Index of content:
Volume 38, Issue 5, May 2011
38(2011); http://dx.doi.org/10.1118/1.3581376View Description Hide Description
38(2011); http://dx.doi.org/10.1118/1.3533902View Description Hide Description
- RADIATION THERAPY PHYSICS
A distance to dose difference tool for estimating the required spatial accuracy of a displacement vector field38(2011); http://dx.doi.org/10.1118/1.3572228View Description Hide DescriptionPurpose
: To introduce a tool, termed distance to dose difference (DTD), which estimates the required spatial accuracy of displacement vector fields (DVFs) used for mapping four dimensional dose values.Methods
: Dose mapping maps dose values from an irradiated geometry to a reference geometry. DVF errors result in dose being mapped from the wrong spatial location in the irradiated geometry, with a dose error equal to the dose difference between the error-free and sampled spatial locations. The DTD, defined as the distance to observe a given dose difference in the irradiated geometry, quantifies the permitted DVF error to ensure a prespecified desired dose mapping accuracy is achieved. To demonstrate the DTD, a treatment plan is generated with a 5 mm internal target volume-to-planning target volume margin for an intensity modulated radiation therapylung patient. The DTD is evaluated for mapping dose from the end of inhale image with a dose error tolerance of 3.30 Gy, which equals 5% of the 66 Gy prescription dose. The DTD is loaded into the treatment planning system to visualize positional dependencies of permissible DVF errors overlaid on the patient’s anatomy and DTD-volume-histograms are generated.Results
: DTD values vary with location in the patient anatomy. For the test case, DTD analysis indicates that accurate DVFs (∼1 mm) are required in high dose gradient regions while large DVF errors (>20 mm) are acceptable in low dose gradient regions. Within the clinical target volume (CTV), tolerated DVF uncertainties range from 1 to 12 mm, depending on location. Ninety percent of the CTV volume had DTD values less than 4 mm.Conclusions
: The DVF spatial accuracy required to meet a dose mapping accuracy tolerance depends on the spatial location within the dose distribution. For dose mapping, DVFs accuracy must be highest in dose gradient regions, while less accurate DVFs can be tolerated in uniform dose regions. The DTD tool provides a useful first estimate of DVF required spatial accuracy.
Initial application of a geometric QA tool for integrated MV and kV imaging systems on three image guided radiotherapy systems38(2011); http://dx.doi.org/10.1118/1.3570768View Description Hide DescriptionPurpose:
Several linacs with integrated kilovoltage (kV) imaging have been developed for delivery of image guided radiation therapy(IGRT). High geometric accuracy and coincidence of kV imagingsystems and megavoltage (MV) beam delivery are essential for successful image guidance. A geometric QA tool has been adapted for routine QA for evaluating and characterizing the geometric accuracy of kV and MV cone-beam imagingsystems. The purpose of this work is to demonstrate the application of methodology to routine QA across three IGRT-dedicated linac platforms.Methods:
It has been applied to a Varian Trilogy (Varian MedicalSystems, Palo Alto, CA), an Elekta SynergyS (Elekta, Stockholm, Sweden), and a Brainlab Vero (Brainlab AG, Feldkirchen, Germany). Both the Trilogy and SynergyS linacs are equipped with a retractable kV x-ray tube and a flat panel detector. The Vero utilizes a rotating, rigid ring structure integrating a MV x-ray head mounted on orthogonal gimbals, an electronic portal imaging device(EPID), two kV x-ray tubes, and two fixed flat panel detectors. This dual kV imagingsystem provides orthogonal radiographs, CBCT images, and real-time fluoroscopic monitoring. Two QA phantoms were built to suit different field sizes. Projection images of a QA phantom were acquired using MV and kV imagingsystems at a series of gantry angles. Software developed for this study was used to analyze the projection images and calculate nine geometric parameters for each projection. The Trilogy was characterized five times over one year, while the SynergyS was characterized four times and the Vero once. Over 6500 individual projections were acquired and analyzed. Quantitative geometric parameters of both MV and kV imagingsystems, as well as the isocenter consistency of the imagingsystems, were successfully evaluated.Results:
A geometric tool has been successfully implemented for calibration and QA of integrated kV and MV across a variety of radiotherapy platforms. X-ray source angle deviations up to 0.8°, and detector center offsets up to 3 mm, were observed for three linacs, with the exception of the Vero, for which a significant center offset of one kV detector (prior to machine commissioning) was observed. In contrast, the gimbal-based MV source positioning of the Vero demonstrated differences between observed and expected source positions of less than 0.2 mm, both with and without gimbal rotation.Conclusions:
This initial application of this geometric QA tool shows promise as a universal, independent tool for quantitative evaluation of geometric accuracies of both MV and integrated kV imagingsystems across a range of platforms. It provides nine geometric parameters of any imagingsystem at every gantry angle as well as the isocenter coincidence of the MV and kV imagesystems.
38(2011); http://dx.doi.org/10.1118/1.3571419View Description Hide DescriptionPurpose:
This study investigates the benefits of a modified flattening filter free (FFF) linac over the standard (STD) linac equipped with the flattening filter. Energy and angular spread of the electron beam of the FFF linac were modified. Modification of FFF beam parameters is explored to maximize the monitor unit efficiency and to minimize the head scatter in IMRT delivery for large target volumes or targets lying away from the central axis.Methods:
The EGSnrc code is used to model FFF and STD linacs and study basic beam properties for both linac types in various beam configurations. Increasing energy of FFF linac results in similar beam attenuation properties and maximized dose rate compared to STD linac. Matching beam attenuation properties allows a more direct exploration of beam flatness of FFF linac in regard to IMRT delivery, especially away from the central axis where the effective dose rate is considerably smaller than the one at the central axis. Flatness of open beamdose profile of FFF linac is improved by increasing the angular spread of the electron beam. The resulting dose rate within the treatment field and outside of the field (peripheral dose) are characterized and compared to the unmodified FFF and STD linacs.Results:
In order to match beam penetration properties, the energy of FFF is adjusted from 6.5 to 8.0 MeV for small to medium field sizes and from 6.5 to 8.5 MeV for larger ones. Dose rate of FFF vs STD linac increased by a factor of 1.9 (6.5 MeV) and 3.4–4.1 (8.0–8.5 MeV). Adjusting the mean angular spread of the electron beam from 0° to 5°–10° resulted in complete flattening of photonbeam for field sizes between 10 × 10 cm2 and 15 × 15 cm2 and partial flattening for field sizes from 15 × 15 cm2 to 30 × 30 cm2. Values of angular spread ≥14° are not recommended as they exceed the opening of the primary collimator, affecting the area at the edges of the field. FFF fields of sizes smaller than 6 × 6 cm2 are already flat and beam flattening is not necessary. Overall, the angular spread of 5°–10° is sufficient and can satisfactorily flatten open beamdose profiles even for larger field sizes. Increasing the electron beam angular spread amounts to a slight decrease of dose rate of FFF linac. However, for angular spread, 5°–10° dose rate factor of FFF vs STD is still about 1.6–2.6, depending on the field size (and the adjusted energy). Similarly, in case of peripheral dose, a moderate increase in dose can be observed for angular spread of 5°–10° and for field sizes 10 × 10 cm2 to 30 × 30 cm2. Lastly, beam flatness of not modified FFF linac can be conveniently described by an analytical function representing a ratio of STD vs FFF doses: 1 + b|r| n .Conclusions:
A modified FFF beamline with increased energy and electron beam angular spread results in satisfactory flattened beam and high dose rate within the field. Peripheral dose remaining at similar (or smaller) level than that of STD linac for the same delivered dose within the treatment field.
38(2011); http://dx.doi.org/10.1118/1.3570575View Description Hide Description
Purpose: To propose a method to calculate the tissue phantom ratio (TPR) using the depth dose and to compare the proposed method with two other methods.Methods: An analytical dose model from Bjärngard was used to describe the depth dose and the TPR. The parameters of the model were derived from depth dose measurements, which were then used to calculate the TPR. The calculated TPR values were compared with actual measurements as well as with TPR values predicted from two methods that also use depth dose, namely, the method proposed by BrainLAB and the conventional method that sets the quotients of the scatter phantom ratios (Sp) to 1.Results: TPR values calculated from the proposed algorithm deviated by −0.2 ± 0.1% (mean deviation) from the experimental measurements, over a range of field sizes and depths.Conclusions: The results of the proposed method were in better agreement with the experimental measurements than were results using the other two methods. Furthermore, the differences between the proposed method and the other methods are statistically significant.
38(2011); http://dx.doi.org/10.1118/1.3570579View Description Hide Description
Purpose: Electron radiation therapy is used frequently for the treatment of skin cancers and superficial tumors especially in the absence of kilovoltage treatment units. Head-and-neck treatment sites require accurate dose distribution calculation to minimize dose to critical structures, e.g., the eye, optic chiasm, nerves, and parotid gland. Monte Carlo simulations can be regarded as the dose calculation method of choice because it can simulate electron transport through any tissue and geometry. In order to use this technique, an accurate electron beam model should be used.Methods: In this study, a two point-source electron beam model developed for an Elekta Precise linear accelerator was validated. Monte Carlo data were benchmarked against measured water tank data for a set of regular and circular fields and at 95, 100, and 110 cm source-to-skin-distance. EDR2 Film dose distribution data were also obtained for a paranasal sinus treatment case using a Rando phantom and compared with corresponding dose distribution data obtained from Monte Carlo simulations and a CMS XiO treatment planning system. A partially shielded electron field was also evaluated using a solid water phantom and EDR2 film measurements against Monte Carlo simulations using the developed source model.Results: The major findings were that it could accurately replicate percentage depth dose and beam profile data for water measurements at source-to-skin-distances ranging between 95 and 110 cm over beam energies ranging from 4 to 15 MeV. This represents a stand-off between 0 and 15 cm. Most percentage depth dose and beam profile data (better than 95%) agreed within 2%/2 mm and nearly 100% of the data compared within 3%/3 mm. Calculated penumbra data were within 2 mm for the 20 × 20 cm2 field compared to water tank data at 95 cm source-to-skin-distance over the above energy range. Film data for the Rando phantom case showed gamma index map data that is similar in comparison with the treatment planning system and the Monte Carlosource model. The gamma index showed good agreement (2%/2 mm) between the Monte Carlosource model and the film data.Conclusions: Percentage depth dose and beam profile data were in most cases within a tolerance of 2%/2 mm. The biggest discrepancies were in most cases recorded in the first 6 mm of the water phantom. Circular fields showed local dose agreement within 3%/3mm. Good agreement was found between calculated dose distributions for a paranasal sinus case between Monte Carlo, film measurements and a CMS XiO treatment planning system. The electron beam model can be easily implemented in the BEAMnrc or DOSXYZnrc Monte Carlo codes enabling quick calculation of electron dose distributions in complex geometries.
A jaw calibration method to provide a homogeneous dose distribution in the matching region when using a monoisocentric beam split technique38(2011); http://dx.doi.org/10.1118/1.3581377View Description Hide Description
Purpose: Asymmetric collimators are currently available in most of linear accelerators. They involve a lot of clinical improvements, such as the monoisocentric beam split technique that is more and more used in many external radiotherapy treatments. The tolerance established for each independent jaw positioning is . Within this tolerance, a gap or overlap of the collimators up to can occur in the half beams matching region, causing dose heterogeneities up to 40%. In order to solve this dosimetric problem, we propose an accurate jaw calibration method based on the Monte Carlo modeling of linacphoton beams.Methods: Simulating different jaw misalignments, the dose distribution occurring in the matching region for each particular configuration is precisely known, so we can relate the misalignment of the jaws with the maximum heterogeneity produced. From experimental measurements using film dosimetry, and taking into account Monte Carlo results, we obtain the actual misalignment of each jaw. By direct inspection of the readings of the potentiometers that control the position of the jaws, high precision correction can be performed, adjusting the obtained misalignments.Results: In the linac studied, the dose heterogeneity in the junction performed with X jaws (those farther from the source), and photon beam was initially over 12%, although each jaw was within the tolerance in position. After jaw calibration, the heterogeneity was reduced to below 3%.Conclusions: With this method, we are able to reduce the positioning accuracy to . Consequently, the dose distribution in the junction of abutted fields is highly smoothed, achieving the maximum dose heterogeneity to be less than 3%.
Influence of dose calculation algorithms on the predicted dose distributions and NTCP values for NSCLC patients38(2011); http://dx.doi.org/10.1118/1.3575418View Description Hide Description
Purpose: To investigate differences in calculated doses and normal tissue complication probability (NTCP) values between different dose algorithms. Methods: Six dose algorithms from four different treatment planning systems were investigated: Eclipse AAA, Oncentra MasterPlan Collapsed Cone and Pencil Beam, Pinnacle Collapsed Cone and XiO Multigrid Superposition, and Fast Fourier Transform Convolution. Twenty NSCLC patients treated in the period 2001–2006 at the same accelerator were included and the accelerator used for treatments were modeled in the different systems. The treatment plans were recalculated with the same number of monitor units and beam arrangements across the dose algorithms. Dose volume histograms of the GTV, PTV, combined lungs (excluding the GTV), and heart were exported and evaluated. NTCP values for heart and lungs were calculated using the relative seriality model and the LKB model, respectively. Furthermore, NTCP for the lungs were calculated from two different model parameter sets. Calculations and evaluations were performed both including and excluding density corrections. Results: There are found statistical significant differences between the calculated dose to heart,lung, and targets across the algorithms. Mean lungdose and V20 are not very sensitive to change between the investigated dose calculation algorithms. However, the different dose levels for the PTV averaged over the patient population are varying up to 11%. The predicted NTCP values for pneumonitis vary between 0.20 and 0.24 or 0.35 and 0.48 across the investigated dose algorithms depending on the chosen model parameter set. The influence of the use of density correction in the dose calculation on the predicted NTCP values depends on the specific dose calculation algorithm and the model parameter set. For fixed values of these, the changes in NTCP can be up to 45%. Conclusions: Calculated NTCP values for pneumonitis are more sensitive to the choice of algorithm than mean lungdose and V20 which are also commonly used for plan evaluation. The NTCP values for heart complication are, in this study, not very sensitive to the choice of algorithm. Dose calculations based on density corrections result in quite different NTCP values than calculations without density corrections. It is therefore important when working with NTCP planning to use NTCP parameter values based on calculations and treatments similar to those for which the NTCP is of interest.
38(2011); http://dx.doi.org/10.1118/1.3576106View Description Hide Description
Purpose: The purpose of this study is to evaluate the dosimetric effect of carbon fiber couches (CFCs) on delivered skindose as well as to explore potential venues for its minimization for volumetric modulated arc (VMAT) treatments.Methods: A carbon fiber couch (BrainLab) was incorporated in Pinnacle treatment planning system (TPS) by autocontouring. A retrospective investigation on five lung and five prostate patient plans was performed. Targets and organs at risk (OARs), together with a 0.3 cm thick skin contour interfacing the CFC, were outlined in each plan. For each patient, two VMAT plans were generated: a single arc with 6 MV photon energy and two or three arcs with 18 MV photon energy for the posterior arc(s) and 6 MV energy for the anterior arc (mixed energy plans). Both plans for each patient case were normalized such that 95% of the PTV was covered by the same prescription dose, ranging from 7600 to 7800 cGy. For each patient, the prescription doses were escalated to the maximum allowed by the OAR constraints. CFC bolus effects on skindoses were tallied by the highest dose to 1% of skin volume. Results: With the utilization of higher energy photons for the posterior arcs, the statistically significant differences in skindose between the two plans were as high as 34% of the prescribed dose, where surface doses changed on average from 3800 to 2940 cGy for 6 MV and mixed energy plans, respectively. In addition, skindoses in excess of 68% and 80% of the prescription doses for mixed and 6 MV energy plans, respectively, were observed in individual cases. Conclusions: The presented findings indicate that mixed energy VMAT plans would result in a substantial skin sparing of more than ∼34% compared to VMAT plans with only 6 MV arc(s). Additionally, the high skindoses in some cases (81% of the prescription dose) suggest that in hypofractionated SRS/SRT treatments, the carbon fiber couch effects on skindoses need to be evaluated when arc delivery is considered as a treatment option.
Effect of mid-scan breathing changes on quality of 4DCT using a commercial phase-based sorting algorithm38(2011); http://dx.doi.org/10.1118/1.3574872View Description Hide Description
Purpose: Though it is known that irregular breathing can introduce artifacts in commercial 4DCT, this has not been systematically explored. The purpose of this study is to investigate the effect of variations in basic parameters of the breathing wave on 4DCT imaging quality.Methods: A four-dimensional motion platform holding an acrylic sphere was scanned while moving in a trajectory modeled from a lungcancer patient. A bellows device was used as a respiratory surrogate, and the images were sorted by a commercial phase-based sorting algorithm. Motion during the first half of the scan was produced at a baseline trajectory with a consistent frequency and amplitude of 15 breaths per minute and 1 cm, peak to peak. The two parameters were then varied mid-scan to new frequency and amplitude values, with frequencies ranging from 7.5 to 22 bpm and amplitudes ranging from 0.5 to 1.5 cm. Image sets representing four respiratory phases were contoured. Each set was analyzed to compare centroid displacement, density homogeneity, and volumetric and geometric distortions of the imaged sphere. Undercoverage of the target ITV and overcoverage of healthy tissue was also evaluated.Results: Changes in amplitude of 25% or more, with or without changes in frequency, consistently caused measurable distortions in shape, position, and density of the imaged sphere. Frequency changes over 50% showed a similar trend.Conclusions: This study suggests that basic breathing statistics can be used to quickly assess the quality of a 4DCT scan prior to image reconstruction. Such information can help give indication of the proper course of action when irregular breathing patterns are observed during CT scanning.
Investigations of interference between electromagnetic transponders and wireless MOSFET dosimeters: A phantom study38(2011); http://dx.doi.org/10.1118/1.3578602View Description Hide Description
Purpose: To evaluate both the Calypso Systems’ (Calypso Medical Technologies, Inc., Seattle, WA) localization accuracy in the presence of wireless metal–oxide–semiconductor field-effect transistor (MOSFET)dosimeters of dose verification system (DVS, Sicel Technologies, Inc., Morrisville, NC) and the dosimeters’ reading accuracy in the presence of wireless electromagnetic transponders inside a phantom.Methods: A custom-made, solid-water phantom was fabricated with space for transponders and dosimeters. Two inserts were machined with positioning grooves precisely matching the dimensions of the transponders and dosimeters and were arranged in orthogonal and parallel orientations, respectively. To test the transponder localization accuracy with/without presence of dosimeters (hypothesis 1), multivariate analyses were performed on transponder-derived localization data with and without dosimeters at each preset distance to detect statistically significant localization differences between the control and test sets. To test dosimeterdose-reading accuracy with/without presence of transponders (hypothesis 2), an approach of alternating the transponder presence in seven identical fraction dose (100 cGy) deliveries and measurements was implemented. Two-way analysis of variance was performed to examine statistically significant dose-reading differences between the two groups and the different fractions. A relative-dose analysis method was also used to evaluate transponder impact on dose-reading accuracy after dose-fading effect was removed by a second-order polynomial fit.Results:Multivariate analysis indicated that hypothesis 1 was false; there was a statistically significant difference between the localization data from the control and test sets. However, the upper and lower bounds of the 95% confidence intervals of the localized positional differences between the control and test sets were less than 0.1 mm, which was significantly smaller than the minimum clinical localization resolution of 0.5 mm. For hypothesis 2, analysis of variance indicated that there was no statistically significant difference between the dosimeter readings with and without the presence of transponders. Both orthogonal and parallel configurations had difference of polynomial-fit dose to measured dose values within 1.75%.Conclusions: The phantom study indicated that the Calypso System’s localization accuracy was not affected clinically due to the presence of DVS wireless MOSFETdosimeters and the dosimeter-measureddoses were not affected by the presence of transponders. Thus, the same patients could be implanted with both transponders and dosimeters to benefit from improved accuracy of radiotherapy treatments offered by conjunctional use of the two systems.
38(2011); http://dx.doi.org/10.1118/1.3574874View Description Hide DescriptionPurpose:
To demonstrate the feasibility of using a knowledge base of prior treatment plans to generate new prostate intensity modulated radiation therapy(IMRT) plans. Each new case would be matched against others in the knowledge base. Once the best match is identified, that clinically approved plan is used to generate the new plan.Methods:
A database of 100 prostate IMRTtreatment plans was assembled into an information-theoretic system. An algorithm based on mutual information was implemented to identify similar patient cases by matching 2D beam’s eye view projections of contours. Ten randomly selected query cases were each matched with the most similar case from the database of prior clinically approved plans. Treatment parameters from the matched case were used to develop new treatment plans. A comparison of the differences in the dose–volume histograms between the new and the original treatment plans were analyzed.Results:
On average, the new knowledge-based plan is capable of achieving very comparable planning target volume coverage as the original plan, to within 2% as evaluated for D98, D95, and D1. Similarly, the dose to the rectum and dose to the bladder are also comparable to the original plan. For the rectum, the mean and standard deviation of the dose percentage differences for D20, D30, and D50 are 1.8% ± 8.5%, −2.5% ± 13.9%, and −13.9% ± 23.6%, respectively. For the bladder, the mean and standard deviation of the dose percentage differences for D20, D30, and D50 are −5.9% ± 10.8%, −12.2% ± 14.6%, and −24.9% ± 21.2%, respectively. A negative percentage difference indicates that the new plan has greater dose sparing as compared to the original plan.Conclusions:
The authors demonstrate a knowledge-based approach of using prior clinically approved treatment plans to generate clinically acceptable treatment plans of high quality. This semiautomated approach has the potential to improve the efficiency of the treatment planning process while ensuring that high quality plans are developed.
38(2011); http://dx.doi.org/10.1118/1.3578607View Description Hide DescriptionPurpose:
To evaluate the accuracy of the patient-positioning function of a newly developed image-guided radiotherapy system, the MHI-TM2000 (Mitsubishi Heavy Industries, Ltd., Japan).Methods:
The isocenter positions prescribed by the lasers, MV treatment beam, and image guidance systems (kV X-rayimage and kV-CBCT) were calculated using a cube phantom with a 10-mm-diameter steel ball fixed to the center of the phantom. Then, their location discrepancies were estimated. In addition, to verify the scale and orientation of the coordinate axes of the kV X-ray imaging system, positional measurements were repeated with the phantom placed at 50 mm off-isocenter along the vertical, longitudinal, and lateral directions, respectively. Further, image fusions of an anthropomorphic phantom image and the corresponding image translated by a pre-determined amount were performed.Results:
The isocenter alignment among the coordinate systems was coincident within 0.5 mm in translation for the vertical, longitudinal, and lateral axes, respectively. The geometrical errors at 50 mm off-isocenter for kV X-rayimages and CBCT were within 0.2 mm and 1.0 mm, respectively. The image fusion errors were within 1.0 mm in translation and 1.0° in rotation, respectively. No significant difference in the image fusion accuracy was observed between the chest and pelvis phantoms.Conclusions:
The isocenter alignment among the coordinate systems was performed with high accuracy. Furthermore, the automatic image fusion function achieved sufficient patient positioning accuracy and precision for image-guided radiotherapy.
38(2011); http://dx.doi.org/10.1118/1.3581406View Description Hide DescriptionPurpose:
To investigate the delivery of biologically adapted high resolution intensity modulated radiotherapy(IMRT) to an anthropomorphic phantom, using dosimetric and radiobiologic measures.Methods:
A compartment based 3D hypoxia map of a highly heterogeneous tumor was imported into the CT planning basis of an anthropomorphic phantom. Biologically adapted IMRT was planned according to the corresponding 3D dose prescription map with elevated dose to the hypoxic regions. Three treatment fractions were delivered to the phantom by means of a conventional linear accelerator equipped with a high resolution micro multileaf collimator (mMLC). EDR2 radiographic films were positioned in two planes of the phantom during irradiation. Software was developed to analyze dose distributions from the prescription, dose plan, and films. Dose distributions were scored within each of four radiobiologic tumor compartments. The treatment effect in each tumor compartment was estimated as the equivalent uniform dose (EUD). A conventional γ analysis was utilized for quantitative comparisons of planned and delivered dose distributions.Results:
The planned and delivered dose maps qualitatively resembled the prescribed dose map. The γ analyzes showed that, on average for all films, more than 95% of the pixels within the tumor passed the 3%/2 mm criteria. For compartments with increasing degree of hypoxia and thereby increased prescribed dose, the planned and delivered EUDs were severely reduced compared to the prescription. The prescribed compartmental doses were met only for the most oxic compartment. The mean tumordose as measured by the films was 6.6% lower than the corresponding planned dose.Conclusions:
Biologically adapted radiotherapy may be delivered with high precision according to the dose plan. However, the large reduction in compartmental EUD values from prescribed to planned treatment indicated lower tumor effect than expected for such a treatment.
38(2011); http://dx.doi.org/10.1118/1.3578927View Description Hide DescriptionPurpose:
To develop a novel four-dimensional (4D) intensity modulated radiation therapy(IMRT)treatment planning methodology based on dynamic virtual patient models.Methods:
The 4D model-based planning (4DMP) is a predictive tracking method which consists of two main steps: (1) predicting the 3D deformable motion of the target and critical structures as a function of time during treatment delivery; (2) adjusting the delivery beam apertures formed by the dynamic multi-leaf collimators (DMLC) to account for the motion. The key feature of 4DMP is the application of a dynamic virtual patient model in motion prediction, treatment beam adjustment, and dose calculation. A lung case was chosen to demonstrate the feasibility of the 4DMP. For the lung case, a dynamic virtual patient model (4D model) was first developed based on the patient’s 4DCT images. The 4D model was capable of simulating respiratory motion of different patterns. A model-based registration method was then applied to convert the 4D model into a set of deformation maps and 4DCT images for dosimetric purposes. Based on the 4D model, 4DMP treatment plans with different respiratory motion scenarios were developed. The quality of 4DMP plans was then compared with two other commonly used 4D planning methods: maximum intensity projection (MIP) and planning on individual phases (IP).Results:
Under regular periodic motion, 4DMP offered similar target coverage as MIP with much better normal tissue sparing. At breathing amplitude of 2 cm, the lung V20 was 23.9% for a MIP plan and 16.7% for a 4DMP plan. The plan quality was comparable between 4DMP and IP: PTV V97 was 93.8% for the IP plan and 93.6% for the 4DMP plan. Lung V20 of the 4DMP plan was 2.1% lower than that of the IP plan and Dmax to cord was 2.2 Gy higher. Under a real time irregular breathing pattern, 4DMP had the best plan quality. PTV V97 was 90.4% for a MIP plan, 88.6% for an IP plan and 94.1% for a 4DMP plan. Lung V20 was 20.1% for the MIP plan, 17.8% for the IP plan and 17.5% for the 4DMP plan. The deliverability of the real time 4DMP plan was proved by calculating the maximum leaf speed of the DMLC.Conclusions:
The 4D model-based planning, which applies dynamic virtual patient models in IMRTtreatment planning, can account for the real time deformable motion of the tumor under different breathing conditions. Under regular motion, the quality of 4DMP plans was comparable with IP and superior to MIP. Under realistic motion in which breathing amplitude and period change, 4DMP gave the best plan quality of the three 4D treatment planning techniques.
Dosimetric comparison of Acuros XB deterministic radiation transport method with Monte Carlo and model-based convolution methods in heterogeneous media38(2011); http://dx.doi.org/10.1118/1.3582690View Description Hide DescriptionPurpose:
The deterministic Acuros XB (AXB) algorithm was recently implemented in the Eclipse treatment planning system. The goal of this study was to compare AXB performance to Monte Carlo(MC) and two standard clinical convolution methods: the anisotropic analytical algorithm (AAA) and the collapsed-cone convolution (CCC) method.Methods:
Homogeneous water and multilayer slab virtual phantoms were used for this study. The multilayer slab phantom had three different materials, representing soft tissue, bone, and lung. Depth dose and lateral dose profiles from AXB v10 in Eclipse were compared to AAA v10 in Eclipse, CCC in Pinnacle3, and EGSnrc MC simulations for 6 and 18 MV photon beams with open fields for both phantoms. In order to further reveal the dosimetric differences between AXB and AAA or CCC, three-dimensional (3D) gamma index analyses were conducted in slab regions and subregions defined by AAPM Task Group 53.Results:
The AXB calculations were found to be closer to MC than both AAA and CCC for all the investigated plans, especially in bone and lung regions. The average differences of depth dose profiles between MC and AXB, AAA, or CCC was within 1.1, 4.4, and 2.2%, respectively, for all fields and energies. More specifically, those differences in bone region were up to 1.1, 6.4, and 1.6%; in lung region were up to 0.9, 11.6, and 4.5% for AXB, AAA, and CCC, respectively. AXB was also found to have better dose predictions than AAA and CCC at the tissue interfaces where backscatter occurs. 3D gamma index analyses (percent of dose voxels passing a 2%/2 mm criterion) showed that the dose differences between AAA and AXB are significant (under 60% passed) in the bone region for all field sizes of 6 MV and in the lung region for most of field sizes of both energies. The difference between AXB and CCC was generally small (over 90% passed) except in the lung region for 18 MV 10 × 10 cm2 fields (over 26% passed) and in the bone region for 5 × 5 and 10 × 10 cm2 fields (over 64% passed). With the criterion relaxed to 5%/2 mm, the pass rates were over 90% for both AAA and CCC relative to AXB for all energies and fields, with the exception of AAA 18 MV 2.5 × 2.5 cm2 field, which still did not pass.Conclusions:
In heterogeneous media, AXB dose prediction ability appears to be comparable to MC and superior to current clinical convolution methods. The dose differences between AXB and AAA or CCC are mainly in the bone, lung, and interface regions. The spatial distributions of these differences depend on the field sizes and energies.
Investigation of a novel algorithm for true 4D-VMAT planning with comparison to tracked, gated and static delivery38(2011); http://dx.doi.org/10.1118/1.3578608View Description Hide DescriptionPurpose:
A novel 4D volumetric modulated arc therapy (4D-VMAT) planning system is presented where radiation sparing of organs at risk (OARs) is enhanced by exploiting respiratory motion of tumor and healthy tissues.Methods:
In conventional radiation therapy, a motion encompassing margin is normally added to the clinical target volume (CTV) to ensure the tumor receives the planned treatmentdose. This results in a substantial increase in dose to the OARs. Our 4D-VMAT algorithm aims to reduce OAR dose by incorporating 4D volumetric target and OAR motions directly into the optimization process. During optimization, phase correlated beam samples are progressively added throughout the full range of gantry rotation. The resulting treatment plans have respiratory phase-optimized apertures whose deliveries are synchronized to the patient’s respiratory cycle. 4D-VMAT plans reduce dose to the OAR by: (1) eliminating the motion margin, (2) selectively redistributing OAR dose over the OAR volume, and (3) timing larger dose contributions (MU) to respiratory phases where greater separations between the target and OAR occur. Our 4D-VMAT algorithm was tested by simulating a variety of tumor motion amplitudes (0.5–2 cm) in the superior/inferior and anterior/posterior directions. 4D-VMAT’s performance was compared against 3D-VMAT, gated VMAT and dynamic multileaf collimator (DMLC) ideal-tracking VMAT.Results:
Results show that OAR sparing of 4D-VMAT was greater than 3D-VMAT in all cases due to the smaller PTV margin. Compared to DMLC ideal-tracking VMAT, 4D-VMAT’s OAR sparing is superior only when the relative distance between the PTV and OAR is changing. For gated VMAT, results compared to 4D-VMAT are phantom dependent. There was negligible difference in plan qualities for the tested case of motion along the anterior/posterior axis. For motions along the superior/inferior axis, gated VMAT’s narrow beam-on window reduces the OAR volume directly irradiated by the linac but also allows higher dose accumulation in the exposed OAR. In contrast, 4D-VMAT can reduce the OAR volume exposed to high doses but at the cost of redistributing the OAR dose over a larger volume. Finally for 4D-VMAT, an increase in tumor motion no longer resulted in greater irradiation of the OAR as seen in conventional 3D radiation therapy. OAR dose levels were preserved for increasing target motion along the anterior/posterior axis. For increasing superior/inferior motion, the volume of OAR exposed to high doses actually decreased due to dose redistribution.Conclusions:
Our investigation demonstrated that the 4D-VMAT system has the potential to improve radiation therapy of periodically moving tumors over 3D-VMAT, gating or tracking methods.
38(2011); http://dx.doi.org/10.1118/1.3574884View Description Hide DescriptionPurpose:
The purpose of this study was to investigate how incorporation of magnetic resonance spectroscopy imaging(MRSI) into radiotherapy planning would increase the target volume for patients with recurrent glioma.Methods:
After prior standard radiotherapy, 25 patients with recurrent glioma were treated with bevacizumab and concurrent hypofractionated stereotactic radiotherapy (HFSRT), delivering 30 Gy in five fractions. MRSI were acquired for 12 patients. Areas with markedly higher choline levels relative to the levels of total creatine and N-acetylaspartate were identified and referred to as MRSI voxels with elevated metabolite ratios (EMR). Gross tumor volume (GTV) consisted of contrast-enhancing tumor on T1-weighted magnetic resonance images(MRI) and computed tomography. Clinical target volume (CTV) was GTV + 5 mm margin and MRSI voxels with EMR. Overall survival (OS) and 6-month progression free survival (PFS) for these patients were reported in a prior publication [Gutinet al., Int. J. Radiat. Oncol., Biol., Phys. 75(1), 156–163 (2009)], and the outcome was correlated with the GTV and the volume of MRSI voxels with EMR in this study.Results:
Seven of the 12 patients had MRSI voxels with EMR. If none of the MRSI voxels with EMR were included, the CTV would range from 9.2 to 73.0 cm3 with a median of 31.0 cm3, whereas if all voxels were included, the CTV would range from 27.4 to 74.4 cm3 with a median of 35.0 cm3. For three of the seven patients, including the voxels with EMR, would have increased the CTV by 14%–23%. For one patient, where the MRSI voxels with EMR did not overlap the GTV, including these voxels would increase the CTV by 198%. No correlation could be found between the OS and PFS and the GTV or the volume of MRSI voxels with EMR.Conclusions:
Seven of 12 patients with recurrent glioma had MRSI voxels with EMR. For four of these seven patients, including the MRSI voxels with EMR, significantly increased the CTV. This study does not have statistical power to conclude on the importance of including areas with MRSI-suspect disease into the radiation target volume.