Index of content:
Volume 37, Issue 10, October 2010
37(2010); http://dx.doi.org/10.1118/1.3470099View Description Hide Description
- RADIATION THERAPY PHYSICS
Sensitivity of low energy brachytherapy Monte Carlo dose calculations to uncertainties in human tissue composition37(2010); http://dx.doi.org/10.1118/1.3477161View Description Hide DescriptionPurpose:
The objective of this work is to assess the sensitivity of Monte Carlo(MC)dose calculations to uncertainties in human tissue composition for a range of low photon energy brachytherapy sources:, , , and an electronic brachytherapy source (EBS). The low energy photons emitted by these sources make the dosimetry sensitive to variations in tissue atomic number due to the dominance of the photoelectric effect. This work reports dose to a small mass of water in medium as opposed to dose to a small mass of medium in medium .Methods:
Mean adipose, mammary gland, and breast tissues (as uniform mixture of the aforementioned tissues) are investigated as well as compositions corresponding to one standard deviation from the mean. Prostate mean compositions from three different literature sources are also investigated. Three sets of MC simulations are performed with theGEANT4 code: (1) Dose calculations for idealized TG-43-like spherical geometries using point sources. Radial dose profiles obtained in different media are compared to assess the influence of compositional uncertainties. (2) Dose calculations for four clinical prostate LDR brachytherapy permanent seed implants using seeds (Model 2301, Best Medical, Springfield, VA). The effect of varying the prostate composition in the planning target volume (PTV) is investigated by comparing PTV values. (3) Dose calculations for four clinical breast LDR brachytherapy permanent seed implants using seeds (Model 2335, Best Medical). The effects of varying the adipose/gland ratio in the PTV and of varying the elemental composition of adipose and gland within one standard deviation of the assumed mean composition are investigated by comparing PTV values. For (2) and (3), the influence of using the mass density from CT scans instead of unit mass density is also assessed.Results:
Results from simulation (1) show that variations in the mean compositions of tissues affect low energy brachytherapydosimetry.Dose differences between mean and one standard deviation of the mean composition increasing with distance from the source are observed. It is established that the and sources are the least sensitive to variations in elemental compositions while is most sensitive. The EBS falls in between and exhibits complex behavior due to significant spectral hardening. Results from simulation (2) show that two prostate compositions are dosimetrically equivalent to water while the third shows differences of up to 4%. Results from simulation (3) show that breast is more sensitive than prostate with dose variations of up to 30% from water for 70% adipose/30% gland breast. The variability of the breast composition adds a ±10% dose variation.Conclusions:
Low energy brachytherapydose distributions in tissue differ from water and are influenced by density, mean tissue composition, and patient-to-patient composition variations. The results support the use of a dose calculation algorithm accounting for heterogeneities such as MC. Since this work shows that variations in mean tissue compositions affect MCdosimetry and result in increased dose uncertainties, the authors conclude that imaging tools providing more accurate estimates of elemental compositions such as dual energy CT would be beneficial.
Fundus image fusion in EYEPLAN software: An evaluation of a novel technique for ocular melanoma radiation treatment planning37(2010); http://dx.doi.org/10.1118/1.3488891View Description Hide DescriptionPurpose:
The purpose of this study is to evaluate a novel approach for treatment planning using digital fundus image fusion inEYEPLAN for proton beam radiation therapy (PBRT) planning for ocular melanoma. The authors used a prototype version of EYEPLANsoftware, which allows for digital registration of high-resolution fundus photographs. The authors examined the improvement in tumor localization by replanning with the addition of fundus photo superimposition in patients with macular area tumors.Methods:
The new version ofEYEPLAN (v3.05) software allows for the registration of fundus photographs as a background image. This is then used in conjunction with clinical examination, tantalum marker clips, surgeon’s mapping, and ultrasound to draw the tumor contour accurately. In order to determine if the fundus image superimposition helps in tumor delineation and treatment planning, the authors identified 79 patients with choroidal melanoma in the macular location that were treated with PBRT. All patients were treated to a dose of 56 GyE in four fractions. The authors reviewed and replanned all 79 macular melanoma cases with superimposition of pretreatment and post-treatment fundus imaging in the new EYEPLANsoftware. For patients with no local failure, the authors analyzed whether fundus photograph fusion accurately depicted and confirmed tumor volumes as outlined in the original treatment plan. For patients with local failure, the authors determined whether the addition of the fundus photograph might have benefited in terms of more accurate tumor volume delineation.Results:
The mean follow-up of patients was. Tumor growth was seen in six eyes of the 79 macular lesions. All six patients were marginal failures or tumor miss in the region of dose fall-off, including one patient with both in-field recurrence as well as marginal. Among the six recurrences, three were managed by enucleation and one underwent retreatment with proton therapy. Three patients developed distant metastasis and all three patients have since died. The replanning of six patients with their original fundus photograph superimposed showed that in four cases, the treatment field adequately covered the tumor volume. In the other two patients, the overlaid fundus photographs indicated the area of marginal miss. The replanning with the fundus photograph showed improved tumor coverage in these two macular lesions. For the remaining patients without local failure, replanning with fundus photograph superimposition confirmed the tumor volume as drawn in the original treatment plan.Conclusions:
Local control was excellent in patients receiving 56 GyE of PBRT for uveal melanomas in the macular region, which traditionally can be more difficult to control. Posterior lesions are better defined with the additional use of fundus image since they can be difficult to mark surgically. In one-third of treatment failing patients, the superposition of the fundus photograph would have clearly allowed improved localization of tumor. The current practice standard is to use the superimposition of the fundus photograph in addition to the surgeon’s clinical and clip mapping of the tumor and ultrasound measurement to draw the tumor volume.
Monte Carlo characterization of skin doses in 6 MV transverse field MRI-linac systems: Effect of field size, surface orientation, magnetic field strength, and exit bolus37(2010); http://dx.doi.org/10.1118/1.3488980View Description Hide DescriptionPurpose:
The main focus of this work is to continue investigations into the Monte Carlo predicted skindoses seen in MRI-guided radiotherapy. In particular, the authors aim to characterize theskindoses over a larger range of magnetic field strength and x-ray field size than in the current literature. The effect of surface orientation on both the entry and exit sides is also studied. Finally, the use of exit bolus is also investigated for minimizing the negative effects of the electron return effect (ERE) on the exit skindose.Methods:
High resolutionGEANT4Monte Carlo simulations of a water phantom exposed to a 6 MV x-ray beam (Varian 2100C) have been performed. Transverse magnetic fields of strengths between 0 and 3 T have been applied to a phantom. This phantom is also altered to have variable entry and exit surfaces with respect to the beam central axis and they range from −75° to . The exit bolus simulated is a 1 cm thick (water equivalent) slab located on the beam exit side.Results:
On the entry side, significant skindoses at the beam central axis are reported for large positive surface angles and strong magnetic fields. However, over the entry surface angle range of −30° to −60°, the entry skindose is comparable to or less than the zero magnetic fieldskindose, regardless of magnetic field strength and field size. On the exit side, moderate to high central axis skindose increases are expected except at large positive surface angles. For exit bolus of 1 cm thickness, the central axis exit skindose becomes an almost consistent value regardless of magnetic field strength or exit surface angle. This is due to the almost complete absorption of the ERE electrons by the bolus.Conclusions:
There is an ideal entry angle range of −30° to −60° where entry skindose is comparable to or less than the zero magnetic fieldskindose. Other than this, the entry skindose increases are significant, especially at higher magnetic fields. On the exit side there is mostly moderate to high skindose increases for 0.2–3 T with the only exception being large positive angles. Exit bolus of 1 cm thickness will have a significant impact on lowering such exit skindose increases that occur as a result of the ERE.
37(2010); http://dx.doi.org/10.1118/1.3488892View Description Hide DescriptionPurpose:
In high energy teletherapy, is known to be a very accurate and efficient Monte Carlo(MC) code. In principle, the MC method is also a powerful dose calculation tool in other areas in radiation oncology, e.g., brachytherapy or orthovoltage radiotherapy. However, is not validated for the low-energy range of such applications. This work aims in the validation of the MC code for photon beams in the energy range between 20 and 1000 keV.Methods:
Dose calculations were performed in different phantoms of different materials. Dose distributions of monoenergetic (ranging from 20 to 1000 keV) and parallel beams were calculated. Voxel sizes of and were used for the dose calculations. The resulting dose distributions were compared to those calculated using EGSnrc, which is used as a golden standard in this work.Results:
At energies between 100 and 1000 keV, EGSnrc and calculated dose distributions agree within the statistical uncertainty of about 1% . At energies, dose differences of up to 1.6% (in % of ) occur when and EGSnrc are compared. Turning off Rayleigh scattering, binding effects for Compton scattering, and the atomic relaxation after photoelectric absorption in EGSnrc (all not implemented in ) leads to an agreement between both MC codes within statistical uncertainty. Further, using the KERMA approximation feature implemented in leads to very efficient simulations in the energy range between 20 and 1000 keV.Conclusions:
Further improvements for very low energies in accuracy of could be achieved by implementing Rayleigh scattering, binding effects for Compton scattering, and the atomic relaxation after photoelectric absorption. Implementation into of KERMA approximation has been validated.
37(2010); http://dx.doi.org/10.1118/1.3484093View Description Hide DescriptionPurpose:
Ultraviolet phototherapy is widely used in the treatment of numerous skin conditions. This treatment is well established and largely beneficial to patients on both physical and psychological levels; however, overexposure to ultraviolet radiation (UVR) can have detrimental effects, such as erythemal responses and ocular damage in addition to the potentially carcinogenic nature of UVR. For these reasons, it is essential to control and quantify the radiationdose incident upon the patient to ensure that it is both biologically effective and has the minimal possible impact on the surrounding unaffected tissue.Methods:
To date, there has been little work on dose modeling, and the output of artificial UVR sources is an area where research has been recommended. This work characterizes these sources by formalizing an approach from first principles and experimentally examining this model.Results:
An implementation of a line source model is found to give impressive accuracy and quantifies the output radiation well.Conclusions:
This method could potentially serve as a basis for a full computational dose model for quantifying patient dose.
37(2010); http://dx.doi.org/10.1118/1.3480480View Description Hide DescriptionPurpose:
A three-parameter semiempirical model for scatter-to-primarydose ratio (SPR) is proposed to fit PDD (or TPR) and beam data. The SPR formula proposed in this study is more accurate than the previously published formula utilizing two parameters, especially for lower energy megavoltage photon beams, because the effect of backscatteredphotons is now taken into account.Methods:
Monte Carlo(MC) calculated SPR for photon energy spectrum between and 24 MV are used to evaluate the accuracy of the models. Based on fitting the MC data, the dependence of the SPR parameters with the attenuation coefficients of the photon beams is obtained and they were incorporated into the authors’ optimization routine. The ability of the optimization routine to fit measured clinic data is examined for photon energies ranging from to 25 MV for all major cobalt and linear accelerator manufacturers.Results:
The authors’ model successfully fits the measured photon beam data for field size, where is photon energy in MV and for clinically usable depths, to 20 cm for , to 30 cm for 4 MV, and to 40 cm for 6 MV and higher photon energies. The maximum error among these fits is better than 2% for photon energies above .Conclusions:
The new SPR formula, along with the optimization routine, can serve as an efficient tool for performing quality control of x-ray beam data that conforms to AAPM Radiation Therapy Committee TG40 and Therapy Physics Committee TG142 reports on beam data requirement.
Clinical development of a failure detection-based online repositioning strategy for prostate IMRT—Experiments, simulation, and dosimetry study37(2010); http://dx.doi.org/10.1118/1.3488887View Description Hide DescriptionPurpose:
To implement and evaluate clinic-ready adaptive imaging protocols for online patient repositioning (motion tracking) during prostate IMRT using treatment beam imaging supplemented by minimal, as-needed use of on-board kV.Methods:
The authors examine the two-step decision-making strategy: (1) Use cine-MV imaging and online-updated characterization of prostate motion to detect target motion that is potentially beyond a predefined threshold and (2) use paired MV-kV 3D localization to determine overthreshold displacement and, if needed, reposition the patient. Two levels of clinical implementation were evaluated: (1) Field-by-field based motion correction for present-day linacs and (2) instantaneous repositioning for new-generation linacs with capabilities of simultaneous MV-kV imaging and remote automatic couch control during treatment delivery. Experiments were performed on a Varian Trilogy linac in clinical mode using a 4D motion phantom programed with prostate motion trajectories taken from patient data. Dosimetric impact was examined using a 2D ion chamber array. Simulations were done for 536 trajectories from 17 patients.Results:
Despite the loss of marker detection efficiency caused by the MLC leaves sometimes obscuring the field at the marker’s projected position on the MV imager, the field-by-field correction halved (from 23% to 10%) the mean percentage of time that target displacement exceeded a 3 mm threshold, as compared to no intervention. This was achieved at minimal cost in additional imaging (average of one MV-kV pair per two to three treatment fractions) and with a very small number of repositionings (once every four to five fractions). Also with low kV usage , the instantaneous repositioning approach reduced overthreshold time by more than 75% (23% to 5%) even with severe MLC blockage as often encountered in current IMRT and could reduce the overthreshold time tenfold (to ) if the MLC blockage problem were relieved. The information acquired for repositioning using combined MV-kV images was found to have submillimeter accuracy.Conclusions:
This work demonstrated with a current clinical setup that substantial reduction of adverse targeting effects of intrafraction prostate motion can be realized. The proposed adaptive imaging strategy incurs minimal imagingdose to the patient as compared to other stereoscopic imaging techniques.
Dosimetric variation due to the photon beam energy in the small-animal irradiation: A Monte Carlo study37(2010); http://dx.doi.org/10.1118/1.3488979View Description Hide DescriptionPurpose:
The impact of photon beam energy and tissue heterogeneities on dose distributions and dosimetric characteristics such as point dose, mean dose, and maximum dose was investigated in the context of small-animal irradiation using Monte Carlo simulations based on the EGSnrc code.Methods:
Three Monte Carlo mouse phantoms, namely, heterogeneous, homogeneous, and bone homogeneous were generated based on the same mouse computed tomographyimage set. These phantoms were generated by overriding the tissue type of none of the voxels (heterogeneous), all voxels (homogeneous), and only the bone voxels (bone homogeneous) to that of soft tissue. Phase space files of the 100 and 225 kVp photon beams based on a small-animal irradiator (XRad225Cx, Precision X-Ray Inc., North Branford, CT) were generated using BEAMnrc. A 360° photon arc was simulated and three-dimensional (3D) dose calculations were carried out using the DOSXYZnrc code through DOSCTP in the above three phantoms. For comparison, the 3D dose distributions, dose profiles, mean, maximum, and point doses at different locations such as the isocenter, lung, rib, and spine were determined in the three phantoms.Results:
The dose gradient resulting from the 225 kVp arc was found to be steeper than for the 100 kVp arc. The mean dose was found to be 1.29 and 1.14 times higher for the heterogeneous phantom when compared to the mean dose in the homogeneous phantom using the 100 and 225 kVp photon arcs, respectively. The bone doses (rib and spine) in the heterogeneous mouse phantom were about five (100 kVp) and three (225 kVp) times higher when compared to the homogeneous phantom. However, the lungdose did not vary significantly between the heterogeneous, homogeneous, and bone homogeneous phantom for the 225 kVp compared to the 100 kVp photon beams.Conclusions:
A significant bone dose enhancement was found when the 100 and 225 kVp photon beams were used in small-animal irradiation. This dosimetric effect, due to the presence of the bone heterogeneity, was more significant than that due to the lung heterogeneity. Hence, for kV photon energies of the range used in small-animal irradiation, the increase of the mean and bone dose due to the photoelectric effect could be a dosimetric concern.
Advanced treatment planning methods for efficient radiation therapy with laser accelerated proton and ion beams37(2010); http://dx.doi.org/10.1118/1.3491406View Description Hide DescriptionPurpose:
Laser plasma acceleration can potentially replace large and expensive cyclotrons or synchrotrons for radiotherapy with protons and ions. On the way toward a clinical implementation, various challenges such as the maximum obtainable energy still remain to be solved. In any case, laser accelerated particles exhibit differences compared to particles from conventional accelerators. They typically have a wide energy spread and the beam is extremely pulsed (i.e., quantized) due to the pulsed nature of the employed lasers. The energy spread leads to depth dose curves that do not show a pristine Bragg peak but a wide high dose area, making precise radiotherapy impossible without an additional energy selection system. Problems with the beam quantization include the limited repetition rate and the number of accelerated particles per laser shot. This number might be too low, which requires a high repetition rate, or it might be too high, which requires an additional fluence selection system to reduce the number of particles. Trying to use laser accelerated particles in a conventional way such as spot scanning leads to long treatment times and a high amount of secondary radiation produced when blocking unwanted particles.Methods:
The authors present methods of beam delivery and treatment planning that are specifically adapted to laser accelerated particles. In general, it is not necessary to fully utilize the energy selection system to create monoenergetic beams for the whole treatment plan. Instead, within wide parts of the target volume, beams with broader energy spectra can be used to simultaneously cover multiple axially adjacent spots of a conventional dose delivery grid as applied in intensity modulated particle therapy. If one laser shot produces too many particles, they can be distributed over a wider area with the help of a scattering foil and a multileaf collimator to cover multiple lateral spot positions at the same time. These methods are called axial and lateral clustering and reduce the number of particles that have to be blocked in the beam delivery system. Furthermore, the optimization routine can be adjusted to reduce the number of dose spots and laser shots. The authors implemented these methods into a research treatment planning system for laser accelerated particles.Results:
The authors’ proposed methods can decrease the amount of secondary radiation produced when blocking particles with wrong energies or when reducing the total number of particles from one laser shot. Additionally, caused by the efficient use of the beam, the treatment time is reduced considerably. Both improvements can be achieved without extensively changing the quality of the treatment plan since conventional intensity modulated particle therapy usually includes a certain amount of unused degrees of freedom which can be used to adapt to laser specific properties.Conclusions:
The advanced beam delivery and treatment planning methods reduce the need to have a perfect laser-based accelerator reproducing the properties of conventional accelerators that might not be possible without increasing treatment time and secondary radiation to the patient. The authors show how some of the differences to conventional beams can be overcome and efficiently used for radiation treatment.
A support vector machine (SVM) for predicting preferred treatment position in radiotherapy of patients with breast cancer37(2010); http://dx.doi.org/10.1118/1.3483264View Description Hide DescriptionPurpose:
NYU 05-181 protocol compared the CT simulation in both supine and prone positions for 400 patients with breast cancer (200 left-breast and 200 right-breast) to identify which setup is better at sparing heart and lung involvement in the treatment process. The results demonstrated that all right-breast patients benefited from the prone treatment position, while for left-breast patients, 85% were better treated prone and 15% were better treated supine. Using the clinical data collected from this protocol, the authors aimed at developing an automated tool capable of identifying which of the left-breast cancer patients are better treated supine without obtaining a second CT scan in the supine position.Methods:
Prone CT scans from 198 of the 200 left-breast cancer patients enrolled in NYU 05-181 protocol were deidentified and exported to a dedicated research planning workstation. Three-dimensional geometric features of the organs at risk and tumor bed were extracted. A two-stage classifier was used to classify patients into the prone class or the supine class. In the first stage, the authors use simple thresholding to divide the patients into two groups based on their in-field heart volume. For patients with in-field heart volume, the prone position was chosen as the preferred treatment position. Patients with in-field heart volume will be further classified in the second stage by a weighted support vector machine (SVM). The weight parameters of the SVM were adjusted to maximize the specificity at the cost of lowering but still maintaining reasonable sensitivity . The authors used -fold cross validations to test the performance of the SVM classifier. A feature selection algorithm was also used to identify features that give the best classification performance.Results:
After the first stage, 49 of the 198 left-breast cancer patients were found to have of in-field heart volume. The three geometric features of heart orientation, distance between heart and tumor, and in-field lung were selected by the feature selection algorithm in the second stage of the two-stage classifier to give the best predefined weighted accuracy. The overall sensitivity and specificity of the proposed method were found to be 90.4% and 99.3%, respectively. Using two-stage classification, the authors reduced the proportion of prone-treated patients that need a second supine CT scan down to 16.3/170 or 9.6%, as compared to 21/170 or 12.4% when the authors use only the first stage (thresholding) for classification.Conclusions:
The authors’ study showed that a feature-based classifier is feasible for predicting the preferred treatment position, based on features extracted from prone CT scans. The two-stage classifier achieved very high specificity at an acceptable expense of sensitivity.
37(2010); http://dx.doi.org/10.1118/1.3488971View Description Hide DescriptionPurpose:
The formalism recommended by Task Group 60 (TG-60) of the American Association of Physicists in Medicine (AAPM) is applicable for sources. Radioactive biocompatible and biodegradable glass seed without encapsulation is a emitter radionuclide with a short half-life and delivers a high dose rate to the tumor in the millimeter range. This study presents the results of Monte Carlo calculations of the dosimetric parameters for the brachytherapy source.Methods:
Version 5 of the (MCNP) Monte Carloradiation transport code was used to calculate two-dimensional dose distributions around the source. The dosimetric parameters of AAPM TG-60 recommendations including the reference dose rate, the radial dose function, the anisotropy function, and the one-dimensional anisotropy function were obtained.Results:
The dose rate value at the reference point was estimated to be. Due to the low energy beta emitted from sources, the dose fall-off profile is sharper than the other beta emitter sources. The calculated dosimetric parameters in this study are compared to several beta and photon emitting seeds.Conclusions:
The results show the advantage of the source in comparison with the other sources because of the rapid dose fall-off of beta ray and high dose rate at the short distances of the seed. The results would be helpful in the development of the radioactive implants using seeds for the brachytherapy treatment.
Apparent absence of a proton beam dose rate effect and possible differences in RBE between Bragg peak and plateau37(2010); http://dx.doi.org/10.1118/1.3490086View Description Hide DescriptionPurpose:
Respiration-gated irradiation for a moving target requires a longer time to deliver single fraction in protonradiotherapy (PRT). Ultrahigh dose rate (UDR) proton beam, which is 10–100 times higher than that is used in current clinical practice, has been investigated to deliver daily dose in single breath hold duration. The purpose of this study is to investigate the survival curve and relative biological effectiveness (RBE) of such an ultrahigh dose rate proton beam and their linear energy transfer (LET) dependence.Methods:
HSG cells were irradiated by a spatially and temporally uniform proton beam at two different dose rates: 8 Gy/min (CDR, clinical dose rate) and 325 Gy/min (UDR, ultrahigh dose rate) at the Bragg peak and 1.75 (CDR) and 114 Gy/min (UDR) at the plateau. To study LET dependence, the cells were positioned at the Bragg peak, where the absorbed dose-averaged LET was, and at the plateau, where it was . After the cell exposure and colony assay, the measured data were fitted by the linear quadratic (LQ) model and the survival curves and RBE at 10% survival were compared.Results:
No significant difference was observed in the survival curves between the two protondose rates. The ratio of the RBE for CDR/UDR was at the Bragg peak and at the plateau. On the other hand, Bragg peak/plateau RBE ratio was for UDR and for CDR.Conclusions:
Present RBE can be consistently used in treatment planning of PRT using ultrahigh dose rate radiation. Because a significant increase in RBE toward the Bragg peak was observed for both UDR and CDR, further evaluation of RBE enhancement toward the Bragg peak and beyond is required.
37(2010); http://dx.doi.org/10.1118/1.3483260View Description Hide DescriptionPurpose:
Applications of cone-beam CT(CBCT) to image-guided radiation therapy (IGRT) are hampered by shading artifacts in the reconstructed images. These artifacts are mainly due to scatter contamination in the projections but also can result from uncorrected beam hardening effects as well as nonlinearities in responses of the amorphous silicon flat panel detectors. While currently, CBCT is mainly used to provide patient geometry information for treatment setup, more demanding applications requiring high-quality CBCTimages are under investigation. To tackle these challenges, many CBCT correction algorithms have been proposed; yet, a standard approach still remains unclear. In this work, we propose a shading correction method for CBCT that addresses artifacts from low-frequency projection errors. The method is consistent with the current workflow of radiation therapy.Methods:
With much smaller inherent scatter signals and more accurate detectors, diagnostic multidetector CT (MDCT) provides high quality CTimages that are routinely used for radiation treatment planning. Using the MDCT image as “free” prior information, we first estimate the primary projections in the CBCT scan via forward projection of the spatially registered MDCT data. Since most of the CBCT shading artifacts stem from low-frequency errors in the projections such as scatter, these errors can be accurately estimated by low-pass filtering the difference between the estimated and raw CBCT projections. The error estimates are then subtracted from the raw CBCT projections. Our method is distinct from other published correction methods that use the MDCT image as a prior because it is projection-based and uses limited patient anatomical information from the MDCT image. The merit of CBCT-based treatment monitoring is therefore retained.Results:
The proposed method is evaluated using two phantom studies on tabletop systems. On the Catphan©600 phantom, our approach reduces the reconstruction error from 348 Hounsfield unit (HU) without correction to 4 HU around the object center after correction, and from 375 HU to 17 HU in the high-contrast regions. In the selected regions of interest (ROIs), the average image contrast is increased by a factor of 3.3. When noise suppression is implemented, the proposed correction substantially improves the contrast-to-noise ratio(CNR) and therefore the visibility of low-contrast objects, as seen in a more challenging pelvis phantom study. Besides a significant improvement in image uniformity, a low-contrast object of, which is otherwise buried in the shading artifacts, can be clearly identified after the proposed correction due to a CNR increase of 3.1. Compared to a kernel-based scatter correction method coupled with an analytical beam hardening correction, our approach also shows an overall improved performance with some residual artifacts.Conclusions:
By providing effective shading correction, our approach has the potential to improve the accuracy of more advanced CBCT-based clinical applications for IGRT, such as tumor delineation and dose calculation.
37(2010); http://dx.doi.org/10.1118/1.3483783View Description Hide Description
Purpose: The system is an image-guided stereotactic positioning system (IGSPS) that provides real-time target localization. This study involves the first use of this system with three camera pods. The authors have evaluated its localization accuracy and tracking ability using a cone-beam computed tomography(CBCT) system and an optical tracking system in a clinical setting.
Methods: A modified Rando head-and-neck phantom and five patients receiving intracranial stereotactic radiotherapy (SRT) were used to evaluate the calibration, registration, and position-tracking accuracies of the system and to study surface reconstruction uncertainties, including the effects due to interfractional and intrafractional motion, skin tone, room light level, camera temperature, and image registration region of interest selection. System accuracy was validated through comparison with the Elekta kV CBCT system (XVI) and the Varian frameless SonArray (FSA) optical tracking system. Surface-image data sets were acquired with the daily for the evaluation of pretreatment and interfractional and intrafractional motion for each patient. Results for two different reference image sets, planning CT surface contours (CT_S) and previously recorded optical surface images (ART_S), are reported.
Results: The system origin displacements for the and XVI systems agreed to within 1.3 mm and 0.7°. Similar results were seen for vs FSA. For the phantom displacements having couch angles of 0°, those that utilized ART_S references resulted in a mean difference of 0.9 mm/0.4° with respect to XVI and 0.3 mm/0.2° with respect to FSA. For phantom displacements of more than ±10 mm and ±3°, the maximum discrepancies between AlignRT and the XVI and FSA systems were 3.0 and 0.4 mm, respectively. For couch angles up to ±90°, the mean (max.) difference between the and FSA was 1.2 (2.3) mm/0.7° (1.2°). For all tests, the mean registration errors obtained using the CT_S references were approximately 1.3 mm/1.0° larger than those obtained using the ART_S references. For the patient study, the mean differences in the pretreatment displacements were 0.3 mm/0.2° between the and XVI systems and 1.3 mm/1° between the FSA and XVI systems. For noncoplanar treatments, interfractional motion displacements obtained using the ART_S and CT_S references resulted in 90th percentile differences within 2.1 mm/0.8° and 3.3 mm/0.3°, respectively, compared to the FSA system. Intrafractional displacements that were tracked for a maximum of 14 min were within 1 mm/1° of those obtained with the FSA system. Uncertainties introduced by the bite-tray were as high as 3 mm/2° for one patient. The combination of gantry, aSi detector panel, and x-ray tube blockage effects during the CBCT acquisition resulted in a registration error of approximately 3 mm. No skin-tone or surface deformation effects were seen with the limited patient sample.
Conclusions: can be used as a nonionizing IGSPS with accuracy comparable to current image/marker-based systems. IGSPS and CBCT can be combined for high-precision positioning without the need for patient-attached localization devices.
37(2010); http://dx.doi.org/10.1118/1.3488900View Description Hide DescriptionPurpose:
Because the small-radius photon beams shaped by cones in stereotactic radiosurgery(SRS) lack lateral electronic equilibrium and a detector’s finite cross section, direct experimental measurement of dosimetric data for these beams can be subject to large uncertainties. As the dose calculation accuracy of a treatment planning system largely depends on how well the dosimetric data are measured during the machine’s commissioning, there is a critical need for an independent method to validate measured results. Therefore, the authors studied the model-based calculation as an approach to validate measured off-axis ratios (OARs).Methods:
The authors previously used a two-component analytical model to calculate central axis dose and associated dosimetric data (e.g., scatter factors and tissue-maximum ratio) in a water phantom and found excellent agreement between the calculated and the measured central axis doses for small 6 MV SRS conic beams. The model was based on that ofNizin and Mooij [“An approximation of central-axis absorbed dose in narrow photon beams,” Med. Phys.24, 1775–1780 (1997)] but was extended to account for apparent attenuation, spectral differences between broad and narrow beams, and the need for stricter scatter dose calculations for clinical beams. In this study, the authors applied Clarkson integration to this model to calculate OARs for conic beams. OARs were calculated for selected cones with radii from 0.2 to 1.0 cm. To allow comparisons, the authors also directly measured OARs using stereotactic diode (SFD), microchamber, and film dosimetry techniques. The calculated results were machine-specific and independent of direct measurement data for these beams.Results:
For these conic beams, the calculated OARs were in excellent agreement with the data measured using an SFD. The discrepancies in radii and in 80%–20% penumbra were within 0.01 cm, respectively. Using SFD-measured OARs as the reference data, the authors found that the calculated OARs were more accurate than those measured with a microchamber or film dosimetry.Conclusions:
The model produces sufficiently accurate conic beam dosimetric data that can be used to validate direct measurement results for such beams.
37(2010); http://dx.doi.org/10.1118/1.3488978View Description Hide DescriptionPurpose:
To investigate the efficacy of two widely used scatter mitigation methods: antiscatter grids (ASGs) and beam modulating with bowtie filters (BTFs), in combination with subtractive scatter correction or zeroth order normalization phantom calibration, for improving imagenoise,contrast,contrast-to-noise ratio(CNR), and image uniformity for on-board cone-beam CT(CBCT)imaging systems used for image-guided radiation therapy.Methods:
PTRAN Monte CarloCBCT x-ray projections of head and pelvic phantoms were calculated for combinations of beam-modulation and scatter rejection methods and images were reconstructed by in-house developed software. In addition, a simple one-dimensional analytic model was developed to predict scatter-to-primary ratio (SPR) and CNR as a function of cylindrical phantom thickness, ASG transmission, and beam modulation with bow-tie filters.Results:
ASGs were found to have slightly negative or no effect on head phantom imageCNR and to modestly improve CNR (10%–20%) in pelvic phantom images. However, scatter subtraction and norm-phantom calibration perform better when applied on data acquired with ASGs. Scatter subtraction improves CT number accuracy, but increases noise, and in high SPR/low primary-photon transmission scenarios can dramatically reduce CNR and introduce streaking artifacts. The BTF is found to reduce SPR and imagenoise, resulting in a better trade-off between CNR and imaging dose, but introduces a circular band artifact.Conclusions:
Our study shows that ASGs have a modest positive impact in pelvic scans and negative in head scans, scatter subtraction improves the HU accuracy but reduces CNR, while BTF has a clearly positive effect.
Measuring output factors of small fields formed by collimator jaws and multileaf collimator using plastic scintillation detectors37(2010); http://dx.doi.org/10.1118/1.3488981View Description Hide DescriptionPurpose:
As the practice of using high-energy photon beams to create therapeutic radiation fields of subcentimeter dimensions (as in intensity-modulated radiotherapy or stereotactic radiosurgery) grows, so too does the need for accurate verification of beam output at these small fields in which standard practices of dose verification break down. This study investigates small-field output factors measured using a small plastic scintillation detector(PSD), as well as aionization chamber. Specifically, output factors were measured with both detectors using small fields that were defined by either the X-Y collimator jaws or the multileaf collimator(MLC).Methods:
A PSD of 0.5 mm diameter and 2 mm length was irradiated with 6 and 18 MV linac beams. The PSD was positioned vertically at a source-to-axis distance of 100 cm, at 10 cm depth in a water phantom, and irradiated with fields ranging in size from to . The field sizes were defined either by the collimator jaws alone or by a MLC alone. The MLC fields were constructed in two ways: with the closed leaves (i.e., those leaves that were not opened to define the square field) meeting at either the field center line or at a 4 cm offset from the center line. Scintillation light was recorded using a CCD camera and an estimation of error in the median-filtered signals was made using the bootstrapping technique. Measurements were made using a CC01 ionization chamber under conditions identical to those used for the PSD.Results:
Output factors measured by the PSD showed close agreement with those measured using the ionization chamber for field sizes of and above. At smaller field sizes, the PSD obtained output factors as much as 15% higher than those found using the ionization chamber by jaw-defined fields. Output factors measured with no offset of the closed MLC leaves were as much as 20% higher than those measured using a 4 cm leaf offset.Conclusions:
The authors’ results suggest that PSDs provide a useful and possibly superior alternative to existing dosimetry systems for small fields, as they are inherently less susceptible to volume-averaging and perturbation effects than larger, air-filled ionization chambers. Therefore, PSDs may provide more accurate small-field output factor determination, regardless of the collimation mechanism.
- RADIATION IMAGING PHYSICS
Metal artifact suppression from reformatted projections in multislice helical CT using dual-front active contours37(2010); http://dx.doi.org/10.1118/1.3462814View Description Hide DescriptionPurpose:
Large metallic implants, such as hip prosthesis and shoulder implants, can cause severe artifacts in CT exams. As such, there have been significant efforts on the design of various image- or projection-based correction methods or iterative reconstruction methods with the hope to reconstruct artifact-free images. Unfortunately, suppression of metal artifacts remains a very challenging problem, in which metal region segmentation is one of the most important steps in assuring the efficiency of artifact suppression. In this article, the authors propose a novel, semiautomatic metal region segmentation algorithm based on a dual-front active contour model and a boundary mapping strategy to detect multiple large metal implants on reformatted projection data and to effectively suppress or eliminate metal artifacts on reconstructed images.Methods:
First, the projections created from helical scan data were reformatted by combining data at the same view angle over the full longitudinal scan range. In this way, the shape, location, and number of the metal structures show up clearly on each reformatted projection, changing only slightly between adjacent projections. Second, an initial boundary on one of the reformatted projections is defined, and a boundary mapping strategy was utilized to map the metal boundary on the first reformatted projection to the next adjacent projection. Third, a novel dual-front active contour model was used to evolve the mapped boundary from the prior projection to the actual boundary in the current projection. By iteratively performing the boundary mapping and boundary evolution procedure, the metal structures (one or multiple) on all the projections can be extracted efficiently and accurately. Finally, a Delaunay triangulation was applied to fill the metal shadows and the corrected projection data were reconstructed with a commercially available algorithm.Results:
Experimental studies on clinical hip and shoulder CT exams and a comparison with a gradient-based threshold method were performed. The results demonstrated that the proposed segmentation strategy was able to segment multiple metal implants more accurately than the threshold method. Soft-tissue visibility was improved dramatically.Conclusions:
In total, the artifacts caused by dense metal implants were suppressed dramatically with the proposed metal artifact suppression technique.
37(2010); http://dx.doi.org/10.1118/1.3477088View Description Hide DescriptionPurpose:
Due to x-ray beam polychromaticity and scattered radiation, attenuation measurements tend to be underestimated. Cupping and beam hardening artifacts become apparent in the reconstructedCTimages. If only one material such as water, for example, is present, these artifacts can be reduced by precorrecting the rawdata. Higher order beam hardening artifacts, as they result when a mixture of materials such as water and bone, or water and bone and iodine is present, require an iterative beam hardening correction where the image is segmented into different materials and those are forward projected to obtain new rawdata. Typically, the forward projection must correctly model the beam polychromaticity and account for all physical effects, including the energy dependence of the assumed materials in the patient, the detector response, and others. We propose a new algorithm that does not require any knowledge about spectra or attenuation coefficients and that does not need to be calibrated. The proposed method corrects beam hardening in single energy CT data.Methods:
The onlya priori knowledge entering EBHC is the segmentation of the object into different materials. Materials other than water are segmented from the original image, e.g., by using simple thresholding. Then, a (monochromatic) forward projection of these other materials is performed. The measured rawdata and the forward projected material-specific rawdata are monomially combined (e.g., multiplied or squared) and reconstructed to yield a set of correction volumes. These are then linearly combined and added to the original volume. The combination weights are determined to maximize the flatness of the new and corrected volume. EBHC is evaluated using data acquired with a modern cone-beam dual-source spiral CTscanner (Somatom Definition Flash, Siemens Healthcare, Forchheim, Germany), with a modern dual-source micro-CT scanner (TomoScope Synergy Twin, CTImaging GmbH, Erlangen, Germany), and with a modern C-arm CTscanner (Axiom Artis dTA, Siemens Healthcare, Forchheim, Germany). A large variety of phantom, small animal, and patient data were used to demonstrate the data and system independence of EBHC.Results:
Although no physics apart from the initial segmentation procedure enter the correction process, beam hardening artifacts were significantly reduced by EBHC. The image quality for clinical CT, micro-CT, and C-arm CT was highly improved. Only in the case of C-arm CT, where high scatter levels and calibration errors occur, the relative improvement was smaller.Conclusions:
The empirical beam hardening correction is an interesting alternative to conventional iterative higher order beam hardening correction algorithms. It does not tend to over- or undercorrect the data. Apart from the segmentation step, EBHC does not require assumptions on the spectra or on the type of material involved. Potentially, it can therefore be applied to any CTimage.