measurements, AAA, and SPB calculations. The optimization procedure for the configuration of the algorithm was successful in reproducing the basic beam data with an overall accuracy of 3%, 1 mm in the build-up region, and 1%, 1 mm elsewhere. Testing of the algorithm in more clinical setups showed comparable results for depth dose curves, profiles, and monitor units of symmetric open and wedged beams below dmax. The electron contamination model was found to be suboptimal to model the dose around dmax, especially for physical wedges at smaller source to phantom distances. For the asymmetric field verification, absolute dose difference of up to 4% were observed for the most extreme asymmetries. Compared to the SPB, the penumbra modeling is considerably improved (1%, 1 mm). At the interface between solid water and cork, profiles show a better agreement with AAA. Depth dose curves in the cork are substantially better with AAA than with SPB. Improvements are more pronounced for 18 MV than for 6 MV. Point dose measurements in the thoracic phantom are mostly within 5%. In general, we can conclude that, compared to SPB, AAA improves the accuracy of dose calculations. Particular progress was made with respect to the penumbra and low dose regions. In heterogeneous materials, improvements are substantial and more pronounced for high (18 MV) than for low (6 MV) energies.

The Octavius1500 2D ion chamber array and its associated phantoms: dosimetric characterization of a new prototype.

PURPOSE: The purpose of the study is to characterize the prototype of the new Octavius1500 (PTW, Freiburg, Germany) 2D ion chamber array, covering its use in different phantom setups, from the most basic solid water sandwich setup to the more complex cylindrical Octavius® 4D (Oct4D) (PTW) phantom/detector combination. The new detector houses nearly twice the amount of ion chambers as its predecessors (Seven29 and Octavius729), thereby tackling one of the most important limitations of ion chamber (or diode) arrays, namely the limited detector density. The 0.06 cm3 cubic ion chambers are now arranged in a checkerboard pattern, leaving no lines (neither longitudinally nor laterally) without detectors. METHODS: All measurements were performed on a dual energy (6 MV and 18 MV) iX Clinac (Varian Medical Systems, Palo Alto, CA) and all calculations were done in the Eclipse treatment planning system (Varian) with the Anisotropic Analytical Algorithm. First, the basic characteristics of the 2D array, such as measurement stability, dose rate dependence and dose linearity were investigated in the solid water sandwich setup. Second, the directional dependence was assessed to allow the evaluation of the new Octavius2D phantom (Oct2D(1500)) for planar verification measurements of composite plans. Third, measurements were performed in the Oct4D phantom to evaluate the impact of the increased detector density on the accuracy of the volumetric dose reconstruction. RESULTS: While showing equally good dose linearity and dose rate independence, the Octavius1500 outperforms the previous models because of its instantaneous measurement stability and its twofold active area coverage. Orthogonal field-by-field measurements immediately benefit from the increased detector density. The 3.9 cm wide compensation cavity in the new Oct2D(1500) phantom prototype adequately corrects for directional dependence from the rear, resulting in good agreement within the target dose. Discrepancies may arise towards the sides of the array because of uncompensated lateral beam incidence. The beneficial impact of the detector density is most prominent in the Oct4D system, for which the average pass rate (PR) is now nearly 100% (99.31±0.37) when using gamma criteria of 2%G,2 mm (10% dose threshold). In search of gamma analysis criteria that are not too lenient to detect possibly relevant deviations, the authors conclude that for our radiotherapy environment, the authors choose to adopt 3%L,3 mm PR97% (threshold 10%) criteria for the Oct2D(1500)/Octavius1500 system and 2%L,3 mm PR97% (threshold 10%) for the Oct4D/Octavius1500 system. These are first line pass/check criteria and plans that fail are not necessarily rejected, but submitted to a more detailed investigation. CONCLUSIONS: When irradiated from the front, the Octavius1500 array has two main advantages over its 729 predecessors: its instantaneous measurement stability and--most importantly-its twofold detector density. In the Oct2D1500 phantom, these advantages are counterbalanced by the more pronounced directional dependence. The measurement-based 3D dose reconstruction in the Oct4D system, however, benefits considerably from the higher detector density in the checkerboard panel design.