Volume 40, Issue 5, May 2013
Index of content:
To improve the accuracy and the robustness of the segmentation in living donor liver transplantation (LDLT) surgery planning system, the authors present a new segmentation framework that addresses challenges induced by the complex shape variations of patients’ livers with cancer. It is designed to achieve the accurate and robust segmentation of hepatic parenchyma, portal veins, hepatic veins, and tumors in the LDLT surgery planning system.Methods:
The segmentation framework proposed in this paper includes two important modules: (1) The robust shape prior modeling for liver, in which the sparse shape composition (SSC) model is employed to deal with the complex variations of liver shapes and obtain patient-specific liver shape priors. (2) The integration of the liver shape prior with a minimally supervised segmentation algorithm to achieve the accurate segmentation of hepatic parenchyma, portal veins, hepatic veins, and tumors simultaneously. The authors apply this segmentation framework to our previously developed LDLT surgery planning system to enhance its accuracy and robustness when dealing with complex cases of patients with liver cancer.Results:
Compared with the principal component analysis, the SSC model shows a great advantage in handling the complex variations of liver shapes. It also effectively excludes gross errors and outliers that appear in the input shape and preserves local details for specific patients. The proposed segmentation framework was evaluated on the clinical image data of liver cancer patients, and the average symmetric surface distance for hepatic parenchyma, portal veins, hepatic veins, and tumors was 1.07 ± 0.76, 1.09 ± 0.28, 0.92 ± 0.35 and 1.13 ± 0.37 mm, respectively. The Hausdorff distance for these four tissues was 7.68, 4.67, 4.09, and 5.36 mm, respectively.Conclusions:
The proposed segmentation framework improves the robustness of the LDLT surgery planning system remarkably when dealing with complex clinical liver shapes. The SSC model is able to handle non-Gaussian errors and preserve local detail information of the input liver shape. As a result, the proposed framework effectively addresses the problems caused by the complex shape variations of livers with cancer. Our framework not only obtains accurate segmentation results for healthy persons and common patients, but also shows high robustness when dealing with specific patients with large variations of liver shapes in complex clinical environments.
40(2013); http://dx.doi.org/10.1118/1.4790690View Description Hide Description
- MEDICAL PHYSICS LETTER
An adaptive planning strategy for station parameter optimized radiation therapy (SPORT): Segmentally boosted VMAT40(2013); http://dx.doi.org/10.1118/1.4802748View Description Hide DescriptionPurpose:
Conventional volumetric modulated arc therapy (VMAT) discretizes the angular space into equally spaced control points during planning and then optimizes the apertures and weights of the control points. The aperture at an angle in between two control points is obtained through interpolation. This approach tacitly ignores the differential need for intensity modulation of different angles. As such, multiple arcs are often required, which may oversample some angle(s) and undersample others. The purpose of this work is to develop a segmentally boosted VMAT scheme to eliminate the need for multiple arcs in VMAT treatment with improved dose distribution and/or delivery efficiency.Methods:
The essence of the new treatment scheme is how to identify the need of individual angles for intensity modulation and to provide the necessary beam intensity modulation for those beam angles that need it. We introduce a “demand metric” at each control point to decide which station or control points need intensity modulation. To boost the modulation at selected stations, additional segments are added in the vicinity of the selected stations. The added segments are then optimized together with the original set of station or control points as a whole. The authors apply the segmentally boosted planning technique to four previously treated clinical cases: two head and neck (HN) cases, one prostate case, and one liver case. The proposed planning technique is compared with conventional one-arc and two-arc VMAT.Results:
The proposed segmentally boosted VMAT technique achieves better critical structure sparing than one-arc VMAT with similar or better target coverage in all four clinical cases. The segmentally boosted VMAT also outperforms two-arc VMAT for the two complicated HN cases, yet with ∼30% reduction in the machine monitor units (MUs) relative to two-arc VMAT, which leads to less leakage/scatter dose to the patient and can potentially translate into faster dose delivery. For the less challenging prostate and liver cases, similar critical structure sparing as the two-arc VMAT plans was obtained using the segmentally boosted VMAT. The benefit for the two simpler cases is the reduction of MUs and improvement of treatment delivery efficiency.Conclusions:
Segmentally boosted VMAT achieves better dose conformality and/or reduced MUs through effective consideration of the need of individual beam angles for intensity modulation. Elimination of the need for multiple arcs in rotational arc therapy while improving the dose distribution should lead to improved workflow and treatment efficacy, thus may have significant implication to radiation oncology practice.
- VISION 20/20
40(2013); http://dx.doi.org/10.1118/1.4802733View Description Hide Description
Approaches to imaging the breast with nuclear medicine and/or molecular imaging methods have been under investigation since the late 1980s when a technique called scintimammography was first introduced. This review charts the progress of nuclear imaging of the breast over the last 20 years, covering the development of newer techniques such as breast specific gamma imaging, molecular breast imaging, and positron emission mammography. Key issues critical to the adoption of these technologies in the clinical environment are discussed, including the current status of clinical studies, the efforts at reducing the radiation dose from procedures associated with these technologies, and the relevant radiopharmaceuticals that are available or under development. The necessary steps required to move these technologies from bench to bedside are also discussed.
- RADIATION THERAPY PHYSICS
40(2013); http://dx.doi.org/10.1118/1.4798980View Description Hide DescriptionPurpose:
Achieving accurate optical-CT 3D dosimetry without the use of viscous refractive index (RI) matching fluids would greatly increase convenience.Methods:
Software has been developed to simulate optical-CT 3D dosimetry for a range of scanning configurations including parallel-beam, point, and converging light sources. For each configuration the efficacy of three refractive media was investigated: air, water, a fluid closely matched to PRESAGE®, and perfect matching (RI = 1.00, 1.33, 1.49, and 1.501 respectively). Reconstructions were performed using both filtered backprojection (FBP) and algebraic reconstruction technique (ART). The efficacy of the three configurations and the two algorithms was evaluated by calculating the usable radius (i.e., the outermost radius where data were accurate to within 2%), and gamma (Γ) analysis. This definition recognizes that for optical-CT imaging, errors are greatest near the edge of the dosimeter, where refraction can be most pronounced. Simulations were performed on three types of dose distribution: uniform, volumetric modulated arc therapy (VMAT), and brachytherapy (Cs-137).Results:
For a uniformly irradiated dosimeter the usable radius achieved with filtered backprojection was 68% for water-matching and 31% for dry-scanning in air. Algebraic reconstruction gave usable radii of 99% for both water and air (dry-scanning), indicating greater recovery of useful data for the uniform distribution. FBP and ART performed equally well for a VMAT dose distribution where less dose is delivered near the edge of the dosimeter. In this case, the usable radius was 86% and 53% for scanning in water and air, respectively. For brachytherapy, the usable radius was 99% and 98% for scanning in water and air, respectively using FBP, and a major decrease was seen with ART. Point source geometry provided 1%–2% larger usable radii than parallel geometry. Converging geometry recovered less usable dosimetry data (up to 10% reduced usable radii) than point and parallel geometries. A further disadvantage of converging geometry was an increased requirement on detector size by up to 18°.Conclusions:
For applications where dose information is not required in the periphery of the dosimeter, some dry and low-viscous matching configurations may be feasible. For all three dose distributions (uniform, VMAT, brachytherapy) the point source geometry produced slightly more favorable results (an extra 1%–2% usable radii) than parallel and converging. When dosimetry is required on the periphery, best results were obtained using close refractive matching and ART. A concern for water or dry-scanning is the increase in required detector size, introducing potential cost penalties for manufacturing.
Feasibility study on inverse four-dimensional dose reconstruction using the continuous dose-image of EPID40(2013); http://dx.doi.org/10.1118/1.4799941View Description Hide DescriptionPurpose:
When an intensity-modulated radiation beam is delivered to a moving target, the interplay effect between dynamic beam delivery and the target motion due to miss-synchronization can cause unpredictable dose delivery. The portal dose image in electronic portal imaging device (EPID) represents radiation attenuated and scattered through target media. Thus, it may possess information about delivered radiation to the target. Using a continuous scan (cine) mode of EPID, which provides temporal dose images related to target and beam movements, the authors’ goal is to perform four-dimensional (4D) dose reconstruction.Methods:
To evaluate this hypothesis, first, the authors have derived and subsequently validated a fast method of dose reconstruction based on virtual beamlet calculations of dose responses using a test intensity-modulated beam. This method was necessary for processing a large number of EPID images pertinent for four-dimensional reconstruction. Second, cine mode acquisition after summation over all images was validated through comparison with integration mode acquisition on EPID (IAS3 and aS1000) for the test beam. This was to confirm the agreement of the cine mode with the integrated mode, specifically for the test beam, which is an accepted mode of image acquisition for dosimetry with EPID. Third, in-phantom film and exit EPID dosimetry was performed on a moving platform using the same beam. Heterogeneous as well as homogeneous phantoms were used. The cine images were temporally sorted at 10% interval. The authors have performed dose reconstruction to the in-phantom plane from the sorted cine images using the above validated method of dose reconstruction. The reconstructed dose from each cine image was summed to compose a total reconstructed dose from the test beam delivery, and was compared with film measurements.Results:
The new method of dose reconstruction was validated showing greater than 95.3% pass rates of the gamma test with the criteria of dose difference of 3% and distance to agreement of 3 mm. The dose comparison of the reconstructed dose with the measured dose for the two phantoms showed pass rates higher than 96.4% given the same criteria.Conclusions:
Feasibility of 4D dose reconstruction was successfully demonstrated in this study. The 4D dose reconstruction demonstrated in this study can be a promising dose validation method for radiation delivery on moving organs.
40(2013); http://dx.doi.org/10.1118/1.4798961View Description Hide DescriptionPurpose:
This paper introduces a new external beam radiotherapy device named GammaPod that is dedicated for stereotactic radiotherapy of breast cancer.Methods:
The design goal of the GammaPod as a dedicated system for treating breast cancer is the ability to deliver ablative doses with sharp gradients under stereotactic image guidance. Stereotactic localization of the breast is achieved by a vacuum-assisted breast immobilization cup with built-in stereotactic frame. Highly focused radiation is achieved at the isocenter due to the cross-firing from 36 radiation arcs generated by rotating 36 individual Cobalt-60 beams. The dedicated treatment planning system optimizes an optimal path of the focal spot using an optimization algorithm borrowed from computational geometry such that the target can be covered by 90%–95% of the prescription dose and the doses to surrounding tissues are minimized. The treatment plan is intended to be delivered with continuous motion of the treatment couch. In this paper the authors described in detail the gamma radiation unit, stereotactic localization of the breast, and the treatment planning system of the GammaPod system.Results:
A prototype GammaPod system was installed at University of Maryland Medical Center and has gone through a thorough functional, geometric, and dosimetric testing. The mechanical and functional performances of the system all meet the functional specifications.Conclusions:
An image-guided breast stereotactic radiotherapy device, named GammaPod, has been developed to deliver highly focused and localized doses to a target in the breast under stereotactic image guidance. It is envisioned that the GammaPod technology has the potential to significantly shorten radiation treatments and even eliminate surgery by ablating the tumor and sterilizing the tumor bed simultaneously.
40(2013); http://dx.doi.org/10.1118/1.4800493View Description Hide DescriptionPurpose:
This study presents a follow-up to a modified calibration procedure for portal dosimetry published byBailey et al. [“An effective correction algorithm for off-axis portal dosimetry errors,” Med. Phys.36, 4089–4094 (Year: 2009)10.1118/1.3187785]. A commercial portal dose prediction system exhibits disagreement of up to 15% (calibrated units) between measured and predicted images as off-axis distance increases. The previous modified calibration procedure accounts for these off-axis effects in most regions of the detecting surface, but is limited by the simplistic assumption of radial symmetry.Methods:
We find that a two-dimensional (2D) matrix correction, applied to each calibrated image, accounts for off-axis prediction errors in all regions of the detecting surface, including those still problematic after the radial correction is performed. The correction matrix is calculated by quantitative comparison of predicted and measured images that span the entire detecting surface. The correction matrix was verified for dose-linearity, and its effectiveness was verified on a number of test fields. The 2D correction was employed to retrospectively examine 22 off-axis, asymmetric electronic-compensation breast fields, five intensity-modulated brain fields (moderate-high modulation) manipulated for far off-axis delivery, and 29 intensity-modulated clinical fields of varying complexity in the central portion of the detecting surface.Results:
Employing the matrix correction to the off-axis test fields and clinical fields, predicted vs measured portal dose agreement improves by up to 15%, producing up to 10% better agreement than the radial correction in some areas of the detecting surface. Gamma evaluation analyses (3 mm, 3% global, 10% dose threshold) of predicted vs measured portal dose images demonstrate pass rate improvement of up to 75% with the matrix correction, producing pass rates that are up to 30% higher than those resulting from the radial correction technique alone. As in the 1D correction case, the 2D algorithm leaves the portal dosimetry process virtually unchanged in the central portion of the detector, and thus these correction algorithms are not needed for centrally located fields of moderate size (at least, in the case of 6 MV beam energy).Conclusion:
The 2D correction improves the portal dosimetry results for those fields for which the 1D correction proves insufficient, especially in the inplane, off-axis regions of the detector. This 2D correction neglects the relatively smaller discrepancies that may be caused by backscatter from nonuniform machine components downstream from the detecting layer.
Optimization of normalized prescription isodose selection for stereotactic body radiation therapy: Conventional vs robotic linac40(2013); http://dx.doi.org/10.1118/1.4798944View Description Hide DescriptionPurpose:
Although modern technology has allowed for target dose escalation by minimizing normal tissue dose, the dose delivered to a tumor and surrounding tissues still depends largely on the inherent characteristics of the radiation delivery platform. This work aims to determine the optimal prescription isodose line that minimizes normal tissue irradiation for stereotactic body radiation therapy (SBRT) for a conventional linear accelerator and a robotic delivery platform.Methods:
Spherical targets with diameters of 10, 20, and 30 mm were constructed in the lungs and liver of a computer based digital torso phantom which simulates respiratory and cardiac motion. Normal tissue contours included normal lung, normal liver, and a concentric 10 mm shell of normal tissue extending from the spherical target surface. For linac planning, noncoplanar, nonopposing three dimensional (3D) conformal beams were designed, and variable prescription isodose lines were achieved by varying the MLC block margin. For CyberKnife planning, variable prescription isodose lines were achieved by inverse planning. True 4D dose calculations were used for the moving target and surrounding tissue based on each of ten phases of a 4D CT dataset. Doses of 60 Gy in three fractions were prescribed to cover 95% of the target tumor. Commonly used conformality, dosimetric, and radiobiological indices for lung and liver SBRT were used to compare different plans and determine the optimally prescribed isodose line for each treatment platform.Results:
For linac plans, the average optimal prescription isodose line based on all indices evaluated occurred between 59% and 69% for lung tumors and between 67% and 77% for liver tumors depending on the tumor size. CyberKnife plans had average optimal prescription isodose lines occurring between 40% and 48% for lung tumors and between 41% and 42% depending on the tumor size. However, prescription isodose lines under 50% are not advised to prevent large heterogeneous dose distributions within the target.Conclusions:
The choice of prescription isodose line was shown to have a significant impact on parameters commonly used as constraints for lung and liver SBRT treatment planning for both linac-based and CyberKnife delivery platforms. By methodically choosing the prescription isodose line, normal tissue toxicities from SBRT may be reduced.
40(2013); http://dx.doi.org/10.1118/1.4799843View Description Hide DescriptionPurpose:
To develop an approach of estimating subject-centered free-response receiver operating characteristic (FROC) curve for providing patient-centered inferences regarding detection-localization characteristics of a diagnostic system.Methods:
The authors examine properties of the conventional, target-centered, FROC curve and demonstrate that in scenarios where the diagnostic performance correlates with the total number of targets on a subject, it may lead to inadequate inferences from the perspective of possible benefits to a patient. Following solutions to patient-centered approaches in other applications, the authors define a subject-centered FROC curve and develop its formulation as a covariate-adjusted FROC curve. The authors also conduct a numerical study illustrating the relative properties of the conventional and subject-centered approach and provide an example.Results:
A simple-to-implement approach for estimating the subject-centered FROC curve and its overall index can be formulated as a type of stratified FROC analysis. The authors demonstrate that when diagnostic performance is associated with the number of targets, the diagnostic system with apparently superior target-centered characteristics (conventional approach) can be actually inferior from the subject-centered perspective. The authors show that under some clinically reasonable conditions the magnitude of disagreement in results could be substantial. An example from an actual observer performance study illustrates the natural setting where the developed approach would be relevant and lead to conclusions that are contradictory to those obtained from conventional analysis.Conclusions:
The authors developed a subject-centered FROC curve and its overall index provides tools for inferences that may be relevant from a perspective of potential benefits to a patient.
40(2013); http://dx.doi.org/10.1118/1.4800492View Description Hide DescriptionPurpose:
The development of fast and accurate source models (SMs) might be of crucial importance for the future clinical implementation of modulated electron radiation therapy (MERT). In this study, a SM is presented for reconstructing phase-space information of modulated electron beams using a few-leaf electron collimator (FLEC) and the photon jaws.Methods:
During a FLEC-based delivery, two collimation devices (jaws and FLEC) modulate the electron beam characteristics dynamically. The SM separates the beam into a primary and a scattered component. The primary component is derived by a fast Monte Carlo (MC) transport calculation in air using the EGSnrc/BEAMnrc code. The scattered beam is modeled analytically. The accelerator was decomposed into its individual leaf components and the scattered beam was characterized at various levels of the accelerator. Scattered particles are assigned an energy and position by sampling pre-calculated probability distributions. The direction is estimated by geometrical arguments. Particles were assumed to emerge from tunable virtual sources on the side of each collimator leaf. A leaf-hit algorithm was developed to dynamically reject particles that are incident on any collimating leaf. Electron transport in air between the two collimation levels was calculated based on a MC-modified version of the Fermi-Eyges scattering theory. Correlations between direction and position were observed and taken into account at the final collimation level.Results:
To validate the model, reconstructed phase-space data were compared with the full accelerator MC phase-space data. The model accurately reproduced the beam characteristics and preserved important correlations. Depth and profile dose distributions in water were derived for square, rectangular, and off-axis field sizes and for a range of clinical energies. Discrepancies in the dose distributions and dose output were within 3% in all cases.Conclusions:
Fast and accurate SMs open the possibility for fast treatment planning in MERT, based on an inverse optimization MC treatment planning scheme.
Maximizing the biological effect of proton dose delivered with scanned beams via inhomogeneous daily dose distributions40(2013); http://dx.doi.org/10.1118/1.4801897View Description Hide DescriptionPurpose:
Biological effect of radiation can be enhanced with hypofractionation, localized dose escalation, and, in particle therapy, with optimized distribution of linear energy transfer (LET). The authors describe a method to construct inhomogeneous fractional dose (IFD) distributions, and evaluate the potential gain in the therapeutic effect from their delivery in proton therapy delivered by pencil beam scanning.Methods:
For 13 cases of prostate cancer, the authors considered hypofractionated courses of 60 Gy delivered in 20 fractions. (All doses denoted in Gy include the proton's mean relative biological effectiveness (RBE) of 1.1.) Two types of plans were optimized using two opposed lateral beams to deliver a uniform dose of 3 Gy per fraction to the target by scanning: (1) in conventional full-target plans (FTP), each beam irradiated the entire gland, (2) in split-target plans (STP), beams irradiated only the respective proximal hemispheres (prostate split sagittally). Inverse planning yielded intensity maps, in which discrete position control points of the scanned beam (spots) were assigned optimized intensity values. FTP plans preferentially required a higher intensity of spots in the distal part of the target, while STP, by design, employed proximal spots. To evaluate the utility of IFD delivery, IFD plans were generated by rearranging the spot intensities from FTP or STP intensity maps, separately as well as combined using a variety of mixing weights. IFD courses were designed so that, in alternating fractions, one of the hemispheres of the prostate would receive a dose boost and the other receive a lower dose, while the total physical dose from the IFD course was roughly uniform across the prostate. IFD plans were normalized so that the equivalent uniform dose (EUD) of rectum and bladder did not increase, compared to the baseline FTP plan, which irradiated the prostate uniformly in every fraction. An EUD-based model was then applied to estimate tumor control probability (TCP) and normal tissue complication probability (NTCP). To assess potential local RBE variations, LET distributions were calculated with Monte Carlo, and compared for different plans. The results were assessed in terms of their sensitivity to uncertainties in model parameters and delivery.Results:
IFD courses included equal number of fractions boosting either hemisphere, thus, the combined physical dose was close to uniform throughout the prostate. However, for the entire course, the prostate EUD in IFD was higher than in conventional FTP by up to 14%, corresponding to the estimated increase in TCP to 96% from 88%. The extent of gain depended on the mixing factor, i.e., relative weights used to combine FTP and STP spot weights. Increased weighting of STP typically yielded a higher target EUD, but also led to increased sensitivity of dose to variations in the proton's range. Rectal and bladder EUD were same or lower (per normalization), and the NTCP for both remained below 1%. The LET distributions in IFD also depended strongly on the mixing weights: plans using higher weight of STP spots yielded higher LET, indicating a potentially higher local RBE.Conclusions:
In proton therapy delivered by pencil beam scanning, improved therapeutic outcome can potentially be expected with delivery of IFD distributions, while administering the prescribed quasi-uniform dose to the target over the entire course. The biological effectiveness of IFD may be further enhanced by optimizing the LET distributions. IFD distributions are characterized by a dose gradient located in proximity of the prostate's midplane, thus, the fidelity of delivery would depend crucially on the precision with which the proton range could be controlled.
Dosimetric comparison of patient setup strategies in stereotactic body radiation therapy for lung cancer40(2013); http://dx.doi.org/10.1118/1.4801926View Description Hide DescriptionPurpose:
In this work, the authors retrospectively compared the accumulated dose over the treatment course for stereotactic body radiation therapy (SBRT) of lung cancer for three patient setup strategies.Methods:
Ten patients who underwent lung SBRT were selected for this study. At each fraction, patients were immobilized using a vacuum cushion and were CT scanned. Treatment plans were performed on the simulation CT. The planning target volume (PTV) was created by adding a 5-mm uniform margin to the internal target volume derived from the 4DCT. All plans were normalized such that 99% of the PTV received 60 Gy. The plan parameters were copied onto the daily CT images for dose recalculation under three setup scenarios: skin marker, bony structure, and soft tissue based alignments. The accumulated dose was calculated by summing the dose at each fraction along the trajectory of a voxel over the treatment course through deformable image registration of each CT with the planning CT. The accumulated doses were analyzed for the comparison of setup accuracy.Results:
The tumor volume receiving 60 Gy was 91.7 ± 17.9%, 74.1 ± 39.1%, and 99.6 ± 1.3% for setup using skin marks, bony structures, and soft tissue, respectively. The isodose line covering 100% of the GTV was 55.5 ± 7.1, 42.1 ± 16.0, and 64.3 ± 7.1 Gy, respectively. The corresponding average biologically effective dose of the tumor was 237.3 ± 29.4, 207.4 ± 61.2, and 258.3 ± 17.7 Gy, respectively. The differences in lung biologically effective dose, mean dose, and V20 between the setup scenarios were insignificant.Conclusions:
The authors’ results suggest that skin marks and bony structure are insufficient for aligning patients in lung SBRT. Soft tissue based alignment is needed to match the prescribed dose delivered to the tumors.
Dosimetric impact of Acuros XB deterministic radiation transport algorithm for heterogeneous dose calculation in lung cancer40(2013); http://dx.doi.org/10.1118/1.4802216View Description Hide DescriptionPurpose:
The novel deterministic radiation transport algorithm, Acuros XB (AXB), has shown great potential for accurate heterogeneous dose calculation. However, the clinical impact between AXB and other currently used algorithms still needs to be elucidated for translation between these algorithms. The purpose of this study was to investigate the impact of AXB for heterogeneous dose calculation in lung cancer for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT).Methods:
The thorax phantom from the Radiological Physics Center (RPC) was used for this study. IMRT and VMAT plans were created for the phantom in the Eclipse 11.0 treatment planning system. Each plan was delivered to the phantom three times using a Varian Clinac iX linear accelerator to ensure reproducibility. Thermoluminescent dosimeters (TLDs) and Gafchromic EBT2 film were placed inside the phantom to measure delivered doses. The measurements were compared with dose calculations from AXB 11.0.21 and the anisotropic analytical algorithm (AAA) 11.0.21. Two dose reporting modes of AXB, dose-to-medium in medium (D m,m) and dose-to-water in medium (D w,m), were studied. Point doses, dose profiles, and gamma analysis were used to quantify the agreement between measurements and calculations from both AXB and AAA. The computation times for AAA and AXB were also evaluated.Results:
For the RPC lung phantom, AAA and AXB dose predictions were found in good agreement to TLD and film measurements for both IMRT and VMAT plans. TLD dose predictions were within 0.4%–4.4% to AXB doses (bothD m,m and D w,m); and within 2.5%–6.4% to AAA doses, respectively. For the film comparisons, the gamma indexes (±3%/3 mm criteria) were 94%, 97%, and 98% for AAA, AXB_D m,m, and AXB_D w,m, respectively. The differences between AXB and AAA in dose–volume histogram mean doses were within 2% in the planning target volume, lung, heart, and within 5% in the spinal cord. However, differences up to 8% between AXB and AAA were found at lung/soft tissue interface regions for individual IMRT fields. AAA was found to be 5–6 times faster than AXB for IMRT, while AXB was 4–5 times faster than AAA for VMAT plan.Conclusions:
AXB is satisfactorily accurate for the dose calculation in lung cancer for both IMRT and VMAT plans. The differences between AXB and AAA are generally small except in heterogeneous interface regions. AXBD w,m and D m,m calculations are similar inside the soft tissue and lung regions. AXB can benefit lung VMAT plans by both improving accuracy and reducing computation time.
Effectiveness of robust optimization in intensity-modulated proton therapy planning for head and neck cancers40(2013); http://dx.doi.org/10.1118/1.4801899View Description Hide DescriptionPurpose:
Intensity-modulated proton therapy (IMPT) is highly sensitive to uncertainties in beam range and patient setup. Conventionally, these uncertainties are dealt using geometrically expanded planning target volume (PTV). In this paper, the authors evaluated a robust optimization method that deals with the uncertainties directly during the spot weight optimization to ensure clinical target volume (CTV) coverage without using PTV. The authors compared the two methods for a population of head and neck (H&N) cancer patients.Methods:
Two sets of IMPT plans were generated for 14 H&N cases, one being PTV-based conventionally optimized and the other CTV-based robustly optimized. For the PTV-based conventionally optimized plans, the uncertainties are accounted for by expanding CTV to PTV via margins and delivering the prescribed dose to PTV. For the CTV-based robustly optimized plans, spot weight optimization was guided to reduce the discrepancy in doses under extreme setup and range uncertainties directly, while delivering the prescribed dose to CTV rather than PTV. For each of these plans, the authors calculated dose distributions under various uncertainty settings. The root-mean-square dose (RMSD) for each voxel was computed and the area under the RMSD-volume histogram curves (AUC) was used to relatively compare plan robustness. Data derived from the dose volume histogram in the worst-case and nominal doses were used to evaluate the plan optimality. Then the plan evaluation metrics were averaged over the 14 cases and were compared with two-sided pairedt tests.Results:
CTV-based robust optimization led to more robust (i.e., smaller AUCs) plans for both targets and organs. Under the worst-case scenario and the nominal scenario, CTV-based robustly optimized plans showed better target coverage (i.e., greater D95%), improved dose homogeneity (i.e., smaller D5% − D95%), and lower or equivalent dose to organs at risk.Conclusions:
CTV-based robust optimization provided significantly more robust dose distributions to targets and organs than PTV-based conventional optimization in H&N using IMPT. Eliminating the use of PTV and planning directly based on CTV provided better or equivalent normal tissue sparing.
40(2013); http://dx.doi.org/10.1118/1.4801902View Description Hide DescriptionPurpose:
Megavoltage grid therapy is currently delivered with step-and-shoot multisegment techniques or using a high attenuation block with divergent holes. However, the commercial availability of grid blocks is limited, their construction is difficult, and step-and-shoot techniques require longer treatment times and are not practical with some multileaf collimators. This work studies the feasibility of a hybrid collimation system for grid therapy that does not require multiple segments and can be easily implemented with widely available technical means.Methods:
The authors have developed a system to generate a grid of beamlets by the simultaneous use of two perpendicular sets of equally spaced leaves that project stripe patterns in orthogonal directions. One of them is generated with the multileaf collimator integrated in the accelerator and the other with an in-house made collimator constructed with a low melting point alloy commonly available at radiation oncology departments. The characteristics of the grid fields for 6 and 18 MV have been studied with a shielded diode, an unshielded diode, and radiochromic film.Results:
The grid obtained with the hybrid collimation is similar to some of the grids used clinically with respect to the beamlet size (about 1 cm) and the percentage of open beam (1/4 of the total field). The grid fields are less penetrating than the open fields of the same energy. Depending on the depth and the direction of the profiles (diagonal or along the principal axes), the measured valley-to-peak dose ratios range from 5% to 16% for 6 MV and from 9% to 20% for 18 MV. All the detectors yield similar results in the measurement of profiles and percent depth dose, but the shielded diode seems to overestimate the output factors.Conclusions:
The combination of two stripe pattern collimators in orthogonal directions is a feasible method to obtain two-dimensional arrays of beamlets and has potential usefulness as an efficient way to deliver grid therapy. The implementation of this method is technically simpler than the construction of a conventional grid block.
40(2013); http://dx.doi.org/10.1118/1.4799842View Description Hide DescriptionPurpose:
Bladder tumor delineation and localization during treatment are challenging problems in radiotherapy for bladder cancer. The purpose of this study is to investigate improvement of tumor delineation by the fusion of cystoscopy images with the planning CT-scan using lipiodol markers injected around the visible tumor during cystoscopy.Methods:
A registration method was developed for the fusion of cystoscopy images with a planning CT-scan and was tested on a phantom and retrospectively on the imaging data of four bladder cancer patients. For the patients, small deposits of lipiodol were injected at the visible margin of the tumor or previous transurethral resection site during cystoscopy. These deposits were clearly visible on the planning CT-scan and served as markers for both tumor delineation and image guidance of the radiotherapy treatment. Here, the markers were used for the registration of cystoscopy images with the planning CT-scan. The registration procedure works as follows: First, coarse registrations were made to orient the cystoscopy image correctly, using the center of gravity of the markers, the center of the CT bladder, and one of N markers as fiducial points in a point matching procedure. Starting from these N orientations, full registrations are performed taking lens deformation into account. Since a cystoscopy image is 2D, each pixel corresponds to a line-of-sight. The distances between the CT markers and the lines-of-sight of the cystoscopy markers were minimized. The final cost function (the root mean square distance between corresponding CT markers and lines-of-sight) was used to quantify the quality of the registration. The registration with the lowest final cost was considered to represent the correct orientation. The CT-based tumor delineation was finally backprojected onto the cystoscopy image.Results:
The fusion of cystoscopy images with a planning CT-scan succeeded for the phantom and three out of four patients. The fiducial registration error (FRE) for the phantom image registration based on five markers was 1.1 mm, while the target registration error was 1.2–1.7 mm. The FREs for the patient images were 0.1–3.6 mm. The registration procedure failed for one patient, since it was not possible to indicate unambiguously the corresponding lipiodol marker locations in the cystoscopy image and the planning CT-scan. The difference between the CT and cystoscopy defined tumor outlines clearly exceeded the registration accuracy.Conclusions:
Registration of cystoscopy images and planning CT-scan is feasible and allows for improvement of tumor delineation. However, the lipiodol injection protocol needs to be improved to facilitate identification of markers on both cystoscopy images and planning CT-scans.
40(2013); http://dx.doi.org/10.1118/1.4801910View Description Hide DescriptionPurpose:
Proton dose distributions can potentially be altered by anatomical changes in the beam path despite perfect target alignment using traditional image guidance methods. In this simulation study, the authors explored the use of dosimetric factors instead of only anatomy to set up patients for proton therapy using in-room volumetric computed tomographic (CT) images.Methods:
To simulate patient anatomy in a free-breathing treatment condition, weekly time-averaged four-dimensional CT data near the end of treatment for 15 lung cancer patients were used in this study for a dose-based isocenter shift method to correct dosimetric deviations without replanning. The isocenter shift was obtained using the traditional anatomy-based image guidance method as the starting position. Subsequent isocenter shifts were established based on dosimetric criteria using a fast dose approximation method. For each isocenter shift, doses were calculated every 2 mm up to ±8 mm in each direction. The optimal dose alignment was obtained by imposing a target coverage constraint that at least 99% of the target would receive at least 95% of the prescribed dose and by minimizing the mean dose to the ipsilateral lung.Results:
The authors found that 7 of 15 plans did not meet the target coverage constraint when using only the anatomy-based alignment. After the authors applied dose-based alignment, all met the target coverage constraint. For all but one case in which the target dose was met using both anatomy-based and dose-based alignment, the latter method was able to improve normal tissue sparing.Conclusions:
The authors demonstrated that a dose-based adjustment to the isocenter can improve target coverage and/or reduce dose to nearby normal tissue.
Image-guided method for TLD-based in vivo rectal dose verification with endorectal balloon in proton therapy for prostate cancer40(2013); http://dx.doi.org/10.1118/1.4801901View Description Hide DescriptionPurpose:
To present a practical image-guided method to position an endorectal balloon that improvesin vivo thermoluminiscent dosimeter (TLD) measurements of rectal doses in proton therapy for prostate cancer.Methods:
TLDs were combined with endorectal balloons to measure dose at the anterior rectal wall during daily proton treatment delivery. Radiopaque metallic markers were employed as surrogates for balloon position reproducibility in rotation and translation. The markers were utilized to guide the balloon orientation during daily treatment employing orthogonal x-ray image-guided patient positioning. TLDs were placed at the 12 o'clock position on the anterior balloon surface at the midprostatic plane. Markers were placed at the 3 and 9 o'clock positions on the balloon to align it with respect to the planned orientation. The balloon rotation along its stem axis, referred to as roll, causes TLD displacement along the anterior-posterior direction. The magnitude of TLD displacement is revealed by the separation distance between markers at opposite sides of the balloon on sagittal x-ray images.Results:
A total of 81in vivo TLD measurements were performed on six patients. Eighty-three percent of all measurements (65 TLD readings) were within +5% and −10% of the planning dose with a mean of −2.1% and a standard deviation of 3.5%. Examination of marker positions with in-room x-ray images of measured doses between −10% and −20% of the planned dose revealed a strong correlation between balloon roll and TLD displacement posteriorly from the planned position. The magnitude of the roll was confirmed by separations of 10–20 mm between the markers which could be corrected by manually adjusting the balloon position and verified by a repeat x-ray image prior to proton delivery. This approach could properly correct the balloon roll, resulting in TLD positioning within 2 mm along the anterior-posterior direction.Conclusions:
Our results show that image-guided TLD-basedin vivo dosimetry for rectal dose verification can be perfomed reliably and reproducibly for proton therapy in prostate cancer.
40(2013); http://dx.doi.org/10.1118/1.4801912View Description Hide DescriptionPurpose:
To examine the dosimetric effect of intrafraction movements occurred during image-guided frameless brain radiosurgery and to derive the optimal margin recipe to compensate the movement.Methods:
The patients’ movements during image-guided radiosurgeries were measured using skull-tracking method incorporated in the CyberKnife system. The dosimetric changes with the movements were computed using the six different dynamic-arc treatment plans based on the dose-grid analysis method. The authors extensively searched the proper relationship between the dose variations and the intrafraction geometric errors. The optimal margin for intrafraction movement was estimated via statistical analysis of the dosimetric changes with 262 actual patients’ data.Results:
The overall geometric effect of intrafraction movements was approximated as 1.0 , where are the average and standard deviation of the movements, respectively. The authors computed the required margins to compensate the movements with various confidence levels and with various estimated times for completing the treatments. The computed optimal margins were calculated as 2.1, 3.2, and 4.2 mm at 90% confidence level when the authors assumed the estimated treatment times of 10, 20, and 30 min, respectively.Conclusions:
The authors provide a quantitative relationship for dosimetric change with the intrafraction movement and derived appropriate margin recipes to ensure the prescribed dose delivery to targeted area for frameless brain radiosurgery.
40(2013); http://dx.doi.org/10.1118/1.4802731View Description Hide DescriptionPurpose:
Several studies have reported methodologies to calculate and correct the transit dose component of the moving radiation source for high dose rate (HDR) brachytherapy planning systems. However, most of these works employ the average source speed, which varies significantly with the measurement technique used, and does not represent a realistic speed profile, therefore, providing an inaccurate dose determination. In this work, the authors quantified the transit dose component of a HDR unit based on the measurement of the instantaneous source speed to produce more accurate dose values.Methods:
The Nucletron microSelectron-HDR Ir-192 source was characterized considering the Task Group 43 (TG-43U1) specifications. The transit dose component was considered through the calculation of the dose distribution using a Monte Carlo particle transport code, MCNP5, for each source position and correcting it by the source speed. The instantaneous source speed measurements were performed in a previous work using two optical fibers connected to a photomultiplier and an oscilloscope. Calculated doses were validated by comparing relative dose profiles with those obtained experimentally using radiochromic films.Results:
TG-43U1 source parameters were calculated to validate the Monte Carlo simulations. These agreed with the literature, with differences below 1% for the majority of the points. Calculated dose profiles without transit dose were also validated by comparison with ONCENTRA® Brachy v. 3.3 dose values, yielding differences within 1.5%. Dose profiles obtained with MCNP5 corrected using the instantaneous source speed profile showed differences near dwell positions of up to 800% in comparison to values corrected using the average source speed, but they are in good agreement with the experimental data, showing a maximum discrepancy of approximately 3% of the maximum dose. Near a dwell position the transit dose is about 22% of the dwell dose delivered by the source dwelling 1 s and reached 104.0 cGy per irradiation in a hypothetical clinical case studied in this work.Conclusions:
The present work demonstrated that the transit dose correction based on average source speed fails to accurately correct the dose, indicating that the correct speed profile should be considered. The impact on total dose due to the transit dose correction near the dwell positions is significant and should be considered more carefully in treatments with high dose rate, several catheters, multiple dwell positions, small dwell times, and several fractions.