Index of content:
Volume 38, Issue 7, July 2011
High intensity focused ultrasound may be superior to radiation therapy for the treatment of early stage prostate cancer38(2011); http://dx.doi.org/10.1118/1.3561500View Description Hide Description
- MEDICAL PHYSICS LETTERS
Case report of a near medical event in stereotactic radiotherapy due to improper units of measure from a treatment planning system38(2011); http://dx.doi.org/10.1118/1.3598444View Description Hide Description
Purpose: The authors hereby notify the Radiation Oncology community of a potentially lethal error due to improper implementation of linear units of measure in a treatment planning system. The authors report an incident in which a patient was nearly mistreated during a stereotactic radiotherapy procedure due to inappropriate reporting of stereotactic coordinates by the radiation therapytreatment planning system in units of centimeter rather than in millimeter. The authors suggest a method to detect such errors during treatment planning so they are caught and corrected prior to the patient positioning for treatment on the treatment machine.Methods: Using pretreatment imaging, the authors found that stereotactic coordinates are reported with improper linear units by a treatment planning system. The authors have implemented a redundant, independent method of stereotactic coordinate calculation.Results: Implementation of a double check of stereotactic coordinates via redundant, independent calculation is simple and accurate. Use of this technique will avoid any future error in stereotactic treatment coordinates due to improper linear units, transcription, or other similar errors.Conclusions: The authors recommend an independent double check of stereotactic treatment coordinates during the treatment planning process in order to avoid potential mistreatment of patients.
38(2011); http://dx.doi.org/10.1118/1.3592646View Description Hide Description
Purpose: Cerenkov emission is induced when a charged particle moves faster than the speed of light in a given medium. Both x-rayphotons and electrons produce optical Cerenkov photons in everyday radiation therapy of tissue; yet, this phenomenon has never been fully documented. This study quantifies the emissions and also demonstrates that the Cerenkov emission can excite a fluorophore, protoporphyrin IX (PpIX), embedded in biological phantoms.Methods: In this study, Cerenkov emission induced by radiation from a clinical linear accelerator is investigated. Biological mimicking phantoms were irradiated with x-rayphotons, with energies of 6 or 18 MV, or electrons at energies 6, 9, 12, 15, or 18 MeV. The Cerenkov emission and the induced molecular fluorescence were detected by a camera or a spectrometer equipped with a fiber optic cable.Results: It is shown that both x-rayphotons and electrons, at MeV energies, produce optical Cerenkov photons in tissue mimicking media. Furthermore, we demonstrate that the Cerenkov emission can excite a fluorophore, protoporphyrin IX (PpIX), embedded in biological phantoms.Conclusions: The results here indicate that molecular fluorescence monitoring during external beam radiotherapy is possible.
- RADIATION THERAPY PHYSICS
Experimental investigation of a moving averaging algorithm for motion perpendicular to the leaf travel direction in dynamic MLC target tracking38(2011); http://dx.doi.org/10.1118/1.3590384View Description Hide Description
Purpose: In dynamic multileaf collimator(MLC) motion tracking with complex intensity-modulated radiation therapy(IMRT) fields, target motion perpendicular to the MLC leaf travel direction can cause beam holds, which increase beam delivery time by up to a factor of 4. As a means to balance delivery efficiency and accuracy, a moving average algorithm was incorporated into a dynamic MLC motion tracking system (i.e., moving average tracking) to account for target motion perpendicular to the MLC leaf travel direction. The experimental investigation of the moving average algorithm compared with real-time tracking and no compensation beam delivery is described.Methods: The properties of the moving average algorithm were measured and compared with those of real-time tracking (dynamic MLC motion tracking accounting for both target motion parallel and perpendicular to the leaf travel direction) and no compensation beam delivery. The algorithm was investigated using a synthetic motion trace with a baseline drift and four patient-measured 3D tumor motion traces representing regular and irregular motions with varying baseline drifts. Each motion trace was reproduced by a moving platform. The delivery efficiency, geometric accuracy, and dosimetric accuracy were evaluated for conformal, step-and-shoot IMRT, and dynamic sliding window IMRT treatment plans using the synthetic and patient motion traces. The dosimetric accuracy was quantified via a γ-test with a 3%/3 mm criterion.Results: The delivery efficiency ranged from 89 to 100% for moving average tracking, 26%–100% for real-time tracking, and 100% (by definition) for no compensation. The root-mean-square geometric error ranged from 3.2 to 4.0 mm for moving average tracking, 0.7–1.1 mm for real-time tracking, and 3.7–7.2 mm for no compensation. The percentage of dosimetric points failing the γ-test ranged from 4 to 30% for moving average tracking, 0%–23% for real-time tracking, and 10%–47% for no compensation.Conclusions: The delivery efficiency of moving average tracking was up to four times higher than that of real-time tracking and approached the efficiency of no compensation for all cases. The geometric accuracy and dosimetric accuracy of the moving average algorithm was between real-time tracking and no compensation, approximately half the percentage of dosimetric points failing the γ-test compared with no compensation.
Verification of the linac isocenter for stereotactic radiosurgery using cine-EPID imaging and arc delivery38(2011); http://dx.doi.org/10.1118/1.3597836View Description Hide Description
Purpose:Verification of the mechanical isocenter position is required as part of comprehensive quality assurance programs for stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments. Several techniques have been proposed for this purpose but each of them has certain drawbacks. In this paper, a new efficient and more comprehensive method using cine-EPID images has been introduced for automatic verification of the isocenter with sufficient accuracy for stereotactic applications.Methods: Using a circular collimator fixed to the gantry head to define the field, EPIDimages of a Winston–Lutz phantom were acquired in cine-imaging mode during 360° gantry rotations. A robust matlab code was developed to analyze the data by finding the center of the field and the center of the ball bearing shadow in each image with sub-pixel accuracy. The distance between these two centers was determined for every image. The method was evaluated by comparison to results of a mechanical pointer and also by detection of a manual shift applied to the phantom position. The repeatability and reproducibility of the method were tested and it was also applied to detect couch and collimator wobble during rotation.Results:The accuracy of the algorithm was 0.03 ± 0.02 mm. The repeatability was less than 3 μm and the reproducibility was less than 86 μm. The time elapsed for the analysis of more than 100 cine images of Varian aS1000 and aS500 EPIDs were ∼65 and 20 s, respectively. Processing of images taken in integrated mode took 0.1 s. The output of the analysis software is printable and shows the isocenter shifts as a function of angle in both in-plane and cross-plane directions. It gives warning messages where the shifts exceed the criteria for SRS/SRT and provides useful data for the necessary adjustments in the system including bearing system and/or room lasers.Conclusions: The comprehensive method introduced in this study uses cine-images, is highly accurate, fast, and independent of the observer. It tests all gantry angles and is suitable for pretreatment QA of the isocenter for stereotactic treatments.
Management of the baseline shift using a new and simple method for respiratory-gated radiation therapy: Detectability and effectiveness of a flexible monitoring system38(2011); http://dx.doi.org/10.1118/1.3598434View Description Hide Description
Purpose: In respiratory-gated radiation therapy, a baseline shift decreases the accuracy of target coverage and organs at risk (OAR) sparing. The effectiveness of audio-feedback and audio-visual feedback in correcting the baseline shift in the breathing pattern of the patient has been demonstrated previously. However, the baseline shift derived from the intrafraction motion of the patient’s body cannot be corrected by these methods. In the present study, the authors designed and developed a simple and flexible system.Methods: The system consisted of a web camera and a computer running our in-house software. The in-house software was adapted to template matching and also to no preimage processing. The system was capable of monitoring the baseline shift in the intrafraction motion of the patient’s body. Another marker box was used to monitor the baseline shift due to the flexible setups required of a marker box for gated signals. The system accuracy was evaluated by employing a respiratory motion phantom and was found to be within AAPM Task Group 142 tolerance (positional accuracy <2 mm and temporal accuracy <100 ms) for respiratory-gated radiation therapy. Additionally, the effectiveness of this flexible and independent system in gated treatment was investigated in healthy volunteers, in terms of the results from the differences in the baseline shift detectable between the marker positions, which the authors evaluated statistically.Results: The movement of the marker on the sternum [1.599 ± 0.622 mm (1 SD)] was substantially decreased as compared with the abdomen [6.547 ± 0.962 mm (1 SD)]. Additionally, in all of the volunteers, the baseline shifts for the sternum [−0.136 ± 0.868 (2 SD)] were in better agreement with the nominal baseline shifts than was the case for the abdomen [−0.722 ± 1.56 mm (2 SD)]. The baseline shifts could be accurately measured and detected using the monitoring system, which could acquire the movement of the marker on the sternum. The baseline shift-monitoring system with the displacement-based methods for highly accurate respiratory-gated treatments should be used to make most of the displacement-based gating methods.Conclusions: The advent of intensity modulated radiation therapy and volumetric modulated radiation therapy facilitates margin reduction for the planning target volumes and the OARs, but highly accurate irradiation is needed to achieve target coverage and OAR sparing with a small margin. The baseline shifts can affect treatment not only with the respiratory gating system but also without the system. Our system can manage the baseline shift and also enables treatment irradiation to be undertaken with high accuracy.
Motion monitoring for cranial frameless stereotactic radiosurgery using video-based three-dimensional optical surface imaging38(2011); http://dx.doi.org/10.1118/1.3596526View Description Hide Description
Purpose: To establish a new clinical procedure in frameless stereotactic radiosurgery(SRS) for patient setup verification at treatment couch angles as well as for head-motion monitoring during treatment using video-based optical surfaceimaging (OSI).Methods: A video-based three-dimensional (3D) OSI system with three ceiling-mounted camera pods was employed to verify setup at treatment couch angles as well as to monitor head motion during treatment. A noninvasive head immobilization device was utilized, which includes an alpha head mold and a dental mouthpiece with vacuum suction; both were locked to the treatment couch. Cone beam computed tomography(CBCT) was used as the standard for image-guided setup. Orthogonal 2D-kV imaging was applied for setup verification before treatment, between couch rotations, and after treatment at zero couch angle. At various treatment couch angles, OSI setup verification was performed, relative to initial OSI setup verification at zero couch angle after CBCT setup through a coordinate transformation. For motion monitoring, the setup uncertainty was decoupled by taking an on-site surfaceimage as new reference to detect motion-induced misalignment in near real-time (1–2 frames per second). Initial thermal instability baseline of the real-time monitoring was corrected. An anthropomorphous head phantom and a 1D positioning platform were used to assess the OSI accuracy in motion detection in longitudinal and lateral directions. Two hypofractionated (9 Gy × 3 and 6 Gy × 5) frameless stereotactic radiotherapy (SRT) patients as well as two single-fraction (21 and 18 Gy) frameless SRS patients were treated using this frameless procedure. For comparison, 11 conventional frame-based SRS patients were monitored using the OSI to serve as clinical standards. Multiple noncoplanar conformal beams were used for planning both frameless and frame-based SRS with a micromultileaf collimator.Results: The accuracy of the OSI in 1D motion detection was found to be 0.1 mm with uncertainty of ±0.1 mm using the head phantom. The OSI registration against simulation computed tomography(CT) external contour was found to be dependent on the CT skin definition with ∼0.4 mm variation. For frame-based SRS patients, head-motion magnitude was detected to be <1.0 mm (0.3 ± 0.2 mm) and <1.0° (0.2° ± 0.2°) for 98% of treatment time, with exception of one patient with head rotation <1.5° for 98% of the time. For frameless SRT/SRS patients, similar motion magnitudes were observed with an average of 0.3 ± 0.2 mm and 0.2° ± 0.1° in ten treatments. For 98% of the time, the motion magnitude was <1.1 mm and 1.0°. Complex head-motion patterns within 1.0 mm were observed for frameless SRT/SRS patients. The OSI setup verification at treatment couch angles was found to be within 1.0 mm.Conclusions: The OSI system is capable of detecting 0.1 ± 0.1 mm 1D spatial displacement of a phantom in near real time and useful in head-motion monitoring. This new frameless SRS procedure using the mask-less head-fixation system provides immobilization similar to that of conventional frame-based SRS. Head-motion monitoring using near-real-time surfaceimaging provides adequate accuracy and is necessary for frameless SRS in case of unexpected head motion that exceeds a set tolerance.
Determination of action thresholds for electromagnetic tracking system-guided hypofractionated prostate radiotherapy using volumetric modulated arc therapy38(2011); http://dx.doi.org/10.1118/1.3596776View Description Hide Description
Purpose: Hypofractionated prostate radiotherapy may benefit from both volumetric modulated arc therapy (VMAT) due to shortened treatment time and intrafraction real-time monitoring provided by implanted radiofrequency(RF) transponders. The authors investigate dosimetrically driven action thresholds (whether treatment needs to be interrupted and patient repositioned) in VMAT treatment with electromagnetic (EM) tracking.Methods: VMAT plans for five patients are generated for prescription doses of 32.5 and 42.5 Gy in five fractions. Planning target volume (PTV) encloses the clinical target volume (CTV) with a 3 mm margin at the prostate-rectal interface and 5 mm elsewhere. The VMAT delivery is modeled using 180 equi-spaced static beams. Intrafraction prostate motion is simulated in the plan by displacing the beam isocenter at each beam assuming rigid organmotion according to a previously recorded trajectory of the transponder centroid. The cumulative dosedelivered in each fraction is summed over all beams. Two sets of 57 prostate motion trajectories were randomly selected to form a learning and a testing dataset. Dosimetric end points including CTV D95%, rectum wall D1cc, bladder wall D1cc, and urethra Dmax, are analyzed against motion characteristics including the maximum amplitude of the anterior–posterior (AP), superior–inferior (SI), and left–right components. Action thresholds are triggered when intrafraction motion causes any violations of dose constraints to target and organs at risk (OAR), so that treatment is interrupted and patient is repositioned.Results: Intrafraction motion has a little effect on CTV D95%, indicating PTV margins are adequate. Tight posterior and inferior action thresholds around 1 mm need to be set in a patient specific manner to spare organs at risk, especially when the prescription dose is 42.5 Gy. Advantages of setting patient specific action thresholds are to reduce false positive alarms by 25% when prescription dose is low, and increase the sensitivity of detecting dose limits violations by 30% when prescription dose is high, compared to a generic 2 mm action box. The sensitivity and specificity calculated from the testing dataset are consistent to the learning set, which indicates that the patient specific approach is reliable and reproducible within the scope of the prostate database.Conclusions: This work introduces a formalism for ensuring a VMAT delivery meets the most clinically important dose requirements by using patient specific and dosimetric-driven action thresholds to hold the beam and reposition the patient when necessary. Such methods can provide improved sensitivity and specificity compared to conventional methods, which assume directionally symmetric action thresholds.
First assessment of three-dimensional quantitative photoacoustic tomography for in vivo detection of osteoarthritis in the finger joints38(2011); http://dx.doi.org/10.1118/1.3598113View Description Hide Description
Purpose: The purpose of this pilot clinical study is to assess three-dimensional (3-D) quantitative photoacoustic tomography (qPAT) for in vivo detection of osteoarthritis (OA) in the finger joints.Methods: All subject data were handled in compliance with the rules and regulations concerning the privacy and security of protected health information under HIPAA. Seven female subjects (two OA patients and five healthy controls) entered the study and their distal interphalangeal (DIP) joints were examined by a 3-D photoacoustic scanner. 3-D optical absorption coefficient images of all the photoacoustically examined joints were recovered using a 3-D qPAT reconstruction algorithm.Results: The recovered quantitative photoacousticimages revealed obvious difference in the optical absorption coefficient of the joint cavity (cartilage and synovial fluid) between the OA and healthy joints. Quantitative analysis of the joints also indicated an apparent difference in the recovered joint spacing between the OA and healthy subjects.Conclusion: This initial clinical evaluation suggests that it is feasible to detect osteoarthritis in the finger joints with our 3-D qPAT approach, which has paved the way to further statistically evaluate the diagnostic performance of the 3-D qPAT approach in comparison with radiography or magnetic resonance imaging (MRI) on a sample of hand osteoarthritis.
Modeling the TrueBeam linac using a CAD to Geant4 geometry implementation: Dose and IAEA-compliant phase space calculations38(2011); http://dx.doi.org/10.1118/1.3598439View Description Hide Description
Purpose: To create an accurate 6 MV Monte Carlo simulation phase space for the Varian TrueBeam treatment head geometry imported from cad (computer aided design) without adjusting the input electron phase space parameters.Methods:geant4 v4.9.2.p01 was employed to simulate the 6 MV beam treatment head geometry of the Varian TrueBeam linac. The electron tracks in the linear accelerator were simulated with Parmela, and the obtained electron phase space was used as an input to the Monte Carlo beam transport and dose calculations. The geometry components are tessellated solids included in geant4 as gdml (generalized dynamic markup language) files obtained via STEP (standard for the exchange of product) export from Pro/Engineering, followed by STEP import in Fastrad, a STEP–gdml converter. The linac has a compact treatment head and the small space between the shielding collimator and the divergent arc of the upper jaws forbids the implementation of a plane for storing the phase space. Instead, an IAEA (International Atomic Energy Agency) compliant phase space writer was implemented on a cylindrical surface. The simulation was run in parallel on a 1200 node Linux cluster. The 6 MV dose calculations were performed for field sizes varying from 4 × 4 to 40 × 40 cm2. The voxel size for the cm3 water phantom was mm3. For the cm2 field, surface buildup calculations were performed using mm3 voxels within 20 mm of the surface.Results: For the depth dose curves, 98% of the calculated data points agree within 2% with the experimental measurements for depths between 2 and 40 cm. For depths between 5 and 30 cm, agreement within 1% is obtained for 99% (), 95% (), 94% ( and ), and 89% () of the data points, respectively. In the buildup region, the agreement is within 2%, except at 1 mm depth where the deviation is 5% for the cm2 open field. For the lateral dose profiles, within the field size for fields up to cm2, the agreement is within 2% for depths up to 10 cm. At 20 cm depth, the in-field maximum dose difference for the cm2 open field is within 4%, while the smaller field sizes agree within 2%. Outside the field size, agreement within 1% of the maximum dose difference is obtained for all fields. The calculated output factors varied from for the cm2 field to for the cm2 field. Their agreement with the experimental output factors is within 1%.Conclusions: The authors have validated a geant4 simulated IAEA-compliant phase space of the TrueBeam linac for the 6 MV beam obtained using a high accuracy geometry implementation from cad. These files are publicly available and can be used for further research.
38(2011); http://dx.doi.org/10.1118/1.3598442View Description Hide Description
Purpose: In breast radiotherapy with intensity modulation, it is a well established practice to extend the dose fluence outside the limit of the body contour to account for small changes in size and position of the target and the rest of the tissues due to respiration or to possible oedema. A simple approach is not applicable with RapidArc volumetric modulated arc therapy not being based on a fixed field fluence delivery. In this study, a viable technical strategy to account for this need is presented.Methods: RapidArc (RA) plans for six breast cancer patients (three right and three left cases), were optimized (PRO version III) on the original CT data set (O) and on an alternative CT (E) generated with an artificial expansion (and assignment of soft-tissue equivalent HU) of 10 mm of the body in the breast region and of the PTV contours toward the external direction. Final dose calculations for the two set of plans were performed on the same original CT data set O, normalizing the dose prescription (50 Gy) to the target mean. In this way, two treatment plans on the same CT set O for each patient were obtained: the no action plan (OO) and the alternative plan based on an expanded optimization (EO). Fixing MU, these two plans were then recomputed on the expanded CT data set and on an intermediate one (with expansion = 5 mm), to mimic, possible changes in size due to edema during treatment or residual displacements due to breathing not properly controlled. Aim of the study was to quantify the robustness of this planning strategy on dose distributions when either the OO or the EO strategies were adopted. For all the combinations, a DVH analysis of all involved structures is reported.Results: I. The two optimization approaches gave comparable dose distributions on the original CT data set. II. When plans were evaluated on the expanded CTs (mimicking the presence of edema), the EO approach showed improved target coverage if compared to OO: on CT_10 mm, D V = 98% [%] = 92.5 ± 0.9 and 68.5 ± 3.1, respectively, for EO and OO. Minor changes were registered in organs at risk sparing for both EO and OO. III. From dose distributions and DVHs, EO approach allowed to irradiate at near to prescription levels also the expanded fraction of the target: this would account also for residual intrafraction movements.Conclusions: The proposed plan strategy could represent a robust approach to account for moderate changes in target or body volume during the course of breast radiotherapy and to account for residual intrafractional respiratory motion in volumetric modulated arc therapy. The strategy, logistically simple to implement requiring only modifications to the standard planning workflow was routinely implemented at author’s institute for treatment of breast patients with RapidArc.
A fast three-dimensional gamma evaluation using a GPU utilizing texture memory for on-the-fly interpolations38(2011); http://dx.doi.org/10.1118/1.3595114View Description Hide Description
Purpose: A widely accepted method to quantify differences in dose distributions is the gamma (γ) evaluation. Currently, almost all γ implementations utilize the central processing unit (CPU). Recently, the graphics processing unit (GPU) has become a powerful platform for specific computing tasks. In this study, we describe the implementation of a 3D γ evaluation using a GPU to improve calculation time.Methods: The γ evaluation algorithm was implemented on an NVIDIA Tesla C2050 GPU using the compute unified device architecture (cuda). First, several cubic virtual phantoms were simulated. These phantoms were tested with varying dose cube sizes and set-ups, introducing artificial dose differences. Second, to show applicability in clinical practice, five patient cases have been evaluated using the 3D dose distribution from a treatment planning system as the reference and the delivered dose determined during treatment as the comparison. A calculation time comparison between the CPU and GPU was made with varying thread-block sizes including the option of using texture or global memory.Results: A GPU over CPU speed-up of 66 ± 12 was achieved for the virtual phantoms. For the patient cases, a speed-up of 57 ± 15 using the GPU was obtained. A thread-block size of 16 × 16 performed best in all cases. The use of texture memory improved the total calculation time, especially when interpolation was applied. Differences between the CPU and GPU γs were negligible.Conclusions: The GPU and its features, such as texture memory, decreased the calculation time for γ evaluations considerably without loss of accuracy.
38(2011); http://dx.doi.org/10.1118/1.3596527View Description Hide DescriptionPurpose:
The CyberKnife uses an online prediction model to improve radiation delivery when treatinglungtumors. This study evaluates the prediction model used by the CyberKnife radiation therapy system in terms of treatment margins about the gross tumor volume (GTV).Methods:
From the data log files produced by the CyberKnife synchrony model, the uncertainty in radiation delivery can be calculated. Modeler points indicate the tracked position of the tumor and Predictor points predict the position about 115 ms in the future. The discrepancy between Predictor points and their corresponding Modeler points was analyzed for 100 treatment model data sets from 23 de-identified lung patients. The treatment margins were determined in each anatomic direction to cover an arbitrary volume of the GTV, derived from the Modeler points, when the radiation is targeted at the Predictor points. Each treatment model had about 30 min of motion data, of which about 10 min constituted treatment time; only these 10 min were used in the analysis. The frequencies of margin sizes were analyzed and truncated Gaussian normal functions were fit to each direction’s distribution. The standard deviation of each Gaussian distribution was then used to describe the necessary margin expansions in each signed dimension in order to achieve the desired coverage. In this study, 95% modeler point coverage was compared to 99% modeler coverage. Two other error sources were investigated: the correlation error and the targeting error. These were added to the prediction error to give an aggregate error for the CyberKnife during treatment of lungtumors.Results:
Considering the magnitude of 2σ from the mean of the Gaussian in each signed dimension, the margin expansions needed for 95% modeler point coverage were 1.2 mm in the lateral (LAT) direction and 1.7 mm in the anterior–posterior (AP) direction. For the superior–inferior (SI) direction, the fit was poor; but empirically, the expansions were 3.5 mm. For 99% modeler point coverage, the AP margin was 3.6 mm and the lateral margin was 2.9 mm. The SI margins for 99% modeler point coverage were highly variable. The aggregate error at 95% was 6.9 mm in the SI direction, 4.6 mm in the AP direction, and 3.5 in the lateral direction.Conclusions:
The Predictor points follow the Modeler points closely. Margins were found in each clinical direction that would provide 95% modeler point coverage for 95% of the models reviewed in this study. Similar margins were found in two clinical directions for 99% modeler point coverage in 95% of models. These results can offer guidance in the selection of CTV margins for treatment with the CyberKnife.
IPIP: A new approach to inverse planning for HDR brachytherapy by directly optimizing dosimetric indices38(2011); http://dx.doi.org/10.1118/1.3598437View Description Hide DescriptionPurpose:
Many planning methods for high dose rate (HDR) brachytherapy require an iterative approach. A set of computational parameters are hypothesized that will give a dose plan that meets dosimetric criteria. A dose plan is computed using these parameters, and if any dosimetric criteria are not met, the process is iterated until a suitable dose plan is found. In this way, the dose distribution is controlled by abstract parameters. The purpose of this study is to develop a new approach for HDR brachytherapy by directly optimizing the dose distribution based on dosimetric criteria.Methods:
The authors developed inverse planning by integer program (IPIP), an optimizationmodel for computing HDR brachytherapydose plans and a fast heuristic for it. They used their heuristic to compute dose plans for 20 anonymized prostate cancerimagedata sets from patients previously treated at their clinic database. Dosimetry was evaluated and compared to dosimetric criteria.Results:
Dose plans computed from IPIP satisfied all given dosimetric criteria for the target and healthy tissue after a single iteration. The average target coverage was 95%. The average computation time for IPIP was 30.1 s on an Intel(R) CoreTM2 Duo CPU 1.67 GHz processor with 3 Gib RAM.Conclusions:
IPIP is an HDR brachytherapy planning system that directly incorporates dosimetric criteria. The authors have demonstrated that IPIP has clinically acceptable performance for the prostate cases and dosimetric criteria used in this study, in both dosimetry and runtime. Further study is required to determine if IPIP performs well for a more general group of patients and dosimetric criteria, including other cancer sites such as GYN.
38(2011); http://dx.doi.org/10.1118/1.3598441View Description Hide Description
Purpose: To validate GPUMCD, a new package for fast Monte Carlodose calculations based on the GPU (graphics processing unit), as a tool for low-energy single seed brachytherapydosimetry for specific seed models. As the currently accepted method of dose calculation in low-energy brachytherapy computations relies on severe approximations, a Monte Carlo based approach would result in more accurate dose calculations, taking in to consideration the patient anatomy as well as interseed attenuation. The first step is to evaluate the capability of GPUMCD to reproduce low-energy, single source, brachytherapy calculations which could ultimately result in fast and accurate, Monte Carlo based, brachytherapydose calculations for routine planning.Methods: A mixed geometry engine was integrated to GPUMCD capable of handling parametric as well as voxelized geometries. In order to evaluate GPUMCD for brachytherapy calculations, several dosimetry parameters were computed and compared to values found in the literature. These parameters, defined by the AAPM Task-Group No. 43, are the radial dose function, the 2D anisotropy function, and the dose rate constant. These three parameters were computed for two different brachytherapy sources: the Amersham OncoSeed 6711 and the Imagyn IsoStar IS-12501.Results: GPUMCD was shown to yield dosimetric parameters similar to those found in the literature. It reproduces radial dose functions to within 1.25% for both sources in the 0.5< r <10 cm range. The 2D anisotropy function was found to be within 3% at r = 5 cm and within 4% at r = 1 cm. The dose rate constants obtained were within the range of other values reported in the literature.Conclusion: GPUMCD was shown to be able to reproduce various TG-43 parameters for two different low-energy brachytherapy sources found in the literature. The next step is to test GPUMCD as a fast clinical Monte Carlobrachytherapydose calculations with multiple seeds and patient geometry, potentially providing more accurate results than the TG-43 formalism while being much faster than calculations using general purpose Monte Carlo codes.
The management of tumor motions in the stereotactic irradiation to lung cancer under the use of Abches to control active breathing38(2011); http://dx.doi.org/10.1118/1.3604151View Description Hide Description
Purpose: Breathing control is crucial to ensuring the accuracy of stereotactic irradiation for lungcancer. This study monitored respiration in patients with inoperable nonsmall-cell lungcancer using a respiration-monitoring apparatus, Abches, and investigated the reproducibility of tumor position in these patients.Methods: Subjects comprised 32 patients with nonsmall-cell lungcancer who were administered stereotactic radiotherapy under breath-holding conditions monitored by Abches. Computed tomography(CT) was performed under breath-holding conditions using Abches (Abches scan) for treatment planning. A free-breathing scan was performed to determine the range of tumor motions in a given position. After the free-breathing scan, Abches scan was repeated and the tumor position thus defined was taken as the intrafraction tumor position. Abches scan was also performed just before treatment, and the tumor position thus defined was taken as the interfraction tumor position. To calculate the errors, tumor positions were compared based on Abches scan for the initial treatment plan. The error in tumor position was measured using the BrainSCAN treatment-planning device, then compared for each lung lobe.Results: Displacements in tumor position were calculated in three dimensions (i.e., superior–inferior (S–I), left–right (L–R), and anterior–posterior (A–P) dimensions) and recorded as absolute values. For the whole lung, average intrafraction tumor displacement was 1.1 mm (L–R), 1.9 mm (A–P), and 2.0 mm (S–I); the average interfraction tumor displacement was 1.1 mm (L–R), 2.1 mm (A–P), and 2.0 mm (S–I); and the average free-breathing tumor displacement was 2.3 mm (L–R), 3.5 mm (A–P), and 7.9 mm (S–I). The difference between using Abches and free breathing could be reduced from approximately 20 mm at the maximum to approximately 3 mm in the S–I direction for both intrafraction and interfraction positions in the lower lobe. In addition, maximum intrafraction tumor displacement with the use of Abches was 4.5 mm (S–I) in the lingular segment. These results suggest that use of the Abches system can reduce deviations in tumor position to levels below those achieved under free breathing, irrespective of the tumor location.Conclusions: Respiratory control with high accuracy and reproducibility is required for high-precision radiotherapy of inoperable nonsmall-cell lungcancer and was achieved using Abches in this study.
A Bayesian approach to real-time 3D tumor localization via monoscopic x-ray imaging during treatment delivery38(2011); http://dx.doi.org/10.1118/1.3598435View Description Hide DescriptionPurpose:
Monoscopic x-ray imaging with on-board kV devices is an attractive approach for real-time image guidance in modern radiation therapy such as VMAT or IMRT, but it falls short in providing reliable information along the direction of imagingx-ray. By effectively taking consideration of projection data at prior times and/or angles through a Bayesian formalism, the authors develop an algorithm for real-time and full 3D tumor localization with a single x-rayimager during treatment delivery.Methods:
First, a prior probability density function is constructed using the 2D tumor locations on the projection images acquired during patient setup. Whenever an x-rayimage is acquired during the treatment delivery, the corresponding 2D tumor location on the imager is used to update the likelihood function. The unresolved third dimension is obtained by maximizing the posterior probability distribution. The algorithm can also be used in a retrospective fashion when all the projection images during the treatment delivery are used for 3D localization purposes. The algorithm does not involve complex optimization of any model parameter and therefore can be used in a “plug-and-play” fashion. The authors validated the algorithm using (1) simulated 3D linear and elliptic motion and (2) 3D tumormotion trajectories of a lung and a pancreas patient reproduced by a physical phantom. Continuous kV images were acquired over a full gantry rotation with the Varian TrueBeam™ on-board imaging system. Three scenarios were considered: fluoroscopic setup, cone beam CT setup, and retrospective analysis.Results:
For the simulation study, the RMS 3D localization error is 1.2 and 2.4 mm for the linear and elliptic motions, respectively. For the phantom experiments, the 3D localization error is < 1 mm on average and < 1.5 mm at 95th percentile in the lung and pancreas cases for all three scenarios. The difference in 3D localization error for different scenarios is small and is not statistically significant.Conclusions:
The proposed algorithm eliminates the need for any population based model parameters in monoscopic image guided radiotherapy and allows accurate and real-time 3D tumor localization on current standard LINACs with a single x-rayimager.
38(2011); http://dx.doi.org/10.1118/1.3601018View Description Hide Description
Purpose: The aim of this work is to carry out mechanical and dosimetric assessments on a commercial dynamic micromulti leaf collimator system to be used for stereotactic radiosurgery(SRS) and stereotactic radiotherapy (SRT). Mechanical parameters such as leaf position accuracy with different gantry angles and leaf position reproducibility were measured. Also dosimetricmeasurements of the interleaf leakage, intraleaf transmission, penumbra width, and light field alignment were carried out. Furthermore, measurements of output factors (Scp) and in-air factors (Sc) for the μMLC system will be reported.Methods: EBT2 films were used to assess the leaf position error with gantry angle and after stress test, penumbra width and light field alignment. Leaf leakage was quantified using both EBT2 film and a pinpoint ion chamber. With regard to output factors, the pinpoint chamber was placed in a water phantom at 10 cm depth and 100 cm SSD. For in-air output factor measurements, 0.2 cm of brass was placed above the photon diode as build-up.Results: Measurements of mechanical parameters gave values of 0.05 cm (SD 0.035) for the average leaf position accuracy for different gantry angles and after stress test. Dosimetricmeasurements, yielded values of 0.22 ± 0.01 and 0.24 ± 0.01 cm, respectively, for side and head leaf penumbras. Also, average leaf abutting, leakage and transmission were found to be 0.65, 0.91, and 0.20%, respectively.Conclusions: (a) The add-on μMLC system in combination with our LINAC has been commissioned to be used for clinical purposes and showed good agreement with published results for different μMLC types. (b) This work has lead to the recommendation that leaves should be recalibrated after ten static beams or after each dynamic arc.
Quantitative analysis of beam delivery parameters and treatment process time for proton beam therapy38(2011); http://dx.doi.org/10.1118/1.3604153View Description Hide DescriptionPurpose:
To evaluate patient census, equipment clinical availability, maximum daily treatment capacity, use factor for major beam delivery parameters, and treatment process time for actual treatmentsdelivered by proton therapy systems.Methods:
The authors have been recording all beam delivery parameters, including delivered dose, energy, range, spread-out Bragg peak widths, gantry angles, and couch angles for every treatment field in an electronic medical record system. We analyzed delivery system downtimes that had been recorded for every equipment failure and associated incidents. These data were used to evaluate the use factor of beam delivery parameters, the size of the patient census, and the equipment clinical availability of the facility. The duration of each treatment session from patient walk-in and to patient walk-out of the treatment room was measured for 82 patients with cancers at various sites.Results:
The yearly average equipment clinical availability in the last 3 yrs (June 2007–August 2010) was 97%, which exceeded the target of 95%. Approximately 2200 patients had been treated as of August 2010. The major disease sites were genitourinary (49%), thoracic (25%), central nervous system (22%), and gastrointestinal (2%). Beams have been delivered in approximately 8300 treatment fields. The use factor for six beam delivery parameters was also evaluated. Analysis of the treatment process times indicated that approximately 80% of this time was spent for patient and equipment setup. The other 20% was spent waiting for beam delivery and beam on. The total treatment process time can be expressed by a quadratic polynomial of the number of fields per session. The maximum daily treatment capacity of our facility using the current treatment processes was estimated to be 133 ± 35 patients.Conclusions:
This analysis shows that the facility has operated at a high performance level and has treated a large number of patients with a variety of diseases. The use factor of beam delivery parameters varies by disease site. Further improvements in efficiency may be realized in the equipment- and patient-related processes of treatment.
38(2011); http://dx.doi.org/10.1118/1.3603189View Description Hide DescriptionPurpose:
A radical radiation therapy treatment for gliomas requires extremely high absorbed doses resulting in subsequent deleterious side effects in healthy tissue. Microbeam radiation therapy(MRT) is an innovative technique based on the fact that normal tissue can withstand high radiationdoses in small volumes without any significant damage. The synchrotron-generated x-ray beam is collimated and delivered to an array of narrow micrometer-sized planar rectangular fields. Several preclinical experiments performed at the Brookhaven National Laboratory (BNL) and at the European Synchrotron Radiation Facility (ESRF) confirmed that MRT yields a higher therapeutic index than nonsegmented beams of the same characteristics. This index can be greatly improved by loading the tumor with high atomic number (Z) contrast agents. The aim of this work is to find the high-Z element that provides optimum dose enhancement.Methods:
Monte Carlo simulations (PENELOPE/penEasy) were performed to assess the peak and valley doses as well as their ratio (PVDR) in healthy tissue and in the tumor, loaded with different contrast agents. The optimization criteria used were maximization of the ratio between the PVDR values in healthy tissue respect to the PVDR in the tumor and minimization of bone and brain valley doses.Results:
Dose enhancement factors, PVDR, and valley doses were calculated for different high-Z elements. A significant decrease of PVDR values in the tumor, accompanied by a gain in the valley doses, was found in the presence of high-Z elements. This enables the deposited dose in the healthy tissue to be reduced. The optimum high-Z element depends on the irradiation configuration. As a general trend, the best outcome is provided by the highest Z contrast agents considered, i.e., gold and thallium. However, lanthanides (especially Lu) and hafnium also offer a satisfactory performance.Conclusions:
The remarkable therapeutic index in microbeam radiation therapy can be further improved by loading the tumor with a high-Z element. This study reports quantitative data on several dosimetric magnitudes in order to find the optimum contrast agent. Although the final choice of the element will also depend on possible cytotoxicity, three elements were found to be worthy of mention: gold,thallium, and lutetium.