Volume 39, Issue 3, March 2012
Index of content:
: A recently completed Phase I clinical trial combined concurrent Mitomycin-C chemotherapy with deep regional heating using BSD-2000 Sigma-Ellipse applicator (BSD Corporation, Salt Lake City, UT, U.S.A.) for the treatment of nonmuscle invasive bladder cancer. This work presents a new treatment planning approach, and demonstrates potential impact of this approach on improvement of treatment quality.Methods
: This study retrospectively analyzes a subset of five patients on the trial. For each treatment, expert operators selected “clinical-optimal” settings based on simple model calculation on the BSD-2000 control console. Computed tomography(CT) scans acquired prior to treatment were segmented to create finite element patient models for retrospective simulations with Sigma-HyperPlan (Dr. Sennewald Medizintechnik GmbH, Munchen, Germany). Since Sigma-HyperPlan does not account for the convective nature of heat transfer within a fluid filled bladder, an effective thermal conductivity for bladder was introduced. This effective thermal conductivity value was determined by comparing simulation results with clinical measurements of bladder and rectum temperatures. Regions of predicted high temperature in normal tissues were compared with patient complaints during treatment.Treatment results using “computed-optimal” settings from the planning system were compared with clinical results using clinical-optimal settings to evaluate potential of treatment improvement by reducing hot spot volume.Results
: For all five patients, retrospective treatment planning indicated improved matches between simulated and measured bladder temperatures with increasing effective thermal conductivity. The differences were mostly within 1.3 °C when using an effective thermal conductivity value above 10 W/K/m. Changes in effective bladder thermal conductivity affected surrounding normal tissues within a distance of ∼1.5 cm from the bladder wall. Rectal temperature differences between simulation and measurement were large due to sensitivity to the sampling locations in rectum. The predicted bladder T90 correlated well with single-point bladder temperature measurement. Hot spot locations predicted by the simulation agreed qualitatively with patient complaints during treatment. Furthermore, comparison between the temperature distributions with clinical and computed-optimal settings demonstrated that the computed-optimal settings resulted in substantially reduced hot spot volumes.Conclusions
: Determination of an effective thermal conductivity value for fluid filled bladder was essential for matching simulation and treatmenttemperatures. Prospectively planning patients using the effective thermal conductivity determined in this work can potentially improve treatment efficacy (compared to manual operator adjustments) by potentially lower discomfort from reduced hot spots in normal tissue.
The 2014 initiative can have potentially unintended negative consequences for medical physics in diagnostic imaging and nuclear medicine39(2012); http://dx.doi.org/10.1118/1.3658741View Description Hide Description
- RADIATION THERAPY PHYSICS
Fast compressed sensing-based CBCT reconstruction using Barzilai-Borwein formulation for application to on-line IGRT39(2012); http://dx.doi.org/10.1118/1.3679865View Description Hide DescriptionPurpose:
Compressed sensing theory has enabled an accurate, low-dose cone-beam computed tomography(CBCT)reconstruction using a minimal number of noisy projections. However, the reconstruction time remains a significant challenge for practical implementation in the clinic. In this work, we propose a novel gradient projection algorithm, based on the Gradient-Projection-Barzilai-Borwein formulation (GP-BB), that handles the total variation (TV)-norm regularization-based least squares problem for the CBCTreconstruction in a highly efficient manner, with speed acceptable for routine use in the clinic.Methods:
CBCT is reconstructed by minimizing an energy function consisting of a data fidelity term and a TV-norm regularization term. Both terms are simultaneously minimized by calculating the gradient projection of the energy function with the step size determined using an approximate Hessian calculation at each iteration, based on the Barzilai-Borwein formulation. To speed up the process, a multiresolution optimization is used. In addition, the entire algorithm was designed to run with a single graphics processing unit (GPU) card. To evaluate the performance, the Shepp-Logan numerical phantom, the CatPhan 600 physical phantom, and a clinically-treated head-and-neck patient were acquired from the TrueBeam™ system (Varian Medical Systems, Palo Alto, CA). For each scan, in total, 364 projections were acquired in a 200° rotation. The imager has 1024 × 768 pixels with 0.388 × 0.388-mm resolution. This was down-sampled to 512 × 384 pixels with 0.776 × 0.776-mm resolution for reconstruction. Evenly spaced angles were subsampled and used for varying the number of projections for the image reconstruction. To assess the performance of our GP-BB algorithm, we have implemented and compared with three compressed sensing-type algorithms, the two of which are popular and published (forward–backward splitting techniques), and the other one with a basic line-search technique. In addition, the conventional Feldkamp-Davis-Kress (FDK) reconstruction of the clinical patient data is compared as well.Results:
In comparison with the other compressed sensing-type algorithms, our algorithm showed convergence in ≤30 iterations whereas other published algorithms need at least 50 iterations in order to reconstruct the Shepp-Logan phantom image. With the CatPhan phantom, the GP-BB algorithm achieved a clinically-reasonable image with 40 projections in 12 iterations, in less than 12.6 s. This is at least an order of magnitude faster in reconstruction time compared with the most recent reports utilizing GPU technology given the same input projections. For the head-and-neck clinical scan, clinically-reasonable images were obtained from 120 projections in 34–78 s converging in 12–30 iterations. In this reconstruction range (i.e., 120 projections) the image quality is visually similar to or better than the conventional FDK reconstructed images using 364 projections. This represents a dose reduction of nearly 67% (120/364 projections) while maintaining a reasonable speed in clinical implementation.Conclusions:
In this paper, we proposed a novel, fast, low-dose CBCTreconstruction algorithm using the Barzilai-Borwein step-size calculation. A clinically viable head-and-neck image can be obtained within ∼34–78 s while simultaneously cutting the dose by approximately 67%. This makes our GP-BB algorithm potentially useful in an on-line image-guided radiation therapy(IGRT).
39(2012); http://dx.doi.org/10.1118/1.3681274View Description Hide DescriptionPurpose
: The success of the preclinical studies in Microbeam Radiation Therapy(MRT) paved the way to the clinical trials under preparation at the Biomedical Beamline of the European Synchrotron Radiation Facility. Within this framework, an accurate determination of the deposited dose is crucial. With that aim, the scatter factors, which translate the absolute dose measured in reference conditions (2 × 2 cm2field size at 2 cm-depth in water) to peak doses, were assessed.Methods
: Monte Carlo(MC) simulations were performed with two different widely used codes,PENELOPE and GEANT4, for the sake of safety. The scatter factors were obtained as the ratio of the doses that are deposited by a microbeam and by a field of reference size, at the reference depth. The calculated values were compared with the experimental data obtained by radiochromic (ISP HD-810) films and a PTW 34070 large area chamber.Results
: The scatter factors for different microbeam field sizes assessed by the two MC codes were in agreement and reproduced the experimental data within uncertainty bars. Those correction factors were shown to be non-negligible for the future MRT clinical settings: an average 30% lower dose was deposited by a 50 μm microbeam with respect to the reference conditions.Conclusions
: For the first time, the scatter factors in MRT were systematically studied. They constitute an essential key to deposit accurate doses in the forthcoming clinical trials in MRT. The good agreement between the different calculations and the experimental data confirms the reliability of this challenging micrometric dose estimation.
Verification of proton range, position, and intensity in IMPT with a 3D liquid scintillator detector system39(2012); http://dx.doi.org/10.1118/1.3681948View Description Hide DescriptionPurpose:
Intensity-modulated proton therapy (IMPT) using spot scanned proton beams relies on the delivery of a large number of beamlets to shape the dose distribution in a highly conformal manner. The authors have developed a 3D system based on liquid scintillator to measure the spatial location, intensity, and depth of penetration (energy) of the proton beamlets in near real-time.Methods:
The detectorsystem consists of a 20 × 20 × 20 cc liquid scintillator (LS) material in a light tight enclosure connected to a CCDcamera. This camera has a field of view of 25.7 by 19.3 cm and a pixel size of 0.4 mm. While the LS is irradiated, the camera continuously acquires images of the light distribution produced inside the LS. Irradiations were made with proton pencil beams produced with a spot-scanning nozzle. Pencil beams with nominal ranges in water between 9.5 and 17.6 cm were scanned to irradiate an area of 10 × 10 cm square on the surface of the LS phantom. Image frames were acquired at 50 ms per frame.Results:
The signal to noise ratio of a typical Bragg peak was about 170. Proton range measured from the light distribution produced in the LS was accurate to within 0.3 mm on average. The largest deviation seen between the nominal and measured range was 0.6 mm. Lateral position of the measured pencil beam was accurate to within 0.4 mm on average. The largest deviation seen between the nominal and measured lateral position was 0.8 mm; however, the accuracy of this measurement could be improved by correcting light scattering artifacts. Intensity of single proton spots were measured with precision ranging from 3 % for the smallest spot intensity (0.005 MU) to 0.5 % for the largest spot (0.04 MU).Conclusions:
Our LS detectorsystem has been shown to be capable of fast, submillimeter spatial localization of proton spots delivered in a 3D volume. This system could be used for beam range, intensity and position verification in IMPT.
39(2012); http://dx.doi.org/10.1118/1.3682172View Description Hide DescriptionPurpose
: To demonstrate the feasibility of photon energy-modulated radiotherapy during beam-on time.Methods:
A cylindrical device made of aluminum was conceptually proposed as an energy modulator. The frame of the device was connected with 20 tubes through which mercury could be injected or drained to adjust the thickness of mercury along the beam axis. In Monte Carlo(MC) simulations, a flattening filter of 6 or 10 MV linac was replaced with the device. The thickness of mercury inside the device varied from 0 to 40 mm at the field sizes of 5 × 5 cm2 (FS5), 10 × 10 cm2 (FS10), and 20 × 20 cm2 (FS20). At least 5 billion histories were followed for each simulation to create phase space files at 100 cm source to surface distance (SSD). In-water beam data were acquired by additional MC simulations using the above phase space files. A treatment planning system (TPS) was commissioned to generate a virtual machine using the MC-generated beam data. Intensity modulated radiation therapy(IMRT) plans for six clinical cases were generated using conventional 6 MV, 6 MV flattening filter free, and energy-modulated photon beams of the virtual machine.Results:
As increasing the thickness of mercury, Percentage depth doses (PDD) of modulated 6 and 10 MV after the depth of dose maximum were continuously increased. The amount of PDD increase at the depth of 10 and 20 cm for modulated 6 MV was 4.8% and 5.2% at FS5, 3.9% and 5.0% at FS10 and 3.2%–4.9% at FS20 as increasing the thickness of mercury from 0 to 20 mm. The same for modulated 10 MV was 4.5% and 5.0% at FS5, 3.8% and 4.7% at FS10 and 4.1% and 4.8% at FS20 as increasing the thickness of mercury from 0 to 25 mm. The outputs of modulated 6 MV with 20 mm mercury and of modulated 10 MV with 25 mm mercury were reduced into 30%, and 56% of conventional linac, respectively. The energy-modulated IMRT plans had less integral doses than 6 MV IMRT or 6 MV flattening filter free plans for tumors located in the periphery while maintaining the similar quality of target coverage, homogeneity, and conformity.Conclusions:
The MC study for the designed energy modulator demonstrated the feasibility of energy-modulated photon beams available during beam-on time. The planning study showed an advantage of energy-and intensity modulated radiotherapy in terms of integral dose without sacrificing any quality of IMRT plan.
39(2012); http://dx.doi.org/10.1118/1.3682313View Description Hide DescriptionPurpose:
This study was undertaken to explore the effects of the jaws and the MLC openings on the neutrondose equivalent (DE) at the maze door and neutron flux at the patient plane.Methods:
The neutrondose equivalent was measured at the maze entrance door of a 15 MV therapy linear accelerator room. All measurements were performed using various field sizes up to 40 cm × 40 cm. Activation detectors constructed from natural Indium (In) were exposed at Cd envelope to neutrons in order to estimate relative changes of epithermal neutron fluences in the patient plane.Results:
Our study showed that the dose equivalent at the maze door is at the highest when the jaw are closed and that maximal jaws opening reduces the DE by more than 20%. The neutrondose equivalent at the maze door measured for radiation fields defined by jaws do not differ significantly from the DE measured when MLC determines the same size radiation field. The epithermal capture reaction rate measured using different jaw openings differs by approximately 10%. When an MLC leaf is inserted into a fixed geometry for one opening of the jaws, an increase of the epithermal neutron capture reaction rate in Indium activation detectors was observed.Conclusions:
There is no significant difference in the neutron DE when MLC defines radiation field instead of jaws. This leads to the conclusion that the overall number of neutrons remains similar and it does not depend on how primary photon beam was stopped—by the jaws or the MLC. An increase of the fast neutron capture reaction rate when MLC leaves are inserted probably originates from the neutron scattering.
Is there a single spot size and grid for intensity modulated proton therapy? Simulation of head and neck, prostate and mesothelioma cases39(2012); http://dx.doi.org/10.1118/1.3683640View Description Hide DescriptionPurpose:
To assess the quality of dose distributions in real clinical cases for different dimensions of scanned proton pencil beams. The distance between spots (i.e., the grid of delivery) is optimized for each dimension of the pencil beam.Methods:
The authors vary the σ of the initial Gaussian size of the spot, from σ x = σ y = 3 mm to σ x = σ y = 8 mm, to evaluate the impact of the proton beam size on the quality of intensity modulated proton therapy (IMPT) plans. The distance between spots, Δx and Δy, is optimized on the spot plane, ranging from 4 to 12 mm (i.e., each spot size is coupled with the best spot grid resolution). In our Hyperion treatment planning system (TPS), constrained optimization is applied with respect to the organs at risk (OARs), i.e., the optimization tries to satisfy the dose objectives in the planning target volume (PTV) as long as all planning objectives for the OARs are met. Three-field plans for a nasopharynx case, two-field plans for a prostate case, and two-field plans for a malignant pleural mesothelioma case are considered in our analysis.Results:
For the head and neck tumor, the best grids (i.e., distance between spots) are 5, 4, 6, 6, and 8 mm for σ = 3, 4, 5, 6, and 8 mm, respectively. σ ≤ 5 mm is required for tumor volumes with low dose and σ ≤ 4 mm for tumor volumes with high dose. For the prostate patient, the best grid is 4, 4, 5, 5, and 5 mm for σ = 3, 4, 5, 6, and 8 mm, respectively. Beams with σ > 3 mm did not satisfy our first clinical requirement that 95% of the prescribed dose is delivered to more than 95% of prostate and proximal seminal vesicles PTV. Our second clinical requirement, to cover the distal seminal vesicles PTV, is satisfied for beams as wide as σ = 6 mm. For the mesothelioma case, the low dose PTV prescription is well respected for all values of σ, while there is loss of high dose PTV coverage for σ > 5 mm. The best grids have a spacing of 6, 7, 8, 9, and 12 mm for σ = 3, 4, 5, 6, and 8 mm, respectively.Conclusions:
The maximum acceptable proton pencil beam σ depends on the volume treated, the protocol of delivery, and optimization of the plan. For the clinical cases, protocol and optimization used in this analysis, acceptable σs are ≤ 4 mm for the head and neck tumor, ≤ 3 mm for the prostate tumor and ≤ 6 mm for the malignant pleural mesothelioma. One can apply the same procedure used in this analysis when given a “class” of patients, a σ and a clinical protocol to determine the optimal grid spacing.
39(2012); http://dx.doi.org/10.1118/1.3684959View Description Hide DescriptionPurpose:
This paper describes a novel method for simultaneous intrafraction tracking of multiple fiducial markers. Although the proposed method is generic and can be adopted for a number of applications including fluoroscopy based patient position monitoring and gated radiotherapy, the tracking results presented in this paper are specific to tracking fiducial markers in a sequence of cone beam CT projection images.Methods:
The proposed method is accurate and robust thanks to utilizing the mean shift and random sampling principles, respectively. The performance of the proposed method was evaluated with qualitative and quantitative methods, using data from two pancreatic and one prostate cancer patients and a moving phantom. The ground truth, for quantitative evaluation, was calculated based on manual tracking preformed by three observers.Results:
The average dispersion of marker position error calculated from the tracking results for pancreas data (six markers tracked over 640 frames, 3840 marker identifications) was 0.25 mm (at iscoenter), compared with an average dispersion for the manual ground truth estimated at 0.22 mm. For prostate data (three markers tracked over 366 frames, 1098 marker identifications), the average error was 0.34 mm. The estimated tracking error in the pancreas data was < 1 mm (2 pixels) in 97.6% of cases where nearby image clutter was detected and in 100.0% of cases with no nearby image clutter.Conclusions:
The proposed method has accuracy comparable to that of manual tracking and, in combination with the proposed batch postprocessing, superior robustness. Marker tracking in cone beam CT(CBCT) projections is useful for a variety of purposes, such as providing data for assessment of intrafraction motion, target tracking during rotational treatment delivery, motion correction of CBCT, and phase sorting for 4D CBCT.
Accuracy of the electron transport in mcnp5 and its suitability for ionization chamber response simulations: A comparison with the egsnrc and penelope codes39(2012); http://dx.doi.org/10.1118/1.3685446View Description Hide DescriptionPurpose:
In this work, accuracy of themcnp5 code in the electron transport calculations and its suitability for ionization chamber (IC) response simulations in photonbeams are studied in comparison to egsnrc and penelope codes.Methods:
The electron transport is studied by comparing the depth dose distributions in a water phantom subdivided into thin layers using incident energies (0.05, 0.1, 1, and 10 MeV) for the broad parallel electron beams. The IC response simulations are studied in water phantom in three dosimetric gas materials (air, argon, and methane based tissue equivalent gas) for photonbeams (60Co source, 6 MV linear medical accelerator, and mono-energetic 2 MeV photon source). Two optional electron transport models of mcnp5 are evaluated: the ITS-based electron energy indexing (mcnp5ITS) and the new detailed electron energy-loss straggling logic (mcnp5new). The electron substep length (ESTEP parameter) dependency in mcnp5 is investigated as well.Results:
For the electron beam studies, large discrepancies (>3%) are observed between themcnp5dose distributions and the reference codes at 1 MeV and lower energies. The discrepancy is especially notable for 0.1 and 0.05 MeV electron beams. The boundary crossing artifacts, which are well known for the mcnp5ITS, are observed for the mcnp5new only at 0.1 and 0.05 MeV beamenergies. If the excessive boundary crossing is eliminated by using single scoring cells, the mcnp5ITS provides dose distributions that agree better with the reference codes than mcnp5new. The mcnp5dose estimates for the gas cavity agree within 1% with the reference codes, if the mcnp5ITS is applied or electron substep length is set adequately for the gas in the cavity using the mcnp5new. The mcnp5new results are found highly dependent on the chosen electron substep length and might lead up to 15% underestimation of the absorbed dose.Conclusions:
Since themcnp5 electron transport calculations are not accurate at all energies and in every medium by general clinical standards, caution is needed, if mcnp5 is used with the current electron transport models for dosimetric applications.
A real-time respiration position based passive breath gating equipment for gated radiotherapy: A preclinical evaluation39(2012); http://dx.doi.org/10.1118/1.3678986View Description Hide DescriptionPurpose:
To develop a passive gating system incorporating with the real-time position management (RPM) system for the gated radiotherapy.Methods:
Passive breath gating (PBG) equipment, which consists of a breath-hold valve, a controller mechanism, a mouthpiece kit, and a supporting frame, was designed. A commercial real-time positioning management system was implemented to synchronize the target motion and radiation delivery on a linear accelerator with the patient’s breathing cycle. The respiratory related target motion was investigated by using the RPM system for correlating the external markers with the internal target motion while using PBG for passively blocking patient’s breathing. Six patients were enrolled in the preclinical feasibility and efficiency study of the PBG system.Results:
PBG equipment was designed and fabricated. The PBG can be manually triggered or released to block or unblock patient’s breathing. A clinical workflow was outlined to integrate the PBG with the RPM system. After implementing the RPM based PBG system, the breath-hold period can be prolonged to 15–25 s and the treatment delivery efficiency for each field can be improved by 200%–400%. The results from the six patients showed that the diaphragm motion caused by respiration was reduced to less than 3 mm and the position of the diaphragm was reproducible for difference gating periods.Conclusions:
A RPM based PBG system was developed and implemented. With the new gating system, the patient’s breath-hold time can be extended and a significant improvement in the treatment delivery efficiency can also be achieved.
Pretreatment quality assurance of flattening filter free beams on 224 patients for intensity modulated plans: A multicentric study39(2012); http://dx.doi.org/10.1118/1.3685461View Description Hide DescriptionPurpose:
Pretreatment quality assurance data from four centers, members of the European TrueBeam council were analyzed with different verification devices to assess reliability of flattening filter free beam delivery for intensity modulated radiotherapy(IMRT) and RapidArc (RA) techniques.Methods:
TrueBeam® (Varian Medical System) is a new linear accelerator designed for delivering flattened, as well as flattening filter free beams. Pretreatment dosimetric validation of plan delivery was performed with different verification devices and responses to high dose rates were tested. Treatment planning was done in Eclipse planning system (PRO 8.9, AAA 8.9). γ evaluation was performed with (dose difference) = 3% and (distance to agreement) = 3 mm scoring the gamma agreement index (GAI, % of field area passing the test). Two hundred and twenty-four patients with 1–6 lesions in various anatomical regions and dose per fraction ranging from 1.8 Gy to 25 Gy were included in the study; 88 were treated with 6 MV flattening filter free (X6FFF) beam energy and 136 with 10 MV flattening filter free (X10FFF) beam. Gafchromic films in solid water, delta4, arccheck, and matrixx phantom were used to verify the dose distributions. Additionally, point measurements were performed using a PinPoint chamber and a Farmer chamber.Results:
Dose calculation as well as dose delivery was equally accurate for IMRT and RA delivery (IMRT: GAI = 99.3% (±1.1); RA: GAI = 98.8% (±1.1) as well as for the two beams evaluated (X6FFF: GAI = 99.1% (±1.0); X10FFF: GAI = 98.8% (±1.2). Only small differences were found for the four verification devices. A point dose verification was performed on 52 cases, obtaining a dose deviation of 0.34%. The GAI variations with number of monitor units were statistically significant.Conclusions:
The TrueBeam FFF modality, analyzed with a variety of verification devices and planned with Eclipse planning system is dosimetrically accurate (within the specified limits 3 mm/3%) for both X6FFF and X10FFF beam energy.
39(2012); http://dx.doi.org/10.1118/1.3685582View Description Hide DescriptionPurpose:
The dosimetric impact of gold fiducial markers (FM) implanted prior to external beam radiotherapy of prostate cancer on low dose rate (LDR) brachytherapy seed implants performed in the context of combined therapy was investigated.Methods:
A virtual water phantom was designed containing a single FM. Single and multi source scenarios were investigated by performing Monte Carlodose calculations, along with the influence of varying orientation and distance of the FM with respect to the sources. Three prostate cancer patients treated with LDR brachytherapy for a recurrence following external beam radiotherapy with implanted FM were studied as surrogate cases to combined therapy. FM and brachytherapy seeds were identified on post implant CT scans and Monte Carlodose calculations were performed with and without FM. The dosimetric impact of the FM was evaluated by quantifying the amplitude of dose shadows and the volume of cold spots.D 90 was reported based on the post implant CT prostate contour.Results:
Large shadows are observed in the single source-FM scenarios. As expected from geometric considerations, the shadows are dependent on source-FM distance and orientation. Large dose reductions are observed at the distal side of FM, while at the proximal side a dose enhancement is observed. In multisource scenarios, the importance of shadows appears mitigated, although FM at the periphery of the seed distribution caused underdosage (<prescription dose). In clinical cases, the FM reduced the dose to some voxels by up to 50% and generated shadows with extents of the order of 4 mm. Within the prostate contour, cold spots (<95% prescription dose) of the order of 20 mm3 were observed. D 90 proved insensitive to the presence of FM for the cases selected.Conclusions:
There is a major local impact of FM present in LDR brachytherapy seed implant dose distributions. Therefore, reduced tumor control could be expected from FM implanted in tumors, although our results are too limited to draw conclusions regarding clinical significance.
39(2012); http://dx.doi.org/10.1118/1.3685581View Description Hide DescriptionPurpose:
To evaluate the geometric accuracy of the isocenter of an image-guidance system, as implemented in theexactrac system from brainlab, relative to the linear accelerator radiation isocenter. Subsequently to correct the x-ray isocenter of the exactrac system for any geometric discrepancies between the two isocenters.Methods:
Five Varian linear accelerators all equipped with electronic imaging devices andexactrac with robotics from brainlab were evaluated. A commercially available Winston-Lutz phantom and an in-house made adjustable base were used in the setup. The electronic portal imaging device of the linear accelerators was used to acquire MV-images at various gantry angles. Stereoscopic pairs of x-rayimages were acquired using the exactrac system. The deviation between the position of the external laser isocenter and the exactrac isocenter was evaluated using the commercial software of the exactrac system. In-house produced software was used to analyze the MV-images and evaluate the deviation between the external laser isocenter and the radiation isocenter of the linear accelerator. Subsequently, the deviation between the radiation isocenter and the isocenter of the exactrac system was calculated. A new method of calibrating the isocenter of the exactrac system was applied to reduce the deviations between the radiation isocenter and the exactrac isocenter.Results:
To evaluate the geometric accuracy a 3D deviation vector was calculated for each relative isocenter position. The 3D deviation between the external laser isocenter and the isocenter of theexactrac system varied from 0.21 to 0.42 mm. The 3D deviation between the external laser isocenter and the linac radiation isocenter ranged from 0.37 to 0.83 mm. The 3D deviation between the radiation isocenter and the isocenter of the exactrac system ranged from 0.31 to 1.07 mm. Using the new method of calibrating the exactrac isocenter the 3D deviation of one linac was reduced from 0.90 to 0.23 mm. The results were complicated due to routine maintenance of the linac, including laser calibration. It was necessary to repeat the measurements in order to perform the calibration of the exactrac isocenter.Conclusions:
The deviations between the linac radiation isocenter and theexactrac isocenter were of an order that may have clinical relevance. An alternative method of calibrating the isocenter of the exactrac system was applied and reduced the deviations between the two isocenters.
Determining the effects of microsphere and surrounding material composition on 90Y dose kernels using egsnrc and mcnp539(2012); http://dx.doi.org/10.1118/1.3685577View Description Hide DescriptionPurpose:
Recent advances in the imaging of90Y using positron emission tomography(PET) and improved uncertainty in the branching ratio for the internal pair production component of 90Y decay allow for a more accurate determination of the activity distribution of 90Y microspheres within a patient. This improved activity distribution can be convolved with the dose kernel of 90Y to calculate the dose distribution within a patient. This work investigates the effects of microsphere and surrounding material composition on 90Y dose kernels using egsnrc and mcnp5 and compares the results of these two transport codes.Methods:
Monte Carlo simulations were performed withegsnrc and mcnp5 to calculate the dose rate at multiple radial distances around various 90Y sources. Point source simulations were completed with mcnp5 to determine the optimal electron transport settings for this work. After determining the optimal settings, point source simulations were completed using egsnrc (user code edknrc) and mcnp5 in water and liver [as defined by the International Commission on Radiation Units and Measurements (ICRU) Report 44]. The results were compared to ICRU Report 72 reference data. Point source simulations were also completed in water with a density of 1.06 g·cm−3 to evaluate the effect of the density of the surrounding material. Glass and resin microsphere simulations were performed with average and maximum diameter and density values (based on values given in the literature) in water and in liver. The results were compared to point source simulation results using the same transport code and in the same surrounding material. All simulations had statistical uncertainties less than 1%.Results:
The optimal transport settings inmcnp5 for this work included using the energy-and step-specific algorithm (DBCN 17J 2) and ESTEP set to 10. These settings were used for all subsequent simulations with mcnp5. The point source simulations in water for both egsnrc and mcnp5 were found to agree within 2% of the ICRU 72 reference data over the investigated range. Point source simulations in liver had large differences relative to ICRU 72, approaching −60% near the maximum range of 90Y. These differences are mostly attributed to the difference in density between water (1.0 g·cm−3) and liver (1.06 g·cm−3). Glass and resin microsphere simulations showed a slight decrease in the dose rate near the maximum range of 90Y relative to the point source simulations. The largest relative differences were approximately −4.2% and −2.8% for the glass and resin microspheres, respectively. Agreement between the egsnrc and mcnp5 simulations results was generally good.Conclusions:
The presence of the microsphere material causes slight differences in the90Y dose kernel compared to those calculated with point sources. Large differences were seen between simulations in water and those in liver. For the most accurate calculation of the dose distribution, the density of the patient’s liver should be accounted for in the calculation of the dose kernel. Lastly, due to the need to determine the optimal transport settings with mcnp5, electron transport with this code should be used with caution.
39(2012); http://dx.doi.org/10.1118/1.3684952View Description Hide DescriptionPurpose:
In a recent paper by Bouchardet al. [Med. Phys. 36(10), 4654–4663 (2009)], a theoretical model of quality correction factors for idealistic so-called plan-class specific reference (PCSR) fields was proposed. The reasoning was founded on the definition of PCSR fields made earlier by Alfonso et al. [Med. Phys. 35(11), 5179–5186 (2008)], requiring the beam to achieve charged particle equilibrium (CPE), in a time-averaged sense, in the reference medium. The relation obtained by Bouchard et al. was derived using Fano’s theorem (1954) which states that if CPE is established in a given medium, the dose is independent of point-to-point density variations. A potential misconception on the achievability of the condition required by Fano (1954) might be responsible for false practical conclusions, both in the definition of PCSR fields as well as the theoretical model of quality correction factor.Methods:
In this paper, the practical achievability of CPE in external beams is treated in detail. The fact that this condition is not achievable in single or composite deliveries is illustrated by an intuitive method and is also formally demonstrated.Conclusions:
Fano’s theorem is not applicable in external beam radiation dosimetry without (virtually) removing attenuation effects, and therefore, the relation conditionally defined by Bouchardet al. (2009) cannot be valid in practice. A definition of PCSR fields in the recent formalism for nonstandard beams proposed by Alfonso et al. (2008) should be modified, revising the criterion of CPE condition. The authors propose reconsidering the terminology used to describe standard and nonstandard beams. The authors argue that quality correction factors of intensity modulated radiation therapy PCSR fields (i.e., ) could be unity under ideal conditions, but it is concluded that further investigation is necessary to confirm that hypothesis.
Evaluation of a lung tumor autocontouring algorithm for intrafractional tumor tracking using low-field MRI: A phantom study39(2012); http://dx.doi.org/10.1118/1.3685578View Description Hide DescriptionPurpose:
The first aim of this study is to investigate the feasibility of online autocontouring of tumor in low field MR images (0.2 and 0.5 T) by means of a phantom and simulation study for tumor-tracking in linac-MR systems. The second aim of this study is to develop an MR compatible, lungtumor motion phantom.Methods:
An autocontouring algorithm was developed to determine both the position and shape of a lungtumor from each intra fractional MR image. To initiate the algorithm, an expert user contours the tumor and its maximum anticipated range of motion (herein termed the Background) using pretreatment scan data. During treatment, the algorithm processes each intrafractional MR image and automatically contours the tumor. To evaluate this algorithm, the authors built a phantom that replicates the low field contrast parameters (proton density,T 1, T 2) of lungtumors and healthy lung parenchyma. This phantom allows simulation of MR images with the expected lungtumorCNR at 0.2 and 0.5 T by using a single 3 T scanner. Dynamic bSSFP images (approximately 4 images per second) are acquired while the phantom undergoes a series of preprogrammed motions based on patient lungtumor motion data. These images are autocontoured off-line using our algorithm. The fidelity of autocontouring is assessed by comparing autocontoured tumor shape and its centroid position to the actual tumor shape and its position.Results:
The algorithm successfully contoured the shape of a moving tumor model from dynamic MR images acquired every 275 ms. Dice’s coefficients of > 0.96 and > 0.93 are achieved in 0.5 and 0.2 T equivalent images, respectively. Also, the algorithm tracked tumor position during dynamic studies, with root mean squared error (RMSE) values of < 0.55 and < 0.92 mm for 0.5 and 0.2 T equivalent images, respectively. Autocontouring speed is approximately 5 ms for each image.Conclusions:
Dice’s coefficients of > 0.96 and > 0.93 are achieved between autocontoured and real tumor shapes, and the position of a tumor can be tracked with RMSE values of < 0.55 and < 0.92 mm in 0.5 and 0.2 T equivalent images, respectively. These results demonstrate the feasibility of lungtumor autocontouring in low field MR images, and, by extension, intrafractional lungtumor tracking with our laboratory’s linac-MR system.
Experimental assessments of intrafractional prostate motion on sequential and simultaneous boost to a dominant intraprostatic lesion39(2012); http://dx.doi.org/10.1118/1.3685586View Description Hide DescriptionPurpose:
To investigate experimentally the impact of intrafractional prostate motion on the delivered dose to a dominant intraprostatic lesion (DIL) using volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy(IMRT) with sequential and simultaneous boost.Methods:
A series of six IMRT and VMAT treatment plans were generated, evaluated, and compared for two patient CT scans with dissimilar anatomies. Plans were generated for the prostate with and without the DIL. Plans were delivered using a Varian CLINAC and 2D dose distributions were measured usingmapcheckTM–mapphanTM system. The effect of the prostate intrafractional motion on the delivery of the plans was studied by delivering the plans to the mapcheckTM–mapphanTM system on a programmable motion platform. Prostate intrafractional motion was simulated based on six different motion patterns from the literature obtained on Calypso system (Calypso System, Calypso Medical, Seattle, WA, USA) in a clinical study that provided continuous, real-time localization, and monitoring of the prostate. Absolute dose differences and Gamma analysis were used to assess the quality of a total of 42 plans with motion and without motion.Results:
Dose escalation to the whole prostate from 76 to 86 Gy caused the rectum and bladder to exceed normal tissue tolerances in both patients. All the DIL boost plans satisfied the planning criteria and delivery quality assurance when motion was not present. For a single fraction, the motion pattern with large constant shift caused the largest dose delivery discrepancy with mean Gamma value (1.14–1.44) and the lowest plan passing percentage (18.9%–35.7%), while the motion pattern with continuous random changes during treatment had the least impact on dose delivery with mean Gamma value (0.33–0.55) and the highest passing percentage (81.9%–100%) for all the investigated plans. For dose escalation to DIL in the presence of intrafractional prostate motion, a significant difference was observed between the different motion patterns (p < 0.05), but no significant difference in the sensitivity to motion between the various plans was observed (p = 0.30). Based on Gamma analysis, treatment courses in which 15% of the fractions are dominated by severe motion proved to be significantly different from those dominated by random motion (p < 0.05).Conclusions:
The impact of intrafractional prostate motion on dose delivery is sensitive to different motion patterns but not to different delivery techniques. Dose escalation to DIL using either sequential or simultaneous boost plans with 7 mm PTV margin is achievable in the presence of intrafractional prostate motion, even if the severe motion comprised 8.6% (3 out of the 35) treatment fractions.
39(2012); http://dx.doi.org/10.1118/1.3683646View Description Hide DescriptionPurpose:
In our clinic, physicists spend from 15 to 60 min to verify the physical and dosimetric integrity of radiotherapy plans before presentation to radiation oncology physicians for approval. The purpose of this study was to design and implement a framework to automate as many elements of this quality control (QC) step as possible.Methods
: A comprehensive computer application was developed to carry out a majority of these verification tasks in the PhilipsPINNACLEtreatment planning system (TPS). This QC tool functions based on both PINNACLE scripting elements and PERL sub-routines. The core of this technique is the method of dynamic scripting, which involves a PERL programming module that is flexible and powerful for treatment plan data handling. Run-time plan data are collected, saved into temporary files, and analyzed against standard values and predefined logical rules. The results were summarized in a hypertext markup language (HTML) report that is displayed to the user.Results
: This tool has been in clinical use for over a year. The occurrence frequency of technical problems, which would cause delays and suboptimal plans, has been reduced since clinical implementation.Conclusions:
In addition to drastically reducing the set of human-driven logical comparisons, this QC tool also accomplished some tasks that are otherwise either quite laborious or impractical for humans to verify, e.g., identifying conflicts amongst IMRT optimization objectives.
The dosimetric impact of inversely optimized arc radiotherapy plan modulation for real-time dynamic MLC tracking delivery39(2012); http://dx.doi.org/10.1118/1.3685583View Description Hide DescriptionPurpose:
Real-time dynamic multileaf collimator(MLC) tracking for management of intrafraction tumormotion can be challenging for highly modulated beams, as the leaves need to travel far to adjust for target motion perpendicular to the leaf travel direction. The plan modulation can be reduced by using a leaf position constraint (LPC) that reduces the difference in the position of adjacent MLC leaves in the plan. The purpose of this study was to investigate the impact of the LPC on the quality of inversely optimized arc radiotherapy plans and the effect of the MLCmotion pattern on the dosimetric accuracy of MLC tracking delivery. Specifically, the possibility of predicting the accuracy of MLC tracking delivery based on the plan modulation was investigated.Methods:
Inversely optimized arc radiotherapy plans were created on CT-data of three lung cancer patients. For each case, five plans with a single 358° arc were generated with LPC priorities of 0 (no LPC), 0.25, 0.5, 0.75, and 1 (highest possible LPC), respectively. All the plans had a prescribed dose of 2 Gy × 30, used 6 MV, a maximum dose rate of 600 MU/min and a collimator angle of 45° or 315°. To quantify the plan modulation, an average adjacent leaf distance (ALD) was calculated by averaging the mean adjacent leaf distance for each control point. The linear relationship between the plan quality [i.e., the calculated dose distributions and the number of monitor units (MU)] and the LPC was investigated, and the linear regression coefficient as well as a two tailed confidence level of 95% was used in the evaluation. The effect of the plan modulation on the performance of MLC tracking was tested by delivering the plans to a cylindrical diode array phantom moving with sinusoidal motion in the superior–inferior direction with a peak-to-peak displacement of 2 cm and a cycle time of 6 s. The delivery was adjusted to the target motion using MLC tracking, guided in real-time by an infrared optical system. The dosimetric results were evaluated using gamma index evaluation with static target measurements as reference.Results:
The plan quality parameters did not depend significantly on the LPC (p ≥ 0.066), whereas the ALD depended significantly on the LPC (p < 0.001). The gamma index failure rate depended significantly on the ALD, weighted to the percentage of the beam delivered in each control point of the plan (ALDw) when MLC tracking was used (p < 0.001), but not for delivery without MLC tracking (p ≥ 0.342). The gamma index failure rate with the criteria of 2% and 2 mm was decreased from > 33.9% without MLC tracking to <31.4% (LPC 0) and <2.2% (LPC 1) with MLC tracking.Conclusions:
The results indicate that the dosimetric robustness of MLC tracking delivery of an inversely optimized arc radiotherapy plan can be improved by incorporating leaf position constraints in the objective function without otherwise affecting the plan quality. The dosimetric robustness may be estimated prior to delivery by evaluating the ALDw of the plan.
39(2012); http://dx.doi.org/10.1118/1.3687864View Description Hide DescriptionPurpose:
To investigate the behavior of plane-parallel ion chambers in high-energy photon beams through measurements and Monte Carlo simulations.Methods:
Ten plane-parallel ion chamber types were obtained from the major ion chamber manufacturers. Absorbed dose-to-water calibration coefficients are measured for these chambers and factors are determined. In the process, the behaviors of the chambers are characterized through measurements of leakage currents, chamber settling in cobalt-60, polarity and ion recombination behavior, and long-term stability. Monte Carlo calculations of the absorbed dose to the air in the ion chamber and absorbed dose to water are obtained to calculate factors. Systematic uncertainties in Monte Carlo calculated factors are investigated by varying material properties and chamber dimensions.Results:
Chamber behavior was variable in MV photon beams, especially with regard to chamber leakage and ion recombination. The plane-parallel chambers did not perform as well as cylindrical chambers. Significant differences up to 1.5% were observed in calibration coefficients after a period of eight months although factors were consistent on average within 0.17%. Chamber-to-chamber variations in factors for chambers of the same type were at the 0.2% level. Systematic uncertainties in Monte Carlo calculated factors ranged between 0.34% and 0.50% depending on the chamber type. Average percent differences between measured and calculated factors were − 0.02%, 0.18%, and − 0.16% for 6, 10, and 25 MV beams, respectively.Conclusions:
Excellent agreement is observed on average at the 0.2% level between measured and Monte Carlo calculated factors. Measurements indicate that the behavior of these chambers is not adequate for their use for reference dosimetry of high-energy photon beams without a more extensive QA program than currently used for cylindrical reference-class ion chambers.