Volume 37, Issue 2, February 2010
Index of content:
37(2010); http://dx.doi.org/10.1118/1.3276730View Description Hide Description
- VISION 20/20
37(2010); http://dx.doi.org/10.1118/1.3250856View Description Hide Description
Clinical outcomes of charged particle therapy are very promising. Currently, several dedicated centers that use scanning beam technology are either close to clinical use or under construction. Since scanned beam treatments of targets that move with respiration most likely result in marked local over- and underdosage due to interplay of target motion and dynamic beam application, dedicated motion mitigation techniques have to be employed. To date, the motion mitigation techniques, rescanning, beam gating, and beam tracking, have been proposed and tested in experimental studies. Rescanning relies on repeated irradiations of the target with the number of particles reduced accordingly per scan to statistically average local misdosage. Specific developments to prohibit temporal correlation between beam scanning and target motion will be required to guarantee adequate averaging. For beam gating, residual target motion within gating windows has to be mitigated in order to avoid local misdosage. Possibly the most promising strategy is to increase the overlap of adjacent particle pencil beams laterally as well as longitudinally to effectively reduce the sensitivity against small residual target motion. The most conformal and potentially most precise motion mitigation technique is beam tracking. Individual particle pencil beams have to be adapted laterally as well as longitudinally according to the target motion. Within the next several years, it can be anticipated that rescanning as well as beam gating will be ready for clinical use. For rescanning, treatment planning margins that incorporate the full extent of target motion as well as motion induced density variations in the beam paths will result in reduced target conformity of the applied dose distributions. Due to the limited precision of motion monitoring devices, it seems likely that beam gating will be used initially to mitigate interplay effects only but not to considerably decrease treatment planning margins. Then, in the next step, beam gating, based on more accurate motion monitoring systems, provides the possibility to restore target conformity as well as steep dose gradients due to reduced treatment planning margins. Accurate motion monitoring systems will be required for beam tracking. Even though beam tracking has already been successfully tested experimentally, full clinical implementation requires direct feedback of the actual target position in quasireal time to the treatment control system and can be anticipated to be several more years ahead.
- RADIATION THERAPY PHYSICS
37(2010); http://dx.doi.org/10.1118/1.3264176View Description Hide DescriptionPurpose:
Intensity modulated radiation therapy(IMRT) allows the delivery of escalated radiationdose to tumor while sparing adjacent critical organs. In doing so, IMRT plans tend to incorporate steep dose gradients at interfaces between the target and the organs at risk. Current quality assurance (QA) verification tools such as 2D diode arrays, are limited by their spatial resolution and conventional films are nonreal time. In this article, the authors describe a novel silicon strip detector (CMRP DMG) of high spatial resolution suitable for measuring the high dose gradients in an IMRT delivery.Methods:
A full characterization of the detector was performed, including dose per pulse effect, percent depth dose comparison with Farmer ion chamber measurements, stem effect, dose linearity, uniformity, energy response, angular response, and penumbra measurements. They also present the application of the CMRP DMG in the dosimetric verification of a clinical IMRT plan.Results:
The detector response changed by 23% for a 390-fold change in the dose per pulse. A correction function is derived to correct for this effect. The strip detector depth dose curve agrees with the Farmer ion chamber within 0.8%. The stem effect was negligible (0.2%). The dose linearity was excellent for the dose range of. A uniformity correction method is described to correct for variations in the individual detector pixel responses. The detector showed an over-response relative to tissue dose at lower photon energies with the maximum dose response at nominal photon energy. Penumbra studies using a Varian Clinac 21EX at 1.5 and depths were measured to be 2.77 and for the secondary collimators, 3.52 and for the multileaf collimator rounded leaf ends, respectively. Point doses measured with the strip detector were compared to doses measured with EBT film and doses predicted by the Philips Pinnacle treatment planning system. The differences were and , respectively. They demonstrated the high temporal resolution capability of the detector readout system, which will allow one to investigate the temporal dose pattern of IMRT and volumetric modulated arc therapy (VMAT) deliveries.Conclusions:
The CMRP silicon strip detectordose magnifying glass interfaced to a TERA ASIC DAQ system has high spatial and temporal resolution. It is a novel and valuable tool for QA in IMRTdose delivery and for VMAT dose delivery.
37(2010); http://dx.doi.org/10.1118/1.3276778View Description Hide DescriptionPurpose:
The design of a 3D in-line side-coupled 6 MV linac waveguide for medical use is given, and the effect of the side-coupling and port irises on the radio frequency (RF), beam dynamics, and dosimetric solutions is examined. This work was motivated by our research on a linac-MR hybrid system, where accurate electron trajectory information for a clinical medical waveguide in the presence of an external magnetic field was needed.Methods:
For this work, the design of the linac waveguide was generated using the finite element method. The design outlined here incorporates the necessary geometric changes needed to incorporate a full-end accelerating cavity with a single-coupling iris, a waveguide-cavity coupling port iris that allows power transfer into the waveguide from the magnetron, as well as a method to control the RF field magnitude within the first half accelerating cavity into which the electrons from the gun are injected.Results:
With the full waveguide designed to resonate at, a full 3D RF field solution was obtained. The accuracy of the 3D RF field solution was estimated through a comparison of important linac parameters ( factor, shunt impedance, transit time factor, and resonant frequency) calculated for one accelerating cavity with the benchmarked program SUPERFISH. It was found that the maximum difference between the 3D solution and SUPERFISH was less than 0.03%. The eigenvalue solver, which determines the resonant frequencies of the 3D side-coupled waveguide simulation, was shown to be highly accurate through a comparison with lumped circuit theory. Two different waveguide geometries were examined, one incorporating a 0.5 mm first side cavity shift and another with a 1.5 mm first side cavity shift. The asymmetrically placed side-coupling irises and the port iris for both models were shown to introduce asymmetries in the RF field large enough to cause a peak shift and skewing (center of gravity minus peak shift) of an initially cylindrically uniform electron beam accelerating within the waveguide. The shifting and skewing of the electron beam were found to be greatest due to the effects of the side-coupling irises on the RF field. A further Monte Carlo study showed that this effect translated into a 1% asymmetry in a field dose profile.Conclusions:
A full 3D design for an in-line side-coupled 6 MV linear accelerator that emulates a common commercial waveguide has been given. The effect of the side coupling on the dose distribution has been shown to create a slight asymmetry, but overall does not affect the clinical applicability of the linac. The 3D in-line side-coupled linac model further provides a tool for the investigation of linac performance within an external magnetic field, which exists in an integrated linac-MR system.
Assessment of the setup dependence of detector response functions for mega-voltage linear accelerators37(2010); http://dx.doi.org/10.1118/1.3284529View Description Hide DescriptionPurpose:
Accurate modeling of beam profiles is important for precise treatment planning dosimetry. Calculated beam profiles need to precisely replicate profiles measured during machine commissioning. Finite detector size introduces perturbations into the measured profiles, which, in turn, impact the resulting modeled profiles. The authors investigate a method for extracting the unperturbed beam profiles from those measured during linear accelerator commissioning.Methods:
In-plane and cross-plane data were collected for an Elekta Synergy linac at 6 MV using ionization chambers of volume 0.01, 0.04, 0.13, and and a diode of surface area . The detectors were orientated with the stem perpendicular to the beam and pointing away from the gantry. Profiles were measured for a field at depths ranging from 0.8 to 25.0 cm and SSDs from 90 to 110 cm. Shaping parameters of a Gaussian response function were obtained relative to the Edge detector. The Gaussian function was deconvolved from the measured ionization chamber data. The Edge detector profile was taken as an approximation to the true profile, to which deconvolved data were compared. Data were also collected with CC13 and Edge detectors for additional fields and energies on an Elekta Synergy, Varian Trilogy, and Siemens Oncor linear accelerator and response functions obtained. Response functions were compared as a function of depth, SSD, and detector scan direction. Variations in the shaping parameter were introduced and the effect on the resulting deconvolution profiles assessed.Results:
Up to 10% setup dependence in the Gaussian shaping parameter occurred, for each detector for a particular plane. This translated to less than a ±0.7 mm variation in the 80%–20% penumbral width. For large volume ionization chambers such as the FC65 Farmer type, where the cavity length to diameter ratio is far from 1, the scan direction produced up to a 40% difference in the shaping parameter between in-plane and cross-plane measurements. This is primarily due to the directional difference in penumbral width measured by the FC65 chamber, which can more than double in profiles obtained with the detector stem parallel compared to perpendicular to the scan direction. For the more symmetric CC13 chamber the variation was only 3% between in-plane and cross-plane measurements.Conclusions:
The authors have shown that the detector response varies with detector type, depth, SSD, and detector scan direction. In-plane vs cross-plane scanning can require calculation of a direction dependent response function. The effect of a 10% overall variation in the response function, for an ionization chamber, translates to a small deviation in the penumbra from that of the Edge detector measured profile when deconvolved. Due to the uncertainties introduced by deconvolution the Edge detector would be preferable in obtaining an approximation of the true profile, particularly for field sizes where the energy dependence of the diode can be neglected. However, an averaged response function could be utilized to provide a good approximation of the true profile for large ionization chambers and for larger fields for which diode detectors are not recommended.
Comparison of Monte Carlo collimator transport methods for photon treatment planning in radiotherapy37(2010); http://dx.doi.org/10.1118/1.3284978View Description Hide DescriptionPurpose:
The aim of this work was a Monte Carlo(MC) based investigation of the impact of different radiation transport methods in collimators of a linear accelerator on photonbeam characteristics, dose distributions, and efficiency. Thereby it is investigated if it is possible to use different simplifications in the radiation transport for some clinical situations in order to save calculation time.Methods:
Within the Swiss Monte Carlo Plan, a GUI-based framework for photonMCtreatment planning, different MC methods are available for the radiation transport through the collimators [secondary jaws and multileaf collimator(MLC)]: EGSnrc (reference),, and Pin (an in-house developed MC code). Additional nonfull transport methods were implemented in order to provide different complexity levels for the MC simulation: Considering collimator attenuation only, considering Compton scatter only or just the firstCompton process, and considering the collimators as totally absorbing. Furthermore, either a simple or an exact geometry of the collimators can be selected for the absorbing or attenuation method. Phasespaces directly above and dose distributions in a water phantom are analyzed for academic and clinical treatment fields using 6 and 15 MV beams, including intensity modulated radiation therapy with dynamic MLC.Results:
For all MC transport methods, differences in the radial mean energy and radial energy fluence are within 1% inside the geometric field. Below the collimators, the energy fluence is underestimated for nonfull MC transport methods ranging from 5% for Compton to 100% for Absorbing. Gamma analysis using EGSnrc calculated doses as reference shows that the percentage of voxels fulfilling a 1% /1 mm criterion is at least 98% when using, Compton, or firstCompton transport methods. When using the methods Pin, Transmission, Flat-Transmission, Flat-Absorbing or Absorbing, the mean value of points fulfilling this criterion over all tested cases is 97%, 88%, 74%, 68%, or 65%, respectively. However, compared to EGSnrc calculations, the gain in efficiency is a factor of up to 10 for and up to 48 for the absorbing method.Conclusions:
The results of this investigation suggest that it is an option to use a simple transport technique in the initial treatment planning process and use more accurate transport methods for the final dose calculation accepting longer calculation times.
37(2010); http://dx.doi.org/10.1118/1.3276775View Description Hide DescriptionPurpose:
To evaluate the utility of a new complexity metric, the modulation complexity score (MCS), in the treatment planning and quality assurance processes and to evaluate the relationship of the metric with deliverability.Methods:
A multisite (breast, rectum, prostate, prostate bed, lung, and head and neck) and site-specific (lung)dosimetric evaluation has been completed. The MCS was calculated for each beam and the overall treatment plan. A 2D diode array (MapCHECK™, Sun Nuclear, Melbourne, FL) was used to acquire measurements for each beam. The measured and planned dose (PINNACLE3, Phillips, Madison, WI) was evaluated using different percent differences and distance to agreement (DTA) criteria (3%/3 mm and 2%/1 mm) and the relationship between the dosimetric results and complexity (as measured by the MCS or simple beam parameters) assessed.Results:
For the multisite analysis (243 plans total), the mean MCS scores for each treatment site were breast (0.92), rectum (0.858), prostate (0.837), prostate bed (0.652), lung (0.631), and head and neck (0.356). The MCS allowed for compilation of treatment site-specific statistics, which is useful for comparing different techniques, as well as for comparison of individual treatment plans with the typical complexity levels. For the six plans selected for dosimetry, the average diode percent pass rate was 98.7% (minimum of 96%) for 3%/3 mm evaluation criteria. The average difference in absolute dose measurement between the planned and measured dose was 1.7 cGy. The detailed lung analysis also showed excellent agreement between the measured and planned dose, as all beams had a diode percentage pass rate for 3%/3 mm criteria of greater than 95.9%, with an average pass rate of 99.0%. The average absolute maximum dose difference for the lung plans was 0.7 cGy. There was no direct correlation between the MCS and simple beam parameters which could be used as a surrogate for complexity level (i.e., number of segments or MU). An evaluation criterion of 2%/1 mm reliably allowed for the identification of beams that are dosimetrically robust. In this study we defined a robust beam or plan as one that maintained a diode percentage pass rate greater than 90% at 2%/1 mm, indicating delivery that was deemed accurate when compared to the planned dose, even under stricter evaluation criterion. MCS and MU threshold criteria were determined by defining a required specificity of 1.0. A MCS threshold of 0.8 allowed for identification of robust deliverability with a sensitivity of 0.36. In contrast, MU had a lower sensitivity of 0.23 for a threshold of 50 MU.Conclusions:
The MCS allows for a quantitative assessment of plan complexity, on a fixed scale, that can be applied to all treatment sites and can provide more information related to dose delivery than simple beam parameters. This could prove useful throughout the entire treatment planning and QA process.
The use of PET images for radiotherapy treatment planning: An error analysis using radiobiological endpoints37(2010); http://dx.doi.org/10.1118/1.3276776View Description Hide DescriptionPurpose:
There is significant current interest in the use of biological image guidance in radiotherapy planning. In lung-cancer treatment, tumor motion due to respiration is known to be a limitation. This is particularly true in PET, where image data are collected over a number of minutes. An in-house-developed 4D PET acquisition mode is described and an analysis of the effects of acquisition parameters on the reconstructed image quality is presented. The potential impact of the resulting biological image quality on radiotherapy planning is then quantified in terms of tumor control probability (TCP).Methods:
Data were acquired using a human torso phantom comprised of a hot-filled spheroidal “tumor” ( in diameter) suspended in an air-filled “lung” cylinder and surrounded by a warm -filled background. Two different sphere-to-background (S/B) ratios were used. The tumor was connected to a 3-axis computer-controlled motion stage and could be moved during PET data acquisition. Images were acquired with a range of count statistics, motion blurring, and CT attenuation correction (CTAC) misalignment. Four simple models were proposed for the assignment of clonogenic cell density according to the voxel value. The impact of image artifacts was then assessed by calculating the TCP, which is the probability that no clonogenic tumor cell remains after a given dose of radiation. TCP was calculated for a uniform dose distribution in the tumor.Results:
Reduced count statistics and misaligned CTAC images had the most detrimental impact on the image fidelity. It was found that in both cases the images became less intense, demonstrated by smaller number of voxels at the maximum values. The maximum TCP difference between images with the least and most noise was 3.4%, and with weakest and strongest CT misalignment artifacts, it was 3.2% . Motion blurring only contributed weakly to the TCP imprecision at 1.7% between best- and worst-case images. However, the model-calculated TCP showed increasing differences from the ground truth as the complexity of the model increased [maximum difference of (model 3)], which could be attributed to the partial volume effect.Conclusions:
Based on the results of this study, it is believed that simple techniques of biologically guided radiotherapy planning for lungcancer should be feasible at intermediate contrast levels (tumor-to-background ratio of) with the clinically achievable image quality.
37(2010); http://dx.doi.org/10.1118/1.3284359View Description Hide DescriptionPurpose:
Recently a commercial Monte Carlo based IMRT planning system (Monaco version 1.0.0) was released. In this study the dosimetric accuracy of this new planning system was validated.Methods:
Absolute dose profiles, depth dose curves, and output factors calculated by Monaco were compared with measurements in a water phantom. Different static on-axis and off-axis fields were tested at various source-skin distances for 6, 10, and 18 MV photon beams. Four clinical IMRT plans were evaluated in a water phantom using a linear diode detector array and another six IMRT plans for different tumor sites in solid water using a 2D detector array. In order to evaluate the accuracy of the dose engine near tissue inhomogeneities absolute dose distributions were measured with Gafchromic EBT film in an inhomogeneous slab phantom. For an end-to-end test a four-field IMRT plan was applied to an anthropomorphic lung phantom with a simulated tumor peripherally located in the right lung. Gafchromic EBT film, placed in and around the tumor area, was used to evaluate the dose distribution.Results:
Generally, the measured and the calculated dose distributions agreed within 2% dose difference or 2 mm distance-to-agreement. But mainly at interfaces with bone, some larger dose differences could be observed.Conclusions:
Based on the results of this study, the authors concluded that the dosimetric accuracy of Monaco is adequate for clinical introduction.
Coverage optimized planning: Probabilistic treatment planning based on dose coverage histogram criteria37(2010); http://dx.doi.org/10.1118/1.3273063View Description Hide Description
This work (i) proposes a probabilistic treatment planning framework, termed coverage optimized planning (COP), based on dose coverage histogram (DCH) criteria; (ii) describes a concrete proof-of-concept implementation of COP within the PINNACLEtreatment planning system; and (iii) for a set of 28 prostate anatomies, compares COP plans generated with this implementation to traditional PTV-based plans generated with planning criteria approximating those in the high dose arm of the Radiation TherapyOncology Group 0126 protocol. Let denote the dose delivered to fractional volume of a structure. In conventional intensity modulated radiation therapy planning, has a unique value derived from the static (planned) dose distribution. In the presence of geometric uncertainties (e.g., setup errors) assumes a range of values. The DCH is the complementary cumulative distribution function of . DCHs are similar to dose volume histograms (DVHs). Whereas a DVH plots volume versus dose, a DCH plots coverage probability versus . For a given patient, is the probability (i.e., percentage of geometric uncertainties) for which the realized value of exceeds . PTV-based treatment plans can be converted to COP plans by replacing DVH optimization criteria with corresponding DCH criteria. In this approach, PTVs and planning organ at risk volumes are discarded, and DCH criteria are instead applied directly to clinical target volumes (CTVs) or organs at risk (OARs). Plans are optimized using a similar strategy as for DVH criteria. The specific implementation is described. COP was found to produce better plans than standard PTV-based plans, in the following sense. While target OAR dose tradeoff curves were equivalent to those for PTV-based plans, COP plans were able to exploit slack in OAR doses, i.e., cases where OAR doses were below their optimization limits, to increase target coverage. Specifically, because COP plans were not constrained by a predefined PTV, they were able to provide wider dosimetric margins around the CTV, by pushing OAR doses up to, but not beyond, their optimization limits. COP plans demonstrated improved target coverage when averaged over all 28 prostate anatomies, indicating that the COP approach can provide benefits for many patients. However, the degree to which slack OAR doses can be exploited to increase target coverage will vary according to the individual patient anatomy. The proof-of-concept COP implementation investigated here utilized a probabilistic DCH criteria only for the CTV minimum dose criterion. All other optimization criteria were conventional DVH criteria. In a mature COP implementation, all optimization criteria will be DCH criteria, enabling direct planning control over probabilistic dose distributions. Further research is necessary to determine the benefits of COP planning, in terms of tumor control probability and/or normal tissue complication probabilities.
37(2010); http://dx.doi.org/10.1118/1.3291622View Description Hide DescriptionPurpose:
The purpose of this study was to quantify the extent of energy dependence of Gafchromic film to x-ray energies ranging in quality from 105 kVp to 6 MV, and relate this dependency to the film’s chemical composition and date of production.Methods:
Lots of Gafchromic EBT film manufactured in 2004 and 2005 together with more recent batches produced in 2007 were evaluated for energy dependence. Multiple batches of EBT-2 film were also evaluated. Energy dependence was quantified as—the ratio of net optical density (netOD) measured at a given energy relative to the netOD measured at 6 MV, as measured on a linear accelerator. was evaluated for beam qualities of 105 and 220 kVp on a clinical orthovoltage unit using two separate techniques—a flatbed scanner (Epson) and a real-time fiber-optic readout system.Neutronactivation analysis for chlorine and bromine content was performed on all the films to determine whether the composition of the film had changed between batches of film exhibiting different energy dependence responses.Results:
For batches of EBT manufactured in 2007, was 0.75 and was 0.85, indicating an under-response at orthovoltage energies. These results were confirmed using both the Epson flatbed scanner as well as the real-time readout system. For batches of EBT film manufactured before 2006, ranged from 0.9 to 1.0. The results from the neutronactivation analysis confirmed a direct relationship between the concentration of chlorine and the magnitude of under-response at orthovoltage energies. EBT-2 film exhibited values ranging from 0.79 (under-response) to 1.20 (over-response) among batches containing varying concentrations of bromine, chlorine, and potassium.Conclusions:
The results of this study indicated that differences in energy response of EBT and EBT-2 films were due to differences in the chemical composition and therefore the effective atomic number of the film, which have changed over time. To achieve an energy independent dosimeter over a range of kilovoltage energies, the effective atomic number of the dosimeter must be closely matched to that of water. Small deviations in chemical composition can lead to large deviations in response as a function of energy.
Investigation of the effects of treatment planning variables in small animal radiotherapy dose distributions37(2010); http://dx.doi.org/10.1118/1.3276738View Description Hide DescriptionPurpose:
Methods used for small animal radiation treatment have yet to achieve the same dose targeting as in clinical radiation therapy. Toward understanding how to better plan small animal radiation using a system recently developed for this purpose, the authors characterized dose distributions produced from conformal radiotherapy of small animals in a microCT scanner equipped with a variable-aperture collimator.Methods:
Dose distributions delivered to a cylindrical solid water phantom were simulated using a Monte Carlo algorithm. Phase-space files for 120 kVp x-ray beams and collimator widths of 1–10 mm at isocenter were generated using BEAMnrc software, and dose distributions for evenly spaced beams numbered from 5 to 80 were generated in DOSXYZnrc for a variety of targets, including centered spherical targets in a range of sizes, spherical targets offset from centered by various distances, and various ellipsoidal targets. Dose distributions were analyzed using dose volume histograms. The dose delivered to a mouse bearing a spontaneous lungtumor was also simulated, and dose volume histograms were generated for the tumor,heart, left lung, right lung, and spinal cord.Results:
Results indicated that for centered, symmetric targets, the number of beams required to achieve a smooth dose volume histogram decreased with increased target size. Dose distributions for noncentered, symmetric targets did not exhibit any significant loss of conformality with increasing offset from the phantom center, indicating sufficient beam penetration through the phantom for targeting superficial targets from all angles. Even with variable collimator widths, targeting of asymmetric targets was found to have less conformality than that of spherical targets. Irradiation of a mouse lungtumor with multiple beam widths was found to effectively deliver dose to the tumor volume while minimizing dose to other critical structures.Conclusions:
Overall, this method of generating and analyzing dose distributions provides a quantitative method for developing practical guidelines for small animal radiotherapytreatment planning. Future work should address methods to improve conformality in asymmetric targets.
37(2010); http://dx.doi.org/10.1118/1.3291648View Description Hide DescriptionPurpose:
Validation of the targeting and dosedelivery of the IRay™ low voltage age-related macular degeneration treatment system.Methods:
Ten human cadaver eyes were obtained for this study and mounted in the IRay™ system. Using gel and vacuum, an I-Guide™ immobilization device was coupled to the eyes and radiochromic film was affixed to the posterior aspect of the globes. Three narrow x-ray beams were delivered through the pars plana to overlap on the predicted nominal fovea. A needle was placed through the center of the film’s beam spot and into the eye to register the film and the inner retina. The process was performed three times for each of the ten eyes (30 simulated treatments; 90 individual beams). The globes were dissected to assess the targeting accuracy by measuring the distances from the needles to the fovea. The dose to the fovea was calculated from the radiochromic film.Results:
X-ray targeting on the retina averaged from the fovea. Repeated treatments on the same eye showed a reproducibility of . The optic nerve was safely avoided, with the 90% isodose edge of the beam spot between 0.4 and 2.6 mm from the edge of the optic disk. Measured dose matched that prescribed.Conclusions:
This study provides confidence that the IRay™, with an average accuracy of 0.6 mm and a precision of 0.4 mm, can reliably treat most AMD lesions centered on the fovea. With the exception of motion, all sources of error are included.
Development of a population-based model of surface segmentation uncertainties for uncertainty-weighted deformable image registrations37(2010); http://dx.doi.org/10.1118/1.3284209View Description Hide DescriptionPurpose:
To develop a population-based model of surface segmentation uncertainties for uncertainty-weighted surface-based deformable registrations.Methods:
The contours of the prostate, the bladder, and the rectum were manually delineated by five observers on fan beam CTimages of four prostate cancer patients. First, patient-specific representations of structure segmentation uncertainties were derived by determining the interobserver variability (i.e., standard deviation) of the structure boundary delineation. This was achieved by (1) generating an average structuresurface mesh from the structure contours drawn by different observers, and (2) calculating three-dimensional standard deviation surface meshes (SDSMs) based on the perpendicular distances from the individual boundary surface meshes to the average surface mesh computed above. Then an average structuresurface mesh was constructed to be the reference mesh for the population-based model. The average structure meshes of the other patients were deformably registered to the reference mesh. The calculated deformable vector fields were used to map the patient-specific SDSMs to the reference mesh to obtain the registered SDSMs. Finally, the population-based SDSM was derived by taking the average of the registered SDSMs in quadrature.Results:
Population-based structuresurface statistical models of the prostate, the bladder, and the rectum were created by mapping the patient-specific SDSMs to the population surface model. Graphical visualization indicates that the boundary uncertainties are dependent on anatomical location.Conclusions:
The authors have developed and demonstrated a general method for objectively constructing surface maps of uncertainties derived from topologically complex structure boundary segmentations from multiple observers. The computed boundary uncertainties have significant spatial variations. They can be used as weighting factors for surface-based probabilistic deformable registration.
Technical Note: Development of a tidal volume surrogate that replaces spirometry for physiological breathing monitoring in 4D CT37(2010); http://dx.doi.org/10.1118/1.3284282View Description Hide DescriptionPurpose:
Spirometry exhibits baseline drift and frequent measurement errors so it cannot be used by itself to provide tidal volume-based image sorting or breathing motion modeling. Other breathing surrogates, in this study an abdominal bellows system, are drift free but do not measure tidal volume. Simultaneously using spirometry and the bellows system allows the user to convert the recorded bellows signal to tidal volume but still relies on spirometry measurements. The authors therefore propose to use CT-based air content, rather than a spirometer, to convert the bellows signal to tidal volume.Methods:
41 4D CTdata sets are acquired, while the breathing cycle is simultaneously measured using spirometry and an abdominal pressure bellows system. The assumptions underlying the conversion of the bellows measurement to tidal volume by CT-based air content are analyzed. This comprises of detailed correlation studies of the spirometry-measured tidal volume, the bellows signal, and CT-based air content.Results:
For patients, the spirometry signals are not consistently acquired during the 4D CT session, so correlating spirometry to bellows measurements and CT-based air content leads to erroneous conversion coefficients. After introducing a minimum correlation threshold to remove these data, good correlations are obtained between the remaining breathing signals. The ratio of CT-based air content to tidal volume is measured to be ; the expected value is 1.11 because room air is 11% more dense than air in the lungs.Conclusions:
The observed problems of spirometry recording illustrate the challenges encountered when using spirometers as breathing surrogate for 4D CT acquisition. The high correlation between spirometry and bellows breathing signals and the verified factor of 1.11 between CT-based air content and tidal volume mean that the bellows measurement (or other equivalent surrogates) can be reliably converted to tidal volume using the CT-based air content, avoiding the need for a spirometer.
37(2010); http://dx.doi.org/10.1118/1.3284249View Description Hide DescriptionPurpose:
Anatomical deformations of prostate-bed, rectum, and bladder can compromise the targeting accuracy in post-prostatectomy cancer patients. In this work, the impact of anatomical interventions on the localization data from post-prostatectomy patients who received image-guided IMRT was analyzed.Methods:
Patients were localized daily with online kilovoltage cone-beam computed tomography (kV-CBCT). The target and the organs at risk (OARs) positional and volumetric changes were evaluated and couch shifts were applied. For patients with large target or OAR volumetric changes, quantified by either a rectal or bladder wall displacement of on the CBCT sagittal images compared to the planning CT, repeated localization CBCT scans were performed following an interventional procedure. The procedure involves insertion of a catheter to deflate the rectum, evacuation of stools, and/or adjustment of bladder filling. The required shifts were then evaluated, and the IMRT treatment was subsequently delivered after proper patient positioning. The pre- and post-intervention shifts were compared in the lateral [left-right (LR)], longitudinal [superior-inferior (SI)], and vertical [anterior-posterior (AP)] directions. The percentage of shifts larger than 5 mm in all directions was also compared. Clinical target volume to planning target volume (CTV-to-PTV) expansion margins were estimated based on the pre- and post-intervention localization data.Results:
Intervention was performed on all patientstreated between October 2008 and March 2009. The number of interventions ranged from 2 to 12 with a median number of 5, resulting in a total of 96 pairs of pre- and post-intervention shifts. The mean value and standard deviation of the shifts from pre- versus post-intervention data were LR, vs ; SI, vs ; and AP, vs . The mean 3D shift distance was vs . The percentage of pre-intervention shifts larger than 5 mm were 7%, 7%, and 45% in the LR, SI, and AP directions, respectively, compared to 8%, 4%, and 21% for post-intervention. Localization data from pre- and post-intervention procedures suggest that treatments that do not include intervention to correct for rectum/bladder anatomical variations require an additional 3.3 mm CTV-to-PTV margin.Conclusions:
Anatomical interventions reduced the localization errors arising from large volume and shape changes in the rectum and/or bladder compared to treatments without interventions.
Dosimetric accuracy of a deterministic radiation transport based brachytherapy treatment planning system. Part I: Single sources and bounded homogeneous geometries37(2010); http://dx.doi.org/10.1118/1.3290630View Description Hide DescriptionPurpose:
The aim of this work is to validate a deterministic radiation transport based treatment planning system (TPS) for singlebrachytherapy source dosimetry in homogeneous water geometries.Methods:
TPS results were obtained using the deterministic radiation transport option of aBRACHYVISION v. 8.8 system for three characteristic source designs (VS2000, GMPlus HDR, and GMPlus PDR) with each source either centered in a 15 cm radius spherical water phantom, or positioned at varying distance away from the phantom center. Corresponding MC simulations were performed using the MCNPX code v.2.5.0 and source geometry models prepared using information provided by the manufacturers.Results:
Comparison in terms of the AAPM TG-43 dosimetric formalism quantities, as well as dose rate distributions per unit air kerma strength with a spatial resolution of 0.1 cm, yielded close agreement between TPS and MC results for the sources centered in the phantom. Besides some regions close to the source longitudinal axes where discrepancies could be characterized as systematic, overall agreement for all three sources studied is comparable to the statistical (type A) uncertainty of MC simulations (1% at the majority of points in the geometry increasing to 2%–3% at points lying both away from the source center and close to the source longitudinal axis). A corresponding good agreement was also found between TPS and MC results for the sources positioned away from the phantom center.Conclusions:
Results of this work attest the capability of the TPS to accurately account for the scatter conditions regardless of the size or shape of a given geometry of dosimetric interest, and the position of a source within it. This is important since, as shown in the literature and summarized also in this work, these factors could introduce a significant dosimetric effect that is currently ignored in clinical treatment planning. It is concluded that the implementation of the deterministic radiation transport option of theBRACHYVISION v. 8.8 system for brachytherapydosimetry in homogeneous water geometries yields results of comparable accuracy to the golden standard of Monte Carlo simulation, in clinically viable calculation times.
Comparison of organ doses for patients undergoing balloon brachytherapy of the breast with HDR or electronic sources using Monte Carlo simulations in a heterogeneous human phantoma)37(2010); http://dx.doi.org/10.1118/1.3292292View Description Hide DescriptionPurpose:
Accelerated partial breast irradiation via interstitial balloon brachytherapy is a fast and effective treatment method for certain early stage breast cancers. The radiation can be delivered using a conventional high-dose rate (HDR) gamma-emitting source or a novel electronic brachytherapy (eBx) source which uses lower energy x rays that do not penetrate as far within the patient. A previous study [A. Dickler, M. C. Kirk, N. Seif, K. Griem, K. Dowlatshahi, D. Francescatti, and R. A. Abrams, “A dosimetric comparison of MammoSite high-dose-rate brachytherapy and Xoft Axxent electronic brachytherapy,” Brachytherapy6, 164–168 (2007)] showed that the target dose is similar for HDR and eBx. This study compares these sources based on the dose received by healthy organs and tissues away from the treatment site.Methods:
A virtual patient with left breast cancer was represented by a whole-body, tissue-heterogeneous female voxel phantom. Monte Carlo methods were used to calculate the dose to healthy organs in a virtual patient undergoing balloon brachytherapy of the left breast with HDR or eBx sources. The dose-volume histograms for a few organs which received large doses were also calculated. Additional simulations were performed with all tissues in the phantom defined as water to study the effect of tissue inhomogeneities.Results:
For both HDR and eBx, the largest mean organdoses were received by the ribs, thymus gland, left lung,heart, and sternum which were close to the brachytherapy source in the left breast. eBx yielded mean healthy organdoses that were more than a factor of smaller than for HDR for all organs considered, except for the three closest ribs. Excluding these ribs, the average and median dose-reduction factors were and , respectively. The volume distribution of doses in nearby soft tissueorgans that were outside the PTV were also improved with eBx. However, the maximum dose to the closest rib with the eBx source was 5.4 times greater than that of the HDR source. The ratio of tissue-to-water maximum rib dose for the eBx source was .Conclusions:
The results of this study indicate that eBx may offer lower toxicity to most healthy tissues, except nearby bone. TG-43 methods have a tendency to underestimate dose to bone, especially the ribs. Clinical studies evaluating the negative health effects caused by irradiating healthy organs are needed so that physicians can better understand when HDR or eBx might best benefit a patient.
The effects of dose calculation resolution on dose accuracy for radiation therapy treatments of the lung. Part I. A Monte Carlo model of the lung37(2010); http://dx.doi.org/10.1118/1.3285042View Description Hide Description
Purpose: The purpose of this work was to create an anatomically detailed EGSnrc Monte Carlo based model of the right lung. The resulting model, called BRANCH, includes an accurate representation of the right bronchial, arterial, and venous branching networks down to a scale of 0.1 mm. The model may be varied to represent lung shape and density at any phase of the respiration cycle.
Methods:Polynomial surfaces were used to approximate the anatomic boundaries that define the right lung surface at several phases of the respiration cycle. A branching network algorithm was used to generate the bronchial, arterial, and venous trees within the anatomic boundaries. The branching networks were modeled as a series of bifurcating cylinders connected by spherical junctions. The validity of the BRANCH dose calculation was verified using an all-water version of the model.
Results: The geometric dimensions of the BRANCH model corresponded well with published data. The bronchial tree model contained 27 798 branches ranging from 0.02 to 0.54 cm in diameter. The arterial tree model had 27 957 branches ranging from 0.02 to 1.2 cm in diameter. The venous model tree had 26 347 branches ranging from 0.02 to 0.34 cm in diameter. A gamma analysis indicated that the all-water BRANCH Monte Carlo code produced dose distributions that agreed within 0.1 cm and 0.5% to conventional DOSXYZnrc results.
Conclusions: The BRANCH model is a useful tool for performing detailed dosimetric studies within a realistic representation of the lung.
The effects of dose calculation resolution on dose accuracy for radiation therapy treatments of the lung. Part II. A comparison of dose distributions from an explicit lung model to dose distributions derived from a CT representation37(2010); http://dx.doi.org/10.1118/1.3285043View Description Hide DescriptionPurpose:
Due to limitations in computer memory and computation time, typical radiation therapytreatments are calculated with a voxel dimension on the order of several millimeters. The anatomy below this practical resolution is approximated as a homogeneous region uniform in atomic composition and density. The purpose of this article is to examine whether the exclusion of anatomic structure below the practical dose calculation resolution produces deviations in the resulting dose distributions.Methods:
EGSnrc calculated dose distributions from the BRANCH lung model of Part I are compared and contrasted to dose distributions from a CT representation of the same BRANCH model for three different phases of the respiration cycle.Results:
The exclusion of branching structures below a CT resolution of resulted in a deviation in dose. The deviation in dose was as high as 14% but was localized around the branching structures. There was no significant variation in the dose deviation as a function of either field size or lung density.Conclusions:
The exclusion of explicit branching structures of the lung in a CT representation creates localized deviations in dose. To ensure accurate dose calculations, CT resolution must be increased.