Volume 42, Issue 5, May 2015
Index of content:
42(2015); http://dx.doi.org/10.1118/1.4903902View Description Hide Description
- RADIATION THERAPY PHYSICS
42(2015); http://dx.doi.org/10.1118/1.4916661View Description Hide DescriptionPurpose:
This paper investigates, via magnetic modeling and Monte Carlo simulation, the ability to deliver proton beams to the treatment zone inside a split-bore MRI-guided proton therapy system.Methods:
Field maps from a split-bore 1 T MRI-Linac system are used as input to geant4 Monte Carlo simulations which model the trajectory of proton beams during their paths to the isocenter of the treatment area. Both inline (along the MRI bore) and perpendicular (through the split-bore gap) orientations are simulated. Monoenergetic parallel and diverging beams of energy 90, 195, and 300 MeV starting from 1.5 and 5 m above isocenter are modeled. A phase space file detailing a 2D calibration pattern is used to set the particle starting positions, and their spatial location as they cross isocenter is recorded. No beam scattering, collimation, or modulation of the proton beams is modeled.Results:
In the inline orientation, the radial symmetry of the solenoidal style fringe field acts to rotate the protons around the beam’s central axis. For protons starting at 1.5 m from isocenter, this rotation is 19° (90 MeV) and 9.8° (300 MeV). A minor focusing toward the beam’s central axis is also seen, but only significant, i.e., 2 mm shift at 150 mm off-axis, for 90 MeV protons. For the perpendicular orientation, the main MRI field and near fringe field act as the strongest to deflect the protons in a consistent direction. When starting from 1.5 m above isocenter shifts of 135 mm (90 MeV) and 65 mm (300 MeV) were observed. Further to this, off-axis protons are slightly deflected toward or away from the central axis in the direction perpendicular to the main deflection direction. This leads to a distortion of the phase space pattern, not just a shift. This distortion increases from zero at the central axis to 10 mm (90 MeV) and 5 mm (300 MeV) for a proton 150 mm off-axis. In both orientations, there is a small but subtle difference in the deflection and distortion pattern between protons fired parallel to the beam axis and those fired from a point source. This is indicative of the 3D spatially variant nature of the MRI fringe field.Conclusions:
For the first time, accurate magnetic and Monte Carlo modeling have been used to assess the transport of generic proton beams toward a 1 T split-bore MRI. Significant rotation is observed in the inline orientation, while more complex deflection and distortion are seen in the perpendicular orientation. The results of this study suggest that due to the complexity and energy-dependent nature of the magnetic deflection and distortion, the pencil beam scanning method will be the only choice for delivering a therapeutic proton beam inside a potential MRI-guided proton therapy system in either the inline or perpendicular orientation. Further to this, significant correction strategies will be required to account for the MRI fringe fields.
Optimization of leaf margins for lung stereotactic body radiotherapy using a flattening filter-free beam42(2015); http://dx.doi.org/10.1118/1.4916683View Description Hide DescriptionPurpose:
The authors sought to determine the optimal collimator leaf margins which minimize normal tissue dose while achieving high conformity and to evaluate differences between the use of a flattening filter-free (FFF) beam and a flattening-filtered (FF) beam.Methods:
Sixteen lung cancer patients scheduled for stereotactic body radiotherapy underwent treatment planning for a 7 MV FFF and a 6 MV FF beams to the planning target volume (PTV) with a range of leaf margins (−3 to 3 mm). Forty grays per four fractions were prescribed as a PTV D95. For PTV, the heterogeneity index (HI), conformity index, modified gradient index (GI), defined as the 50% isodose volume divided by target volume, maximum dose (Dmax), and mean dose (Dmean) were calculated. Mean lung dose (MLD), V20 Gy, and V5 Gy for the lung (defined as the volumes of lung receiving at least 20 and 5 Gy), mean heart dose, and Dmax to the spinal cord were measured as doses to organs at risk (OARs). Paired t-tests were used for statistical analysis.Results:
HI was inversely related to changes in leaf margin. Conformity index and modified GI initially decreased as leaf margin width increased. After reaching a minimum, the two values then increased as leaf margin increased (“V” shape). The optimal leaf margins for conformity index and modified GI were −1.1 ± 0.3 mm (mean ± 1 SD) and −0.2 ± 0.9 mm, respectively, for 7 MV FFF compared to −1.0 ± 0.4 and −0.3 ± 0.9 mm, respectively, for 6 MV FF. Dmax and Dmean for 7 MV FFF were higher than those for 6 MV FF by 3.6% and 1.7%, respectively. There was a positive correlation between the ratios of HI, Dmax, and Dmean for 7 MV FFF to those for 6 MV FF and PTV size (R = 0.767, 0.809, and 0.643, respectively). The differences in MLD, V20 Gy, and V5 Gy for lung between FFF and FF beams were negligible. The optimal leaf margins for MLD, V20 Gy, and V5 Gy for lung were −0.9 ± 0.6, −1.1 ± 0.8, and −2.1 ± 1.2 mm, respectively, for 7 MV FFF compared to −0.9 ± 0.6, −1.1 ± 0.8, and −2.2 ± 1.3 mm, respectively, for 6 MV FF. With the heart inside the radiation field, the mean heart dose showed a V-shaped relationship with leaf margins. The optimal leaf margins were −1.0 ± 0.6 mm for both beams. Dmax to the spinal cord showed no clear trend for changes in leaf margin.Conclusions:
The differences in doses to OARs between FFF and FF beams were negligible. Conformity index, modified GI, MLD, lung V20 Gy, lung V5 Gy, and mean heart dose showed a V-shaped relationship with leaf margins. There were no significant differences in optimal leaf margins to minimize these parameters between both FFF and FF beams. The authors’ results suggest that a leaf margin of −1 mm achieves high conformity and minimizes doses to OARs for both FFF and FF beams.
42(2015); http://dx.doi.org/10.1118/1.4916685View Description Hide DescriptionPurpose:
During the first part of the 20th century, 226Ra was the most used radionuclide for brachytherapy. Retrospective accurate dosimetry, coupled with patient follow up, is important for advancing knowledge on long-term radiation effects. The purpose of this work was to dosimetrically characterize two 226Ra sources, commonly used in Sweden during the first half of the 20th century, for retrospective dose–effect studies.Methods:
An 8 mg 226Ra tube and a 10 mg 226Ra needle, used at Radiumhemmet (Karolinska University Hospital, Stockholm, Sweden), from 1925 to the 1960s, were modeled in two independent Monte Carlo (MC) radiation transport codes: geant4 and mcnp5. Absorbed dose and collision kerma around the two sources were obtained, from which the TG-43 parameters were derived for the secular equilibrium state. Furthermore, results from this dosimetric formalism were compared with results from a MC simulation with a superficial mould constituted by five needles inside a glass casing, placed over a water phantom, trying to mimic a typical clinical setup. Calculated absorbed doses using the TG-43 formalism were also compared with previously reported measurements and calculations based on the Sievert integral. Finally, the dose rate at large distances from a 226Ra point-like-source placed in the center of 1 m radius water sphere was calculated with geant4.Results:
TG-43 parameters [including gL (r), F(r, θ), Λ, and sK ] have been uploaded in spreadsheets as additional material, and the fitting parameters of a mathematical curve that provides the dose rate between 10 and 60 cm from the source have been provided. Results from TG-43 formalism are consistent within the treatment volume with those of a MC simulation of a typical clinical scenario. Comparisons with reported measurements made with thermoluminescent dosimeters show differences up to 13% along the transverse axis of the radium needle. It has been estimated that the uncertainty associated to the absorbed dose within the treatment volume is 10%–15%, whereas uncertainty of absorbed dose to distant organs is roughly 20%–25%.Conclusions:
The results provided here facilitate retrospective dosimetry studies of 226Ra using modern treatment planning systems, which may be used to improve knowledge on long term radiation effects. It is surely important for the epidemiologic studies to be aware of the estimated uncertainty provided here before extracting their conclusions.
42(2015); http://dx.doi.org/10.1118/1.4914863View Description Hide DescriptionPurpose:
The authors investigated the potential of optimized noncoplanar irradiation trajectories for volumetric modulated arc therapy (VMAT) treatments of nasopharyngeal patients and studied the trade-off between treatment plan quality and delivery time in radiation therapy.Methods:
For three nasopharyngeal patients, the authors generated treatment plans for nine different delivery scenarios using dedicated optimization methods. They compared these scenarios according to dose characteristics, number of beam directions, and estimated delivery times. In particular, the authors generated the following treatment plans: (1) a 4π plan, which is a not sequenced, fluence optimized plan that uses beam directions from approximately 1400 noncoplanar directions and marks a theoretical upper limit of the treatment plan quality, (2) a coplanar 2π plan with 72 coplanar beam directions as pendant to the noncoplanar 4π plan, (3) a coplanar VMAT plan, (4) a coplanar step and shoot (SnS) plan, (5) a beam angle optimized (BAO) coplanar SnS IMRT plan, (6) a noncoplanar BAO SnS plan, (7) a VMAT plan with rotated treatment couch, (8) a noncoplanar VMAT plan with an optimized great circle around the patient, and (9) a noncoplanar BAO VMAT plan with an arbitrary trajectory around the patient.Results:
VMAT using optimized noncoplanar irradiation trajectories reduced the mean and maximum doses in organs at risk compared to coplanar VMAT plans by 19% on average while the target coverage remains constant. A coplanar BAO SnS plan was superior to coplanar SnS or VMAT; however, noncoplanar plans like a noncoplanar BAO SnS plan or noncoplanar VMAT yielded a better plan quality than the best coplanar 2π plan. The treatment plan quality of VMAT plans depended on the length of the trajectory. The delivery times of noncoplanar VMAT plans were estimated to be 6.5 min in average; 1.6 min longer than a coplanar plan but on average 2.8 min faster than a noncoplanar SnS plan with comparable treatment plan quality.Conclusions:
The authors’ study reconfirms the dosimetric benefits of noncoplanar irradiation of nasopharyngeal tumors. Both SnS using optimized noncoplanar beam ensembles and VMAT using an optimized, arbitrary, noncoplanar trajectory enabled dose reductions in organs at risk compared to coplanar SnS and VMAT. Using great circles or simple couch rotations to implement noncoplanar VMAT, however, was not sufficient to yield meaningful improvements in treatment plan quality. The authors estimate that noncoplanar VMAT using arbitrary optimized irradiation trajectories comes at an increased delivery time compared to coplanar VMAT yet at a decreased delivery time compared to noncoplanar SnS IMRT.
Optical eye tracking system for real-time noninvasive tumor localization in external beam radiotherapy42(2015); http://dx.doi.org/10.1118/1.4915921View Description Hide DescriptionPurpose:
External beam radiotherapy currently represents an important therapeutic strategy for the treatment of intraocular tumors. Accurate target localization and efficient compensation of involuntary eye movements are crucial to avoid deviations in dose distribution with respect to the treatment plan. This paper describes an eye tracking system (ETS) based on noninvasive infrared video imaging. The system was designed for capturing the tridimensional (3D) ocular motion and provides an on-line estimation of intraocular lesions position based on a priori knowledge coming from volumetric imaging.Methods:
Eye tracking is performed by localizing cornea and pupil centers on stereo images captured by two calibrated video cameras, exploiting eye reflections produced by infrared illumination. Additionally, torsional eye movements are detected by template matching in the iris region of eye images. This information allows estimating the 3D position and orientation of the eye by means of an eye local reference system. By combining ETS measurements with volumetric imaging for treatment planning [computed tomography (CT) and magnetic resonance (MR)], one is able to map the position of the lesion to be treated in local eye coordinates, thus enabling real-time tumor referencing during treatment setup and irradiation. Experimental tests on an eye phantom and seven healthy subjects were performed to assess ETS tracking accuracy.Results:
Measurements on phantom showed an overall median accuracy within 0.16 mm and 0.40° for translations and rotations, respectively. Torsional movements were affected by 0.28° median uncertainty. On healthy subjects, the gaze direction error ranged between 0.19° and 0.82° at a median working distance of 29 cm. The median processing time of the eye tracking algorithm was 18.60 ms, thus allowing eye monitoring up to 50 Hz.Conclusions:
A noninvasive ETS prototype was designed to perform real-time target localization and eye movement monitoring during ocular radiotherapy treatments. The device aims at improving state-of-the-art invasive procedures based on surgical implantation of radiopaque clips and repeated acquisition of X-ray images, with expected positive effects on treatment quality and patient outcome.
42(2015); http://dx.doi.org/10.1118/1.4916092View Description Hide DescriptionPurpose:
In left-sided tangential breast intensity modulated radiation therapy (IMRT), the heart may enter the radiation field and receive excessive radiation while the patient is breathing. The patient’s breathing pattern is often irregular and unpredictable. We verify the clinical applicability of a heart-sparing robust optimization approach for breast IMRT. We compare robust optimized plans with clinical plans at free-breathing and clinical plans at deep inspiration breath-hold (DIBH) using active breathing control (ABC).Methods:
Eight patients were included in the study with each patient simulated using 4D-CT. The 4D-CT image acquisition generated ten breathing phase datasets. An average scan was constructed using all the phase datasets. Two of the eight patients were also imaged at breath-hold using ABC. The 4D-CT datasets were used to calculate the accumulated dose for robust optimized and clinical plans based on deformable registration. We generated a set of simulated breathing probability mass functions, which represent the fraction of time patients spend in different breathing phases. The robust optimization method was applied to each patient using a set of dose-influence matrices extracted from the 4D-CT data and a model of the breathing motion uncertainty. The goal of the optimization models was to minimize the dose to the heart while ensuring dose constraints on the target were achieved under breathing motion uncertainty.Results:
Robust optimized plans were improved or equivalent to the clinical plans in terms of heart sparing for all patients studied. The robust method reduced the accumulated heart dose (D10cc) by up to 801 cGy compared to the clinical method while also improving the coverage of the accumulated whole breast target volume. On average, the robust method reduced the heart dose (D10cc) by 364 cGy and improved the optBreast dose (D99%) by 477 cGy. In addition, the robust method had smaller deviations from the planned dose to the accumulated dose. The deviation of the accumulated dose from the planned dose for the optBreast (D99%) was 12 cGy for robust versus 445 cGy for clinical. The deviation for the heart (D10cc) was 41 cGy for robust and 320 cGy for clinical.Conclusions:
The robust optimization approach can reduce heart dose compared to the clinical method at free-breathing and can potentially reduce the need for breath-hold techniques.
Ultrashort echo-time MRI versus CT for skull aberration correction in MR-guided transcranial focused ultrasound: In vitro comparison on human calvaria42(2015); http://dx.doi.org/10.1118/1.4916656View Description Hide DescriptionPurpose:
Transcranial magnetic resonance-guided focused ultrasound (TcMRgFUS) brain treatment systems compensate for skull-induced beam aberrations by adjusting the phase and amplitude of individual ultrasound transducer elements. These corrections are currently calculated based on a preacquired computed tomography (CT) scan of the patient’s head. The purpose of the work presented here is to demonstrate the feasibility of using ultrashort echo-time magnetic resonance imaging (UTE MRI) instead of CT to calculate and apply aberration corrections on a clinical TcMRgFUS system.Methods:
Phantom experiments were performed in three ex-vivo human skulls filled with tissue-mimicking hydrogel. Each skull phantom was imaged with both CT and UTE MRI. The MR images were then segmented into “skull” and “not-skull” pixels using a computationally efficient, threshold-based algorithm, and the resulting 3D binary skull map was converted into a series of 2D virtual CT images. Each skull was mounted in the head transducer of a clinical TcMRgFUS system (ExAblate Neuro, Insightec, Israel), and transcranial sonications were performed using a power setting of approximately 750 acoustic watts at several different target locations within the electronic steering range of the transducer. Each target location was sonicated three times: once using aberration corrections calculated from the actual CT scan, once using corrections calculated from the MRI-derived virtual CT scan, and once without applying any aberration correction. MR thermometry was performed in conjunction with each 10-s sonication, and the highest single-pixel temperature rise and surrounding-pixel mean were recorded for each sonication.Results:
The measured temperature rises were ∼45% larger for aberration-corrected sonications than for noncorrected sonications. This improvement was highly significant (p < 10−4). The difference between the single-pixel peak temperature rise and the surrounding-pixel mean, which reflects the sharpness of the thermal focus, was also significantly larger for aberration-corrected sonications. There was no significant difference between the sonication results achieved using CT-based and MR-based aberration correction.Conclusions:
The authors have demonstrated that transcranial focal heating can be significantly improved in vitro by using UTE MRI to compute skull-induced ultrasound aberration corrections. Their results suggest that UTE MRI could be used instead of CT to implement such corrections on current 0.7 MHz clinical TcMRgFUS devices. The MR image acquisition and segmentation procedure demonstrated here would add less than 15 min to a clinical MRgFUS treatment session.
42(2015); http://dx.doi.org/10.1118/1.4916684View Description Hide DescriptionPurpose:
Nonuniform spatiotemporal radiotherapy fractionation schemes, i.e., delivering distinct dose distributions in different fractions can potentially improve the therapeutic ratio. This is possible if the dose distributions are designed such that similar doses are delivered to normal tissues (exploit the fractionation effect) while hypofractionating subregions of the tumor. In this paper, the authors develop methodology for treatment planning with nonuniform fractions and demonstrate this concept in the context of intensity-modulated proton therapy (IMPT).Methods:
Treatment planning is performed by simultaneously optimizing (possibly distinct) IMPT dose distributions for multiple fractions. This is achieved using objective and constraint functions evaluated for the cumulative biologically equivalent dose (BED) delivered at the end of treatment. BED based treatment planning formulations lead to nonconvex optimization problems, such that local gradient based algorithms require adequate starting positions to find good local optima. To that end, the authors develop a combinatorial algorithm to initialize the pencil beam intensities.Results:
The concept of nonuniform spatiotemporal fractionation schemes is demonstrated for a spinal metastasis patient treated in two fractions using stereotactic body radiation therapy. The patient is treated with posterior oblique beams with the kidneys being located in the entrance region of the beam. It is shown that a nonuniform fractionation scheme that hypofractionates the central part of the tumor allows for a skin and kidney BED reduction of approximately 10%–20%.Conclusions:
Nonuniform spatiotemporal fractionation schemes represent a novel approach to exploit fractionation effects that deserves further exploration for selected disease sites.
Neural-network based autocontouring algorithm for intrafractional lung-tumor tracking using Linac-MR42(2015); http://dx.doi.org/10.1118/1.4916657View Description Hide DescriptionPurpose:
To develop a neural-network based autocontouring algorithm for intrafractional lung-tumor tracking using Linac-MR and evaluate its performance with phantom and in-vivo MR images.Methods:
An autocontouring algorithm was developed to determine both the shape and position of a lung tumor from each intrafractional MR image. A pulse-coupled neural network was implemented in the algorithm for contrast improvement of the tumor region. Prior to treatment, to initiate the algorithm, an expert user needs to contour the tumor and its maximum anticipated range of motion in pretreatment MR images. During treatment, however, the algorithm processes each intrafractional MR image and automatically generates a tumor contour without further user input. The algorithm is designed to produce a tumor contour that is the most similar to the expert’s manual one. To evaluate the autocontouring algorithm in the author’s Linac-MR environment which utilizes a 0.5 T MRI, a motion phantom and four lung cancer patients were imaged with 3 T MRI during normal breathing, and the image noise was degraded to reflect the image noise at 0.5 T. Each of the pseudo-0.5 T images was autocontoured using the author’s algorithm. In each test image, the Dice similarity index (DSI) and Hausdorff distance (HD) between the expert’s manual contour and the algorithm generated contour were calculated, and their centroid positions were compared (Δd centroid).Results:
The algorithm successfully contoured the shape of a moving tumor from dynamic MR images acquired every 275 ms. From the phantom study, mean DSI of 0.95–0.96, mean HD of 2.61–2.82 mm, and mean Δd centroid of 0.68–0.93 mm were achieved. From the in-vivo study, the author’s algorithm achieved mean DSI of 0.87–0.92, mean HD of 3.12–4.35 mm, as well as Δd centroid of 1.03–1.35 mm. Autocontouring speed was less than 20 ms for each image.Conclusions:
The authors have developed and evaluated a lung tumor autocontouring algorithm for intrafractional tumor tracking using Linac-MR. The autocontouring performance in the Linac-MR environment was evaluated using phantom and in-vivo MR images. From the in-vivo study, the author’s algorithm achieved 87%–92% of contouring agreement and centroid tracking accuracy of 1.03–1.35 mm. These results demonstrate the feasibility of lung tumor autocontouring in the author’s laboratory’s Linac-MR environment.
Dynamic trajectory-based couch motion for improvement of radiation therapy trajectories in cranial SRT42(2015); http://dx.doi.org/10.1118/1.4917165View Description Hide DescriptionPurpose:
To investigate potential improvement in external beam stereotactic radiation therapy plan quality for cranial cases using an optimized dynamic gantry and patient support couch motion trajectory, which could minimize exposure to sensitive healthy tissue.Methods:
Anonymized patient anatomy and treatment plans of cranial cancer patients were used to quantify the geometric overlap between planning target volumes and organs-at-risk (OARs) based on their two-dimensional projection from source to a plane at isocenter as a function of gantry and couch angle. Published dose constraints were then used as weighting factors for the OARs to generate a map of couch-gantry coordinate space, indicating degree of overlap at each point in space. A couch-gantry collision space was generated by direct measurement on a linear accelerator and couch using an anthropomorphic solid-water phantom. A dynamic, fully customizable algorithm was written to generate a navigable ideal trajectory for the patient specific couch-gantry space. The advanced algorithm can be used to balance the implementation of absolute minimum values of overlap with the clinical practicality of large-scale couch motion and delivery time. Optimized cranial cancer treatment trajectories were compared to conventional treatment trajectories.Results:
Comparison of optimized treatment trajectories with conventional treatment trajectories indicated an average decrease in mean dose to the OARs of 19% and an average decrease in maximum dose to the OARs of 12%. Degradation was seen for homogeneity index (6.14% ± 0.67%–5.48% ± 0.76%) and conformation number (0.82 ± 0.02–0.79 ± 0.02), but neither was statistically significant. Removal of OAR constraints from volumetric modulated arc therapy optimization reveals that reduction in dose to OARs is almost exclusively due to the optimized trajectory and not the OAR constraints.Conclusions:
The authors’ study indicated that simultaneous couch and gantry motion during radiation therapy to minimize the geometrical overlap in the beams-eye-view of target volumes and the organs-at-risk can have an appreciable dose reduction to organs-at-risk.
Technical Note: Experimental carbon ion range verification in inhomogeneous phantoms using prompt gammas42(2015); http://dx.doi.org/10.1118/1.4917225View Description Hide DescriptionPurpose:
The purpose of this study was to experimentally assess the possibility to monitor carbon ion range variations—due to tumor shift and/or elongation or shrinking—using prompt-gamma (PG) emission with inhomogeneous phantoms. Such a study is related to the development of PG monitoring techniques to be used in a carbon ion therapy context.Methods:
A 95 MeV/u carbon ion beam was used to irradiate phantoms with a variable density along the ion path to mimic the presence of bone and lung in homogeneous humanlike tissue. PG profiles were obtained after a longitudinal scan of the phantoms. A setup comprising a narrow single-slit collimator and two detectors placed at 90° with respect to the beam axis was used. The time of flight technique was applied to allow the selection between PG and background events.Results:
Using the positions at 50% entrance and 50% falloff of the PG profiles, a quantity called prompt-gamma profile length (PGPL) is defined. It is possible to observe shifts in the PGPL when there are absolute ion range shifts as small as 1–2 mm. Quantitatively, for an ion range shift of −1.33 ± 0.46 mm (insertion of a Teflon slab), a PGPL difference of −1.93 ± 0.58 mm and −1.84 ± 1.27 mm is obtained using a BaF2 and a NaI(Tl) detector, respectively. In turn, when an ion range shift of 4.59 ± 0.42 mm (insertion of a lung-equivalent material slab) is considered, the difference is of 4.10 ± 0.54 and 4.39 ± 0.80 mm for the same detectors.Conclusions:
Herein, experimental evidence of the usefulness of employing PG to monitor carbon ion range using inhomogeneous phantoms is presented. Considering the homogeneous phantom as reference, the results show that the information provided by the PG emission allows for detecting ion range shifts as small as 1–2 mm. When considering the expected PG emission from an energy slice in a carbon ion therapy scenario, the experimental setup would allow to retrieve the same PGPL as the high statistics of the full experimental dataset in 58% of the times. However, this success rate increases to 93% when using a better optimized setup by means of Monte Carlo simulations.
42(2015); http://dx.doi.org/10.1118/1.4917524View Description Hide DescriptionPurpose:
Respiratory motion may affect the accuracy of image guidance of radiation treatment of lung cancer. A cone beam computed tomography (CBCT) image spans several breathing cycles, resulting in a blurred object with a theoretical size equal to the sum of tumor size and breathing motion. However, several factors may affect this theoretical relationship. The objective of this study was to analyze the effect of tumor motion on megavoltage (MV)-CBCT images, by comparing target sizes on simulation and pretreatment images of a large cohort of lung cancer patients.Methods:
Ninety-three MV-CBCT images from 17 patients were analyzed. Internal target volumes were contoured on each MV-CBCT dataset [internal target volume (ITVCB)]. Their extent in each dimension was compared to that of two volumes contoured on simulation 4-dimensional computed tomography (4D-CT) images: the combination of the tumor contours of each phase of the 4D-CT (ITV4D) and the volume contoured on the average CT calculated from the 4D-CT phases (ITVave). Tumor size and breathing amplitude were assessed by contouring the tumor on each CBCT raw projection where it could be unambiguously identified. The effect of breathing amplitude on the quality of the MV-CBCT image reconstruction was analyzed.Results:
The mean differences between the sizes of ITVCB and ITV4D were −1.6 ± 3.3 mm (p < 0.001), −2.4 ± 3.1 mm (p < 0.001), and −7.2 ± 5.3 mm (p < 0.001) in the anterior/posterior (AP), left/right (LR), and superior/inferior (SI) directions, respectively, showing that MV-CBCT underestimates the full target size. The corresponding mean differences between ITVCB and ITVave were 0.3 ± 2.6 mm (p = 0.307), 0.0 ± 2.4 mm (p = 0.86), and −4.0 ± 4.3 mm (p < 0.001), indicating that the average CT image is more representative of what is visible on MV-CBCT in the AP and LR directions. In the SI directions, differences between ITVCB and ITVave could be separated into two groups based on tumor motion: −3.2 ± 3.2 mm for tumor motion less than 15 mm and −10.9 ± 6.3 mm for tumor motion greater than 15 mm. Deviations of measured target extents from their theoretical values derived from tumor size and motion were correlated with motion amplitude similarly for both MV-CBCT and average CT images, suggesting that the two images were subject to similar motion artifacts for motion less than 15 mm.Conclusions:
MV-CBCT images are affected by tumor motion and tend to under-represent the full target volume. For tumor motion up to 15 mm, the volume contoured on average CT is comparable to that contoured on the MV-CBCT. Therefore, the average CT should be used in image registration for localization purposes, and the standard 5 mm PTV margin seems adequate. For tumor motion greater than 15 mm, an additional setup margin may need to be used to account for the increased uncertainty in tumor localization.
42(2015); http://dx.doi.org/10.1118/1.4917479View Description Hide DescriptionPurpose:
Range and probability of nonelastic nuclear interactions (NNIs) for protons can be found only for a limited number of compounds and mixtures in nuclear data tables, and the proton-related analytical studies are therefore restricted to those materials for which the data are provided in these documents. In this paper, the authors present general solutions for calculating the proton range and probability of NNIs for desired compounds and mixtures.Methods:
Benefiting from the Bragg–Kleeman approximation of mass stopping power, the authors derive a concise formula for calculating the proton range in materials with arbitrary number of constituent elements. Additionally, the authors propose another relation for obtaining the probability of undergoing NNIs which is suggested to be additive.Results:
The examination of the formula presented shows that the authors’ method can be considered as general solutions for analytical evaluation of the range in compounds and mixtures. The formula proposed for probability of NNIs is valid for almost every compound except for those materials containing H. It is shown that this formula can be modified so that it covers these materials.Conclusions:
The authors present a general analytical method for calculating the range and probability of NNIs for protons which are mathematically easy to handle and valid for desired compounds or mixtures composed of an arbitrary number of constituent elements, including materials of interest for proton radiotherapy purposes.
A Monte Carlo simulation framework for electron beam dose calculations using Varian phase space files for TrueBeam Linacs42(2015); http://dx.doi.org/10.1118/1.4916896View Description Hide DescriptionPurpose:
To develop a framework for accurate electron Monte Carlo dose calculation. In this study, comprehensive validations of vendor provided electron beam phase space files for Varian TrueBeam Linacs against measurement data are presented.Methods:
In this framework, the Monte Carlo generated phase space files were provided by the vendor and used as input to the downstream plan-specific simulations including jaws, electron applicators, and water phantom computed in the EGSnrc environment. The phase space files were generated based on open field commissioning data. A subset of electron energies of 6, 9, 12, 16, and 20 MeV and open and collimated field sizes 3 × 3, 4 × 4, 5 × 5, 6 × 6, 10 × 10, 15 × 15, 20 × 20, and 25 × 25 cm2 were evaluated. Measurements acquired with a CC13 cylindrical ionization chamber and electron diode detector and simulations from this framework were compared for a water phantom geometry. The evaluation metrics include percent depth dose, orthogonal and diagonal profiles at depths R 100, R 50, Rp , and R p+ for standard and extended source-to-surface distances (SSD), as well as cone and cut-out output factors.Results:
Agreement for the percent depth dose and orthogonal profiles between measurement and Monte Carlo was generally within 2% or 1 mm. The largest discrepancies were observed within depths of 5 mm from phantom surface. Differences in field size, penumbra, and flatness for the orthogonal profiles at depths R 100, R 50, and Rp were within 1 mm, 1 mm, and 2%, respectively. Orthogonal profiles at SSDs of 100 and 120 cm showed the same level of agreement. Cone and cut-out output factors agreed well with maximum differences within 2.5% for 6 MeV and 1% for all other energies. Cone output factors at extended SSDs of 105, 110, 115, and 120 cm exhibited similar levels of agreement.Conclusions:
We have presented a Monte Carlo simulation framework for electron beam dose calculations for Varian TrueBeam Linacs. Electron beam energies of 6 to 20 MeV for open and collimated field sizes from 3 × 3 to 25 × 25 cm2 were studied and results were compared to the measurement data with excellent agreement. Application of this framework can thus be used as the platform for treatment planning of dynamic electron arc radiotherapy and other advanced dynamic techniques with electron beams.
- RADIATION IMAGING PHYSICS
Technical Note: Measuring contrast- and noise-dependent spatial resolution of an iterative reconstruction method in CT using ensemble averaging42(2015); http://dx.doi.org/10.1118/1.4916802View Description Hide DescriptionPurpose:
The spatial resolution of iterative reconstruction (IR) in computed tomography (CT) is contrast- and noise-dependent because of the nonlinear regularization. Due to the severe noise contamination, it is challenging to perform precise spatial-resolution measurements at very low-contrast levels. The purpose of this study was to measure the spatial resolution of a commercially available IR method using ensemble-averaged images acquired from repeated scans.Methods:
A low-contrast phantom containing three rods (7, 14, and 21 HU below background) was scanned on a 128-slice CT scanner at three dose levels (CTDIvol = 16, 8, and 4 mGy). Images were reconstructed using two filtered-backprojection (FBP) kernels (B40 and B20) and a commercial IR method (sinogram affirmed iterative reconstruction, SAFIRE, Siemens Healthcare) with two strength settings (I40-3 and I40-5). The same scan was repeated 100 times at each dose level. The modulation transfer function (MTF) was calculated based on the edge profile measured on the ensemble-averaged images.Results:
The spatial resolution of the two FBP kernels, B40 and B20, remained relatively constant across contrast and dose levels. However, the spatial resolution of the two IR kernels degraded relative to FBP as contrast or dose level decreased. For a given dose level at 16 mGy, the MTF 50% value normalized to the B40 kernel decreased from 98.4% at 21 HU to 88.5% at 7 HU for I40-3 and from 97.6% to 82.1% for I40-5. At 21 HU, the relative MTF 50% value decreased from 98.4% at 16 mGy to 90.7% at 4 mGy for I40-3 and from 97.6% to 85.6% for I40-5.Conclusions:
A simple technique using ensemble averaging from repeated CT scans can be used to measure the spatial resolution of IR techniques in CT at very low contrast levels. The evaluated IR method degraded the spatial resolution at low contrast and high noise levels.
42(2015); http://dx.doi.org/10.1118/1.4916655View Description Hide DescriptionPurpose:
To investigate the efficacy of a new family of measurements made on individual pulmonary cysts extracted from computed tomography (CT) for assessing the severity of lymphangioleiomyomatosis (LAM).Methods:
CT images were analyzed using thresholding to identify a cystic region of interest from chest CT of LAM patients. Individual cysts were then extracted from the cystic region by the watershed algorithm, which separates individual cysts based on subtle edges within the cystic regions. A family of measurements were then computed, which quantify the amount, distribution, and boundary appearance of the cysts. Sequential floating feature selection was used to select a small subset of features for quantification of the severity of LAM. Adjusted R 2 from multiple linear regression and R 2 from linear regression against measurements from spirometry were used to compare the performance of our proposed measurements with currently used density based CT measurements in the literature, namely, the relative area measure and the D measure.Results:
Volumetric CT data, performed at total lung capacity and residual volume, from a total of 49 subjects enrolled in the MILES trial were used in our study. Our proposed measures had adjusted R 2 ranging from 0.42 to 0.59 when regressing against the spirometry measures, with p < 0.05. For previously used density based CT measurements in the literature, the best R 2 was 0.46 (for only one instance), with the majority being lower than 0.3 or p > 0.05.Conclusions:
The proposed family of CT-based cyst measurements have better correlation with spirometric measures than previously used density based CT measurements. They show potential as a sensitive tool for quantitatively assessing the severity of LAM.
42(2015); http://dx.doi.org/10.1118/1.4917227View Description Hide DescriptionPurpose:
In this research, a high-energy in-line phase contrast tomosynthesis prototype was developed and characterized through quantitative investigations and phantom studies.Methods:
The prototype system consists of an x-ray source, a motorized rotation stage, and a CMOS detector with a pixel pitch of 0.05 mm. The x-ray source was operated at 120 kVp for this study, and the objects were mounted on the rotation stage 76.2 cm (R1) from the source and 114.3 cm (R2) from the detector. The large air gap between the object and detector guarantees sufficient phase-shift effects. The quantitative evaluation of this prototype included modulation transfer function and noise power spectrum measurements conducted under both projection mode and tomosynthesis mode. Phantom studies were performed including three custom designed phantoms with complex structures: a five-layer bubble wrap phantom, a fishbone phantom, and a chicken breast phantom with embedded fibrils and mass structures extracted from an ACR phantom. In-plane images of the phantoms were acquired to investigate their image qualities through observation, intensity profile plots, edge enhancement evaluations, and/or contrast-to-noise ratio calculations. In addition, the robust phase-attenuation duality (PAD)-based phase retrieval method was applied to tomosynthesis for the first time in this research. It was utilized as a preprocessing method to fully exhibit phase contrast on the angular projection before reconstruction.Results:
The resolution and noise characteristics of this high-energy in-line phase contrast tomosynthesis prototype were successfully investigated and demonstrated. The phantom studies demonstrated that this imaging prototype can successfully remove the structure overlapping in phantom projections, obtain delineate interfaces, and achieve better contrast-to-noise ratio after applying phase retrieval to the angular projections.Conclusions:
This research successfully demonstrated a high-energy in-line phase contrast tomosynthesis prototype. In addition, the PAD-based method of phase retrieval was combined with tomosynthesis imaging for the first time, which demonstrated its capability in significantly improving the contrast-to-noise ratios in the images.
- RADIATION MEASUREMENT PHYSICS
42(2015); http://dx.doi.org/10.1118/1.4916659View Description Hide DescriptionPurpose:
Targeted and whole-brain irradiation in humans can result in significant side effects causing decreased patient quality of life. To adequately investigate structural and functional alterations after stereotactic radiosurgery, preclinical studies are needed. The purpose of this work is to establish a robust standardized method of targeted irradiation on small regions of the rat brain.Methods:
Euthanized male Fischer rats were imaged in a stereotactic bed, by computed tomography (CT), to estimate positioning variations relative to the bregma skull reference point. Using a rat brain atlas and the stereotactic bregma coordinates obtained from CT images, different regions of the brain were delimited and a treatment plan was generated. A single isocenter treatment plan delivering ≥100 Gy in 100% of the target volume was produced by Leksell GammaPlan using the 4 mm diameter collimator of sectors 4, 5, 7, and 8 of the Gamma Knife unit. Impact of positioning deviations of the rat brain on dose deposition was simulated by GammaPlan and validated with dosimetric measurements.Results:
The authors’ results showed that 90% of the target volume received 100 ± 8 Gy and the maximum of deposited dose was 125 ± 0.7 Gy, which corresponds to an excellent relative standard deviation of 0.6%. This dose deposition calculated with GammaPlan was validated with dosimetric films resulting in a dose-profile agreement within 5%, both in X- and Z-axes.Conclusions:
The authors’ results demonstrate the feasibility of standardizing the irradiation procedure of a small volume in the rat brain using a Gamma Knife.
Impact of the differential fluence distribution of brachytherapy sources on the spectroscopic dose-rate constant42(2015); http://dx.doi.org/10.1118/1.4918325View Description Hide DescriptionPurpose:
To investigate why dose-rate constants for 125I and 103Pd seeds computed using the spectroscopic technique, Λspec, differ from those computed with standard Monte Carlo (MC) techniques. A potential cause of these discrepancies is the spectroscopic technique’s use of approximations of the true fluence distribution leaving the source, φ full. In particular, the fluence distribution used in the spectroscopic technique, φ spec, approximates the spatial, angular, and energy distributions of φ full. This work quantified the extent to which each of these approximations affects the accuracy of Λspec. Additionally, this study investigated how the simplified water-only model used in the spectroscopic technique impacts the accuracy of Λspec.Methods:
Dose-rate constants as described in the AAPM TG-43U1 report, Λfull, were computed with MC simulations using the full source geometry for each of 14 different 125I and 6 different 103Pd source models. In addition, the spectrum emitted along the perpendicular bisector of each source was simulated in vacuum using the full source model and used to compute Λspec. Λspec was compared to Λfull to verify the discrepancy reported by Rodriguez and Rogers. Using MC simulations, a phase space of the fluence leaving the encapsulation of each full source model was created. The spatial and angular distributions of φ full were extracted from the phase spaces and were qualitatively compared to those used by φ spec. Additionally, each phase space was modified to reflect one of the approximated distributions (spatial, angular, or energy) used by φ spec. The dose-rate constant resulting from using approximated distribution i, Λapprox,i , was computed using the modified phase space and compared to Λfull. For each source, this process was repeated for each approximation in order to determine which approximations used in the spectroscopic technique affect the accuracy of Λspec.Results:
For all sources studied, the angular and spatial distributions of φ full were more complex than the distributions used in φ spec. Differences between Λspec and Λfull ranged from −0.6% to +6.4%, confirming the discrepancies found by Rodriguez and Rogers. The largest contribution to the discrepancy was the assumption of isotropic emission in φ spec, which caused differences in Λ of up to +5.3% relative to Λfull. Use of the approximated spatial and energy distributions caused smaller average discrepancies in Λ of −0.4% and +0.1%, respectively. The water-only model introduced an average discrepancy in Λ of −0.4%.Conclusions:
The approximations used in φ spec caused discrepancies between Λapprox,i and Λfull of up to 7.8%. With the exception of the energy distribution, the approximations used in φ spec contributed to this discrepancy for all source models studied. To improve the accuracy of Λspec, the spatial and angular distributions of φ full could be measured, with the measurements replacing the approximated distributions. The methodology used in this work could be used to determine the resolution that such measurements would require by computing the dose-rate constants from phase spaces modified to reflect φ full binned at different spatial and angular resolutions.