Volume 35, Issue 2, February 2008
- medical physics letters
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- optical physics
- ultrasound physics
- infrared and microwave imaging
- tissue measurements
- anatomy and physiology
- radiation protection physics
- radiation biology
- anniversary papers
- books and publications
Index of content:
Cone beam x-ray CT will be superior to digital x-ray tomosynthesis in imaging the breast and delineating cancer35(2008); http://dx.doi.org/10.1118/1.2825612View Description Hide Description
- MEDICAL PHYSICS LETTERS
Prior image constrained compressed sensing (PICCS): A method to accurately reconstruct dynamic CT images from highly undersampled projection data sets35(2008); http://dx.doi.org/10.1118/1.2836423View Description Hide Description
When the number of projections does not satisfy the Shannon/Nyquist sampling requirement, streaking artifacts are inevitable in x-ray computed tomography (CT) images reconstructed using filtered backprojection algorithms. In this letter, the spatial-temporal correlations in dynamic CT imaging have been exploited to sparsify dynamic CT image sequences and the newly proposed compressed sensing (CS) reconstruction method is applied to reconstruct the target image sequences. A prior image reconstructed from the union of interleaved dynamical data sets is utilized to constrain the CS image reconstruction for the individual time frames. This method is referred to as prior image constrained compressed sensing (PICCS). In vivo experimental animal studies were conducted to validate the PICCS algorithm, and the results indicate that PICCS enables accurate reconstruction of dynamic CT images using about 20 view angles, which corresponds to an undersampling factor of 32. This undersampling factor implies a potential radiation dose reduction by a factor of 32 in myocardial CT perfusion imaging.
- RADIATION THERAPY PHYSICS
35(2008); http://dx.doi.org/10.1118/1.2820902View Description Hide Description
In 1996 Swensson published an observer model that predicted receiver operating characteristic (ROC), localization ROC (LROC), free-response ROC (FROC) and alternative FROC (AFROC) curves, thereby achieving “unification” of different observer performance paradigms. More recently a model termed initial detection and candidate analysis (IDCA) has been proposed for fitting computer aided detection (CAD) generated FROC data, and recently a search model for human observer FROC data has been proposed. The purpose of this study was to derive IDCA and the search model based expressions for operating characteristics, and to compare the predictions to the Swensson model. For three out of four mammography CAD data sets all models yielded good fits in the high-confidence region, i.e., near the lower end of the plots. The search model and IDCA tended to better fit the data in the low-confidence region, i.e., near the upper end of the plots, particularly for FROC curves for which the Swensson model predictions departed markedly from the data. For one data set none of the models yielded satisfactory fits. A unique characteristic of search model and IDCA predicted operating characteristics is that the operating point is not allowed to move continuously to the lowest confidence limit of the corresponding Swensson model curves. This prediction is actually observed in the CAD raw data and it is the primary reason for the poor FROC fits of the Swensson model in the low-confidence region.
Testing the GLAaS algorithm for dose measurements on low- and high-energy photon beams using an amorphous silicon portal imager35(2008); http://dx.doi.org/10.1118/1.2828182View Description Hide Description
The GLAaS algorithm for pretreatment intensity modulation radiation therapy absolute dose verification based on the use of amorphous silicondetectors, as described in Nicolini et al. [G. Nicolini, A. Fogliata, E. Vanetti, A. Clivio, and L. Cozzi, Med. Phys.33, 2839–2851 (2006)], was tested under a variety of experimental conditions to investigate its robustness, the possibility of using it in different clinics and its performance. GLAaS was therefore tested on a low-energy Varian Clinac (6 MV) equipped with an amorphous silicon Portal Vision PV-aS500 with electronic readout IAS2 and on a high-energy Clinac (6 and 15 MV) equipped with a PV-aS1000 and IAS3 electronics. Tests were performed for three calibration conditions: A: adding buildup on the top of the cassette such that and comparing measurements with corresponding doses computed at , B: without adding any buildup on the top of the cassette and considering only the intrinsic water-equivalent thickness of the electronic portal imaging devices device (0.8 cm), and C: without adding any buildup on the top of the cassette but comparing measurements against doses computed at . This procedure is similar to that usually applied when in vivodosimetry is performed with solid state diodes without sufficient buildup material. Quantitatively, the gamma index , as described by Low et al. [D. A. Low, W. B. Harms, S. Mutic, and J. A. Purdy, Med. Phys.25, 656–660 (1998)], was assessed. The index was computed for a distance to agreement (DTA) of 3 mm. The dose difference was considered as 2%, 3%, and . As a measure of the quality of results, the fraction of field area with gamma larger than 1 was scored. Results over a set of 50 test samples (including fields from head and neck, breast, prostate, anal canal, and brain cases) and from the long-term routine usage, demonstrated the robustness and stability of GLAaS. In general, the mean values of remain below for equal or larger than , while they are slightly larger for with in the range from 3% to . Since its introduction in routine practice, 1453 fields have been verified with GLAaS at the authors’ institute (6 MV beam). Using a DTA of 3 mm and a of the authors obtained for the entire data set while, stratifying according to the dose calculation algorithm, they observed: for fields computed with the analytical anisotropic algorithm and for pencil-beam based fields with a statistically significant difference between the two groups. If data are stratified according to field splitting, they observed for split fields and for nonsplit fields without any significant difference.
35(2008); http://dx.doi.org/10.1118/1.2828203View Description Hide Description
We have developed a high resolution, quantitative, two-dimensional optical film scanner for use with a commercial high sensitivity radiochromic film (RCF) for measuring single fraction external-beam radiotherapy dose distributions. The film scanner was designed to eliminate artifacts commonly observed in RCF dosimetry. The scanner employed a stationary light source and detector with a moving antireflective glass film platen attached to a high precision computerized translation stage. An ultrabright red light emitting diode(LED) with a peak output at 633 nm and full width at half maximum (FWHM) of 16 nm was selected as the scanner light source to match the RCF absorption peak. A dual detector system was created using two silicon photodiode detectors to simultaneously measure incident and transmitted light. The LED light output was focused to a submillimeter (FWHM 0.67 mm) spot size, which was determined from a scanning knife-edge technique for measuring Gaussian optical beams. Data acquisition was performed with a 16-bit A/D card in conjunction with commercial software. The linearity of the measured densities on the scanner was tested using a calibrated neutral-density step filter. Sensitometric curves and three IMRT field scans were acquired with a spatial resolution of 1 mm for both radiographic film and RCF. The results were compared with measurements taken with a commercial diode array under identical delivery conditions. The RCF was rotated by 90 deg and rescanned to study orientation effects. Comparison between the RCF and the diode array measurements using percent dose difference and distance-to-agreement criteria produced average passing rates of using criteria and using criteria. The same comparison between the radiographic film and diode array measurements resulted in average passing rates and for the above two criteria, respectively. No measurable light-scatter or interference scanner artifacts were observed. The RCF rotated by 90 deg showed no measurable orientation effect. A scan of a area with 1 mm resolution required 22 min to acquire. The LEDdensitometer provides an accurate film dosimetry system with 1 mm or better resolution. The scanner eliminates the orientation dependence of RCF dosimetry that was previously reported with commercial flatbed scanners.
35(2008); http://dx.doi.org/10.1118/1.2825619View Description Hide Description
The purpose of this study is to establish a comprehensive set of dose measurements data obtained from the X-ray Volumetric Imager (XVI®, Elekta Oncology Systems) and the On-Board Imager (OBI®, Varian Medical Systems) cone-beam CT(CBCT) systems. To this end, two uniform-density cylindrical acrylic phantoms with diameters of 18 cm (head phantom) and 30 cm (body phantom) were used for all measurements. Both phantoms included ion chamber placement holes in the center and at periphery (2 cm below surface). For the XVI unit, the four standard manufacturer-supplied protocols were measured. For the OBI unit, the full bow tie and half bow tie (and no bow tie) filters were used in combination with the two scanning modes; namely, full-fan and half-fan. The total (mA s) setting was also varied for each protocol to establish the linear relationship between the dose deposited and the mA s used (with all other factors being held constant). Half-value layers in aluminum(Al) were also measured for beam characteristic determination. For the XVI unit, the average dose ranged from 0.1 to 3.5 cGy with the highest dose measured using the “prostate” protocol with the body phantom. For the OBI unit, the average dose ranged from 1.1 to 8.3 cGy with the highest dose measured using the full-fan protocol with the head phantom. The measured doses were highly linear as a function of mA s, for both units, where the measurement points followed a linear relationship very closely with for all cases. Half-value layers were between 4.6- and 7.0-mm-Al for the two CBCT units where XVI generally had more penetrating beams at the similar kVp settings. In conclusion, a comprehensive series of dose measurements were performed on the XVI and the OBI CBCT units. In the process, many of the important similarities and differences between the two systems were observed and summarized in this work.
35(2008); http://dx.doi.org/10.1118/1.2825622View Description Hide Description
The purposes of this study are: (i) to design field flattening filters for the Leipzig applicators of 2 and 3 cm of inner diameter with the source traveling parallel to the applicator contact surface, which are accessories of the microSelectron-HDR afterloader (Nucletron, Veenendaal, The Netherlands). These filters, made of tungsten, aim to flatten the heterogeneous dose distribution obtained with the Leipzig applicators. (ii) To estimate the dose rate distributions for these applicators by means of the Monte Carlo(MC) method. (iii) To experimentally verify these distributions for prototypes of these new applicators, and (iv) to obtain the correspondence factors to measure the output of the applicators by the user using an insert into a well chamber. The MCGEANT4 code has been used to design the filters and to obtain the dose rate distributions in liquid water for the two applicators. In order to validate this specific application and to guarantee that realistic source-applicator geometry has been considered, an experimental verification procedure was implemented in this study, in accordance with the updated recommendations of the American Association of Physicists in Medicine Task Group No. 43 U1 Report. Thermoluminescent dosimeters, radiochromic film, and a pin-point ionization chamber in a plastic [polymethylmethacrylate (PMMA)] phantom were used to verify the MC results for the two applicators of a microSelectron-HDR afterloader with the mHDR-v2 source. To verify the output of the applicators, correspondence factors were deduced for the well chambers HDR100-plus (Standard Imaging, Inc., Middleton, WI) and TM33004 (PTW, Freiburg, Germany) using a specific insert for both applicators. The doses measured in the PMMA phantom agree within experimental uncertainties with the dose obtained by the MC calculations. Percentage depth dose and off-axis profiles were obtained normalized at a depth of 3 mm along the central applicator axis in a cylindrical water phantom. A table of output factors, normalized to 1 U of source air kerma strength at this depth, is presented. Correspondence factors were obtained for the two well chambers considered. The matrix data obtained in the MC simulation with a grid separation of 0.5 mm has been used to build a data set in a convenient format to model these distributions for routine use with a brachytherapytreatment planning system.
35(2008); http://dx.doi.org/10.1118/1.2828195View Description Hide Description
The scope of this study was to estimate total scatter factors of the three smallest collimators of the Cyberknife radiosurgery system (5–10 mm in diameter), combining experimental measurements and Monte Carlo simulation. Two microchambers, a diode, and a diamonddetector were used to collect experimental data. The treatment head and the detectors were simulated by means of a Monte Carlo code in order to calculate correction factors for the detectors and to estimate total scatter factors by means of a consistency check between measurement and simulation. Results for the three collimators were: , , , all relative to the 60 mm collimator at 80 cm source-to-detector distance. The method also allows the full width at half maximum of the electron beam to be estimated; estimations made with different collimators and different detectors were in excellent agreement and gave a value of 2.1 mm. Correction factors to be applied to the detectors for the measurement of were consistent with a prevalence of volume effect for the microchambers and the diamond and a prevalence of scattering from high- material for the diode detector. The proposed method is more sensitive to small variations of the electron beam diameter with respect to the conventional method used to commission Monte Carlo codes, i.e., by comparison with measured percentage depth doses (PDD) and beam profiles. This is especially important for small fields (less than 10 mm diameter), for which measurements of PDD and profiles are strongly affected by the type of detector used. Moreover, this method should allow of Cyberknife systems different from the unit under investigation to be estimated without the need for further Monte Carlo calculation, provided that one of the microchambers or the diode detector of the type used in this study are employed. The results for the diamond are applicable only to the specific detector that was investigated due to excessive variability in manufacturing.
35(2008); http://dx.doi.org/10.1118/1.2828187View Description Hide Description
Purpose : To compare calibration of the Leksell™ Gamma Knife according to the American Association of Physicists in Medicine Task Groups 21 and 51 protocols. A new phantom was fabricated for this purpose. Its design, physical properties, and composition are described. Materials and methods: The Gamma Knife TG-51 calibration phantom is designed to be filled with water and support an ionization chamber positioned at its center. The phantom is thimble-shaped, with a 2 mm plastic wall to contain water. The phantom and chamber assembly was mounted in a LeksellTM stereotactic frame. The location of the chamber’s sensitive volume was determined using computed tomography. The chamber-phantom assembly was attached to the 18 mm helmet in the Gamma Knife by the stereotactic frame. The phantom’s geometry allowed radiation beams from each of the 201 Gamma Knife cobalt-60 sources to converge after an 8 cm path to the ionization chamber’s sensitive volume. This is similar to the arrangement by which one calibrates the Gamma Knife using the manufacturer-supplied polystyrene phantom. Results: The phantom was attached to the Gamma Knife so that the ionization chamber was reproducibly positioned at the convergence of the radiation beams. Because of the phantom’s design, the phantom could be affixed to either trunnions or the automatic patient positioning system, once mounted in the LeksellTM stereotectic frame. Comparisons using different phantoms and protocols resulted in the following calibration ratios for TG-21 in the polystyrene sphere phantom, TG-21 in the water phantom, and TG-51 in the water phantom, respectively: 1.000, 1.008, 0.986, when corrected for transmission through the plastic water reservoir wall and using the same ionization chamber. Transmission measurements using a 1 cm thickness of the same material in the Co-60 beam determined that the phantom’s 2 mm plastic wall resulted in a reduction in the measured the output by . Conclusions:Calibration of the Gamma Knife can be performed in liquid water using the AAPM TG-51 protocol and this new phantom, thereby eliminating uncertainties with respect to the composition of the manufacturer’s phantom. Perturbation of calibration measurements by nonwater materials was characterized and could be corrected. Calibration values for the Gamma Knife that were obtained using the three methods for our phantoms agree to within . TG21 and TG51 calibration of the Gamma Knife using the water phantom agreed to within .
Comparison of mega-voltage cone-beam computed tomography prostate localization with online ultrasound and fiducial markers methods35(2008); http://dx.doi.org/10.1118/1.2830381View Description Hide Description
The online image-guided localization data from 696 ultrasound(US), 598 mega-voltage cone-beam computed tomography (MV-CBCT), and 393 seed markers (SMs) couch alignments for patients undergoing intensity modulation radiotherapy of the prostate were analyzed. Daily US, MV-CBCT and SM images were acquired for 19, 17 and 12 patients, respectively, after each patient was immobilized in a vacuum cradle and setup to skin markers as the center of mass. The couch shifts applied in the lateral (left-right/LR), vertical (anterior-posterior/AP), and longitudinal (superior-inferior/SI) directions, along with the magnitude of the three-dimensional (3D) shift vector, were analyzed and compared for all three methods. The percentage of shifts larger than in all directions was also compared. Clinical target volume-planning target volume (CTV-to-PTV) expansion margins were estimated based on the localization data with US, CB, and SM image guidance. Results show the US data have greater variability. Systematic and random shifts were (LR), (SI) and (AP) for US, (LR), (SI) and (AP) for CB, and (LR), (SI) and (AP) for SM. The mean 3D shift distance was larger using US compared to CB and SM ( and , respectively). The percentage of US shifts larger than were 34%, 31%, and 38% in the LR, SI, and AP directions, respectively, compared to 18%, 6%, and 16% for CB and 14%, 10%, and 20% for SM. MV-CBCT and SM localization data suggest a different distribution of prostate center-of-mass shifts with smaller variability, compared to US. The online MV-CBCT and SM image-guidance data show that for treatments that do not include daily prostate localization, one can use a CTV-to-PTV margin that is smaller than the one suggested by US data, hence allowing more rectum and bladder sparing and potentially improving the therapeutic ratio.
35(2008); http://dx.doi.org/10.1118/1.2826558View Description Hide Description
This work introduces a new concept—the dosimetric margin distribution (DMD)—and uses it to explain the sensitivity of a group of prostate IMRTtreatment plans to patient setup errors. Prior work simulated the effect of setup errors on 27 prostate IMRTtreatment plans and found the plans could tolerate larger setup errors than predicted by the van Herk margin formula. The conjectured reason for this disagreement was a breakdown in van Herk’s assumption that the planned dose distribution conforms perfectly to target structures. To resolve the disagreement, this work employed the same 27 plans to evaluate the actual margin distributions that exist between: (i) the clinical target volume (CTV) and planning target volume (PTV) and (ii) the CTV and PTV minimum dose isodose surface. These distributions were evaluated for both prostate and nodal targets. Distribution (ii) is the DMD. The dosimetric margin in a given direction determines the probability that the CTV will be underdosed due to setup errors in that direction. Averaging over gives the overall probability of CTV coverage. Minimum doses for prostate and nodal PTVs were obtained from dose volume histograms. Corresponding isodose surfaces were created and converted to regions of interest (ROIs). CTV, PTV, and isodose ROIs were saved as mesh files and then imported into a computational geometry application which calculated distances between meshes (i.e., margins) in 614 discrete directions covering in 10 deg increments. Measured prostate CTV-to-PTV margins were close to the nominal value of 0.5 cm specified in the treatment planning protocol. However, depending on direction, prostate dosimetric margins ranged from 0.5 to 3 cm, reflecting the imperfect conformance of the planned dose distribution to the prostate PTV. For the nodal CTV, the nominal CTV-to-PTV margin employed in treatment planning was again 0.5 cm. However, due to the planning protocol, the nodal PTV follows the surface of the nodal CTV in several places, ensuring that there is no room for rigid body motion of the nodal CTV inside the nodal PTV. Measured nodal CTV-to-PTV margins were therefore zero, while nodal dosimetric margins ranged from 0.2 to 2.8 cm. Prostate and nodal target coverage were found to be well correlated with the measured DMDs, thereby resolving the apparent disagreement with our prior results. The principal conclusion is that target coverage in the presence of setup errors should be evaluated using the DMD, rather than the CTV-to-PTV margin distribution. The DMD is a useful planning metric, which generalizes the ICRU conformity index. DMDs could vary with number of beams, beam arrangements, TPS, and treatment site.
The grid-dose-spreading algorithm for dose distribution calculation in heavy charged particle radiotherapy35(2008); http://dx.doi.org/10.1118/1.2829878View Description Hide Description
A new variant of the pencil-beam (PB) algorithm for dose distribution calculation for radiotherapy with protons and heavier ions, the grid-dose spreading (GDS) algorithm, is proposed. The GDS algorithm is intrinsically faster than conventional PB algorithms due to approximations in convolution integral, where physical calculations are decoupled from simple grid-to-grid energy transfer. It was effortlessly implemented to a carbon-ion radiotherapytreatment planning system to enable realistic beam blurring in the field, which was absent with the broad-beam (BB) algorithm. For a typical prostate treatment, the slowing factor of the GDS algorithm relative to the BB algorithm was 1.4, which is a great improvement over the conventional PB algorithms with a typical slowing factor of several tens. The GDS algorithm is mathematically equivalent to the PB algorithm for horizontal and vertical coplanar beams commonly used in carbon-ion radiotherapy while dose deformation within the size of the pristine spread occurs for angled beams, which was within 3 mm for a single 150-MeV proton pencil beam of incidence, and needs to be assessed against the clinical requirements and tolerances in practical situations.
Increasing the speed of DOSXYZnrc Monte Carlo simulations through the introduction of nonvoxelated geometries35(2008); http://dx.doi.org/10.1118/1.2829874View Description Hide Description
This article presents a method for increasing the speed of DOSXYZnrc Monte Carlo simulations through the introduction of nonvoxelated geometries defined in any coordinate system. Nonvoxelated geometries are used to isolate regions of uniform density and composition from the scoring grid. Particle transport within these geometric regions is not restricted by the boundary constraints of the scoring grid. This allows for larger particle steps, which in turn reduces the calculation time. A water tank phantom, water-lung interface phantom, cylindrical calibration phantom, and CT phantom were each used to test the application of the nonvoxelated approach. Each phantom was simulated using both the original DOSXYZnrc code and the new nonvoxelated code. The equivalence between the original and nonvoxelated simulations were quantified using a analysis. To within the statistical uncertainty, the voxelated and nonvoxelated simulations were found to give nearly identical results, regardless of boundary crossing algorithm. The speed increase was found to be a function of both voxel dimension and field size. Using nonvoxelated geometries and the EXACT boundary crossing algorithm, the speed increase was as high as 9.0, 5.1, 5.7, and 1.3 times faster for the water tank, water-lung interface, cylindrical calibration, and CT phantoms, respectively. If the PRESTA-I boundary crossing algorithm was used, the calculation speed increase was up to 6.0, 2.7, 3.3, and 1.2 times faster. These results clearly show that the nonvoxelated technique greatly increases simulation speed without any loss in dose accuracy.
Automatic registration between reference and on-board digital tomosynthesis images for positioning verification35(2008); http://dx.doi.org/10.1118/1.2831903View Description Hide Description
The authors developed a hybrid multiresolution rigid-body registration technique to automatically register reference digital tomosynthesis (DTS) images with on-board DTS images to guide patient positioning in radiation therapy. This hybrid registration technique uses a faster but less accurate static method to achieve an initial registration, followed by a slower but more accurate adaptive method to fine tune the registration. A multiresolution scheme is employed in the registration to further improve the registration accuracy, robustness, and efficiency. Normalized mutual information is selected as the criterion for the similarity measure and the downhill simplex method is used as the search engine. This technique was tested using image data both from an anthropomorphic chest phantom and from eight head-and-neck cancer patients. The effects of the scan angle and the region-of-interest (ROI) size on the registration accuracy and robustness were investigated. The necessity of using the adaptive registration method in the hybrid technique was validated by comparing the results of the static method and the hybrid method. With a scan angle and a large ROI covering the entire DTS volume, the average of the registration capture ranges in single-axis simulations was between and for rotations and between and for translations in the phantom study, and between and for rotations and between and for translations in the patient study. Decreasing the DTS scan angle from to mainly degraded the registration accuracy and robustness for the out-of-plane rotations. Decreasing the ROI size from the entire DTS volume to the volume surrounding the spinal cord reduced the capture ranges to between and for rotations and between and for translations in the phantom study, and between and for rotations and between and for translations in the patient study. Results also showed that the hybrid registration technique had much larger capture ranges than the static method alone in registering the out-of-plane rotations.
35(2008); http://dx.doi.org/10.1118/1.2828196View Description Hide Description
This paper focuses on the accuracy, in absolute dose measurements, with GafChromic™ EBT film achievable in water for a photon beam up to a dose of . Motivation is to get an absolute dose detection system to measure up dose distributions in a (water) phantom, to check dose calculations. An Epson 1680 color (red green blue) transmission flatbed scanner has been used as film scanning system, where the response in the red color channel has been extracted and used for the analyses. The influence of the flatbed filmscanner on the film based dose detection process was investigated. The scan procedure has been optimized; i.e. for instance a lateral correction curve was derived to correct the scan value, up to 10%, as a function of optical density and lateral position. Sensitometric curves of different film batches were evaluated in portrait and landscape scan mode. Between various batches important variations in sensitometric curve were observed. Energy dependence of the film is negligible, while a slight variation in dose response is observed for very large angles between film surface and incident photon beam. Improved accuracy in absolute dose detection can be obtained by repetition of a film measurement to tackle at least the inherent presence of film inhomogeneous construction. We state that the overall uncertainty is random in absolute EBT filmdose detection and of the order of 1.3% (1 SD) under the condition that the film is scanned in a limited centered area on the scanner and at least two films have been applied. At last we advise to check a new film batch on its characteristics compared to available information, before using that batch for absolute dose measurements.
35(2008); http://dx.doi.org/10.1118/1.2828197View Description Hide Description
Reconstruction of four-dimensional (4D) imaging typically requires an externally measurable surrogate to represent the real-time relative phase of respiration. A common method is to use a reflective marker on the external surface of the patient which moves with respiration and can be tracked in real time. The location of the marker is often chosen to maximize the observable motion, though this location may not be at the region of interest. We evaluate the importance of infrared (IR) marker placement location on breathing phase definition for the purpose of respiratory gating and 4D computed tomography(CT)image reconstruction. Data were collected for ten patients enrolled on an approved IRB protocol. Real-time position data were collected during CTimaging and daily treatments for two external IR reflective markers: one placed near the xyphoid and another at the approximate location of the treatment isocenter. Motion traces from the markers were compared using cross-correlation coefficient and by estimating the relative respiratory phase, based on either marker, as would be used for 4D-CT reconstruction. Cross-correlation analysis revealed differences in the motion waveform, as well as phase differences, both of which were variable between patients as well as day to day for the same patient. Estimated relative phases from each marker were compared by the percentage amount of time the estimated phase for each marker was different, binned based on increments of 10% of a full cycle. For all collected data combined, the frequency with which breathing phase mismatch led to different bin allocation in steps of 10% was as follows: , , , and . Based on ten images per cycle, this indicates that 4D reconstructions would be influenced, depending on which marker was used, by at least 1 bin 34.9% of the time. This number was noticeably higher for some patients; the maximum was 71% of the time for one patient of ten. In conclusion, the respiratory amplitude and relative phase depend significantly on the location of the IR marker used to monitor respiration. For some patients the xyphoid and isocentric markers may be completely out of phase. More importantly, this relationship varies day to day, suggesting that a single marker may be inadequate for the purposes of respiratory gating.
35(2008); http://dx.doi.org/10.1118/1.2828206View Description Hide Description
Superficial doses were measured for static TomoTherapy Hi-Art® beams for normal and oblique incidence. Dose was measured at depths along the central axis of and beams at normal incidence for source to detector distances (SDDs) of 55, 70, and 85 cm. Measurements were also made at depths normal to the phantom surface for the same beams at oblique angles of , , , , and from the normal. Data were collected with a Gammex/RMI model 449 parallel-plate chamber embedded in a solid water phantom and with LiF thermoluminescent dosimeters(TLDs) in the form of powder. For comparison, measurements were made on a conventional 6 MV beam (Varian Clinac 2100C) at normal incidence and at an oblique angle of from the normal. TomoTherapy surface dose varied with the distance from the source and the angle of incidence. For normal incidence, surface dose increased from 0.16 to 0.43 cGy/MU as the distance from the source decreased from 85 to 55 cm for the field and increased from 0.12 to 0.32 cGy/MU for the field. As the angle of incidence increased from to , surface dose increased from 0.24 to 0.63 cGy/MU for the field and from 0.18 to 0.58 cGy/MU for the field. For normal incidence at 55 cm SDD, the surface dose relative to the dose at for the TomoTherapy Hi-Art beam was less than that from a conventional, flattening filter based linear accelerator. These data should prove useful in accessing the accuracy of the TomoTherapy treatment planning system to predict the dose at superficial depths for a static beam delivery.
35(2008); http://dx.doi.org/10.1118/1.2828378View Description Hide Description
Deformable registration is needed for a variety of tasks in establishing the voxel correspondence between respiratory phases. Most registration algorithms assume or imply that the deformation field is smooth and continuous everywhere. However, the lungs are contained within closed invaginated sacs called pleurae and are allowed to slide almost independently along the chest wall. This sliding motion is characterized by a discontinuous vector field, which cannot be generated using standard deformable registration methods. The authors have developed a registration method that can create discontinuous vector fields at the boundaries of anatomical subregions. Registration is performed independently on each subregion, with a boundary-matching penalty used to prevent gaps. This method was implemented and tested using both the B-spline and Demons registration algorithms in the Insight Segmentation and Registration Toolkit. The authors have validated this method on four patient 4DCT data sets for registration of the end-inhalation and end-exhalation volumes. Multiple experts identified homologous points in the lungs and along the ribs in the two respiratory phases. Statistical analyses of the mismatch of the homologous points before and after registration demonstrated improved overall accuracy for both algorithms.
35(2008); http://dx.doi.org/10.1118/1.2825616View Description Hide Description
Modeling and predicting tumor motion caused by respiration is challenging due to temporal variations in breathing patterns. Treatment approaches such as gating or adaptive bed adjustment/alignment may not require full knowledge of instantaneous position, but might benefit from tracking the general trend of the motion. One simple method for tracking mean tumor position is to apply moving average filters with window sizes corresponding to the breathing periods. Yet respiratory motion is only semiperiodic, so such methods require reliable phase estimation, which is difficult in the presence of noise. This article describes a robust method to track the mean position of respiratory motion without explicitly estimating instantaneous phase. We form a state vector from the respiration signal values at the current instant and at a previous time, and fit an ellipse model to training data. Ellipse eccentricity and orientation potentially capture hysteresis in respiratory motion. Furthermore, we provide two recursive online algorithms for real time mean position tracking: a windowed version with an adaptive window size and another one with temporal discounting. We test the proposed method with simulated breathing traces, as well as with real time-displacement (RPM, Varian) signals. Estimation traces are compared with retrospectively generated moving average results to illustrate the performance of the proposed approach.