Biomedical subjects

James F Dempsey

Publications and source records attributed to James F Dempsey.

At least 19 recordsLinked to original sources

PubMed2006

The impact of heterogeneity correction on dosimetric parameters that predict for radiation pneumonitis.

PURPOSE: To determine if heterogeneity correction significantly affects commonly measured dosimetric parameters predicting pulmonary toxicity in patients receiving radiation for lung cancer. METHODS AND MATERIALS: Sixty-eight patients treated for lung cancer were evaluated. The conformal treatment technique mostly employed anteroposterior/posterior-anterior fields and off-cord obliques. The percent total lung volume receiving 20 Gy or higher (V20) and mean lung dose (MLD) were correlated with the incidence of radiation pneumonitis. Parameters from both heterogeneity-corrected and heterogeneity-uncorrected plans were used to assess this risk. RESULTS: Univariate analysis revealed a significant correlation between the development of radiation pneumonitis and both V20 and MLD. A best-fit line to a plot of V20 from the homogeneous plan against the corresponding V20 heterogeneous value produced a slope of 1.00 and zero offset, indicating no difference between the two parameters. For MLD, a similarly significant correlation is seen between the heterogeneous and homogeneous parameters, indicating a 4% difference when correcting for heterogeneity. A significant correlation was also observed between the MLD and V20 parameters (p < 0.0001). CONCLUSIONS: A high degree of correlation exists between heterogeneity-corrected and heterogeneity-uncorrected dosimetric parameters for lung and the risk of developing pneumonitis. Either V20 or MLD predicts the pneumonitis risk with similar effect.

Adult↗

PubMed2006

A high-speed scintillation-based electronic portal imaging device to quantitatively characterize IMRT delivery.

We have developed an electronic portal imaging device (EPID) employing a fast scintillator and a high-speed camera. The device is designed to accurately and independently characterize the fluence delivered by a linear accelerator during intensity modulated radiation therapy (IMRT) with either step-and-shoot or dynamic multileaf collimator (MLC) delivery. Our aim is to accurately obtain the beam shape and fluence of all segments delivered during IMRT, in order to study the nature of discrepancies between the plan and the delivered doses. A commercial high-speed camera was combined with a terbium-doped gadolinium-oxy-sulfide (Gd2O2S:Tb) scintillator to form an EPID for the unaliased capture of two-dimensional fluence distributions of each beam in an IMRT delivery. The high speed EPID was synchronized to the accelerator pulse-forming network and gated to capture every possible pulse emitted from the accelerator, with an approximate frame rate of 360 frames-per-second (fps). A 62-segment beam from a head-and-neck IMRT treatment plan requiring 68 s to deliver was recorded with our high speed EPID producing approximately 6 Gbytes of imaging data. The EPID data were compared with the MLC instruction files and the MLC controller log files. The frames were binned to provide a frame rate of 72 fps with a signal-to-noise ratio that was sufficient to resolve leaf positions and segment fluence. The fractional fluence from the log files and EPID data agreed well. An ambiguity in the motion of the MLC during beam on was resolved. The log files reported leaf motions at the end of 33 of the 42 segments, while the EPID observed leaf motions in only 7 of the 42 segments. The static IMRT segment shapes observed by the high speed EPID were in good agreement with the shapes reported in the log files. The leaf motions observed during beam-on for step-and-shoot delivery were not temporally resolved by the log files.

Computer Systems↗

PubMed2006

Fast voxel and polygon ray-tracing algorithms in intensity modulated radiation therapy treatment planning.

We present work on combining three algorithms to improve ray-tracing efficiency in radiation therapy dose computation. The three algorithms include: An improved point-in-polygon algorithm, incremental voxel ray tracing algorithm, and stereographic projection of beamlets for voxel truncation. The point-in-polygon and incremental voxel ray-tracing algorithms have been used in computer graphics and nuclear medicine applications while the stereographic projection algorithm was developed by our group. These algorithms demonstrate significant improvements over the current standard algorithms in peer reviewed literature, i.e., the polygon and voxel ray-tracing algorithms of Siddon for voxel classification (point-in-polygon testing) and dose computation, respectively, and radius testing for voxel truncation. The presented polygon ray-tracing technique was tested on 10 intensity modulated radiation therapy (IMRT) treatment planning cases that required the classification of between 0.58 and 2.0 million voxels on a 2.5 mm isotropic dose grid into 1-4 targets and 5-14 structures represented as extruded polygons (a.k.a. Siddon prisms). Incremental voxel ray tracing and voxel truncation employing virtual stereographic projection was tested on the same IMRT treatment planning cases where voxel dose was required for 230-2400 beamlets using a finite-size pencil-beam algorithm. Between a 100 and 360 fold cpu time improvement over Siddon's method was observed for the polygon ray-tracing algorithm to perform classification of voxels for target and structure membership. Between a 2.6 and 3.1 fold reduction in cpu time over current algorithms was found for the implementation of incremental ray tracing. Additionally, voxel truncation via stereographic projection was observed to be 11-25 times faster than the radial-testing beamlet extent approach and was further improved 1.7-2.0 fold through point-classification using the method of translation over the cross product technique.

Algorithms↗

PubMed2006

A fourier analysis on the maximum acceptable grid size for discrete proton beam dose calculation.

We developed an analytical method for determining the maximum acceptable grid size for discrete dose calculation in proton therapy treatment plan optimization, so that the accuracy of the optimized dose distribution is guaranteed in the phase of dose sampling and the superfluous computational work is avoided. The accuracy of dose sampling was judged by the criterion that the continuous dose distribution could be reconstructed from the discrete dose within a 2% error limit. To keep the error caused by the discrete dose sampling under a 2% limit, the dose grid size cannot exceed a maximum acceptable value. The method was based on Fourier analysis and the Shannon-Nyquist sampling theorem as an extension of our previous analysis for photon beam intensity modulated radiation therapy [J. F. Dempsey, H. E. Romeijn, J. G. Li, D. A. Low, and J. R. Palta, Med. Phys. 32, 380-388 (2005)]. The proton beam model used for the analysis was a near monoenergetic (of width about 1% the incident energy) and monodirectional infinitesimal (nonintegrated) pencil beam in water medium. By monodirection, we mean that the proton particles are in the same direction before entering the water medium and the various scattering prior to entrance to water is not taken into account. In intensity modulated proton therapy, the elementary intensity modulation entity for proton therapy is either an infinitesimal or finite sized beamlet. Since a finite sized beamlet is the superposition of infinitesimal pencil beams, the result of the maximum acceptable grid size obtained with infinitesimal pencil beam also applies to finite sized beamlet. The analytic Bragg curve function proposed by Bortfeld [T. Bortfeld, Med. Phys. 24, 2024-2033 (1997)] was employed. The lateral profile was approximated by a depth dependent Gaussian distribution. The model included the spreads of the Bragg peak and the lateral profiles due to multiple Coulomb scattering. The dependence of the maximum acceptable dose grid size on the orientation of the beam with respect to the dose grid was also investigated. The maximum acceptable dose grid size depends on the gradient of dose profile and in turn the range of proton beam. In the case that only the phantom scattering was considered and that the beam was aligned with the dose grid, grid sizes from 0.4 to 6.8 mm were required for proton beams with ranges from 2 to 30 cm for 2% error limit at the Bragg peak point. A near linear relation between the maximum acceptable grid size and beam range was observed. For this analysis model, the resolution requirement was not significantly related to the orientation of the beam with respect to the grid.

Algorithms↗

PubMed2005

Novel breathing motion model for radiotherapy.

PURPOSE: An accurate model of breathing motion under quiet respiration is desirable to obtain the most accurate and conformal dose distributions for mobile lung cancer lesions. On the basis of recent lung motion measurements and the physiologic functioning of the lungs, we have determined that the motion of lung and lung tumor tissues can be modeled as a function of five degrees of freedom, the position of the tissues at a user-specified reference breathing phase, tidal volume and its temporal derivative airflow (tidal volume phase space). Time is an implicit variable in this model. METHODS AND MATERIALS: To test this hypothesis, a mathematical model of motion was developed that described the motion of objects p in the lungs as linear functions of tidal volume and airflow. The position of an object was described relative to its position -->P0 at the reference tidal volume and zero airflow, and the motion of the object was referenced to this position. Hysteresis behavior was hypothesized to be caused by pressure imbalances in the lung during breathing and was, in this model, a function of airflow. The motion was modeled as independent tidal volume and airflow displacement vectors, with the position of the object at time t equal to the vector sum -->rP(t) = -->rv(t) + -->rf(t) where -->rv(t) and -->rf(t) were displacement vectors with magnitudes approximated by linear functions of the tidal volume and airflow. To test this model, we analyzed five-dimensional CT scans (CT scans acquired with simultaneous real-time monitoring of the tidal volume) of 4 patients. The scans were acquired throughout the lungs, but the trajectories were analyzed in the couch positions near the diaphragm. A template-matching algorithm was implemented to identify the positions of the points throughout the 15 scans. In total, 76 points throughout the 4 patients were tracked. The lateral motion of these points was minimal; thus, the model was described in two spatial dimensions, with a total of six parameters necessary to describe the 30 degrees of freedom inherent in the 15 positions. RESULTS: For the 76 evaluated points, the average discrepancy (the distance between the measured and prediction positions) of the 15 locations for each tracked point was 0.75 +/- 0.25 mm, with an average maximal discrepancy of 1.55 +/- 0.54 mm. The average discrepancy was also tabulated as a fraction of the breathing motion. Discrepancies of <10% and 15% of the overall motion occurred in 73% and 95% of the tracked points, respectively. CONCLUSION: The motion tracking algorithms are being improved and automated to provide more motion data to test the models. This may allow a measurement of the motion-fitting parameters throughout the lungs. If the parameters vary smoothly, interpolation may be possible, yielding a continuous mathematical model of the breathing motion throughout the lungs. The utility of the model will depend on its stability as a function of time. If the model is only robust during the measurement session, it may be useful for determining lung function. If it is robust for weeks, it may be useful for treatment planning and gating of lung treatments. The use of tidal volume phase space for characterizing breathing motion appears to have provided, for the first time, the potential for a patient-specific mathematical model of breathing motion.

Humans↗

PubMed2005

Late xerostomia after intensity-modulated radiation therapy versus conventional radiotherapy.

PURPOSE: To evaluate xerostomia following intensity-modulated radiation therapy (IMRT). MATERIALS AND METHODS: An institutional review board-approved study of xerostomia was conducted in 210 head and neck cancer patients who were beyond the acute recovery phase (> 1 year) after radiation therapy. Xerostomia was evaluated with the 8-question XQ developed and validated at the University of Michigan. RESULTS: Median XQ scores (normalized so that no xerostomia = 0 and maximum xerostomia = 100) were larynx-only group, 4 points, ipsilateral radiotherapy, 34 points; bilateral radiotherapy, 64 points; IMRT with mean contralateral parotid dose more than 26 Gy, 44 points; and IMRT with mean contralateral parotid dose 26 Gy or less, 32 points (P < 0.05 for all major comparisons). CONCLUSION: The XQ is an excellent tool for identifying important differences in xerostomia after radiation therapy for head and neck cancer. The authors' IMRT program decreases the severity of xerostomia symptoms compared with what is typically seen after conventional radiation therapy.

Adult↗

PubMed2005

A fourier analysis of the dose grid resolution required for accurate IMRT fluence map optimization.

We present a theoretical and empirical analysis of the errors associated with the spatial discretization of the dose grid employed in optimized intensity modulated radiation therapy (IMRT) treatment plans. An information theory based Fourier analysis of the accuracy of discrete representations of three-dimensional dose distributions is presented. When applied to beamlet-based IMRT dose distributions, the theory produces analytic integrals that can bound worst case aliasing errors that can occur regardless of the location and orientation of the dose grid. The predictions of this theory are compared to empirical results obtained by solving a linear-programming based fluence-map optimization model to global optimality. A reasonable agreement between worst case estimates and the empirical results is attributed to the fact that the optimization takes advantage of aliasing to produce an optimal plan. We predicted and empirically demonstrated that an isotropic dose grid with <2.5 mm spacing is sufficient to prevent dose errors larger than a percent. However, we noted that in practice this resolution is mostly needed in high-dose target regions. Finally, a multiresolution 2-4-6 mm spacing model was developed and empirically tested where these spacings were applied to targets, structures, and tissue, respectively.

Algorithms↗

PubMed2005

Evaluation of surface and build-up region dose for intensity-modulated radiation therapy in head and neck cancer.

Despite much development, there remains dosimetric uncertainty in the surface and build-up regions in intensity-modulated radiation therapy treatment plans for head and neck cancers. Experiments were performed to determine the dosimetric discrepancies in the surface and build-up region between the treatment planning system (TPS) prediction and experimental measurement using radiochromic film. A head and neck compression film phantom was constructed from two semicylindrical solid water slabs. Treatment plans were generated using two commercial TPSs (PINNACLE3 and CORVUS) for two cases, one with a shallow (approximately 0.5 cm depth) target and another with a deep (approximately 6 cm depth) target. The plans were evaluated for a 54 Gy prescribed dose. For each case, two pieces of radiochromic film were used for dose measurement. A small piece of film strip was placed on the surface and another was inserted within the phantom. Overall, both TPSs showed good agreement with the measurement. For the shallow target case, the dose differences were within +/- 300 cGy (5.6% with respect to the prescribed dose) for PINNACLE3 and +/- 240 cGy (4.4%) for CORVUS in 90% of the region of interest. For the deep target case, the dose differences were +/- 350 (6.5%) for PINNACLE3 and +/- 260 cGy (4.8%) for CORVUS in 90% of the region of interest. However, it was found that there were significant discrepancies from the surface to about 0.2 cm in depth for both the shallow and deep target cases. It was concluded that both TPSs overestimated the surface dose for both shallow and deep target cases. The amount of overestimation ranges from 400 to 1000 cGy (approximately 7.4% to 18.5% with respect to the prescribed dose, 5400 cGy).

Algorithms↗

PubMed2004

A unifying framework for multi-criteria fluence map optimization models.

Models for finding treatment plans for intensity modulated radiation therapy are usually based on a number of structure-based treatment plan evaluation criteria, which are often conflicting. Rather than formulating a model that a priori quantifies the trade-offs between these criteria, we consider a multi-criteria optimization approach that aims at finding the so-called undominated treatment plans. We present a unifying framework for studying multi-criteria optimization problems for treatment planning that establishes conditions under which treatment plan evaluation criteria can be transformed into convex criteria while preserving the set of undominated treatment plans. Such transformations are identified for many of the criteria that have been proposed to date, establishing equivalences between these criteria. In addition, it is shown that the use of a nonconvex criterion can often be avoided by transformation to an equivalent convex criterion. In particular, we show that models employing criteria such as tumour control probability, normal tissue complication probability, probability of uncomplicated tumour control, as well as sigmoidal transformations of (generalized) equivalent uniform dose are equivalent to models formulated in terms of separable voxel-based criteria that penalize dose in individual voxels.

Dose-Response Relationship, Radiation↗

PubMed2004

Verification of step-and-shoot IMRT delivery using a fast video-based electronic portal imaging device.

We present an investigation into the use of a fast video-based electronic portal-imaging device (EPID) to study intensity modulated radiation therapy (IMRT) delivery. The aim of this study is to test the feasibility of using an EPID system to independently measure the orchestration of collimator leaf motion and beam fluence; simultaneously measuring both the delivered field fluence and shape as it exits the accelerator head during IMRT delivery. A fast EPID that consists of a terbium-doped gadolinium oxysulphide (GdO2S:Tb) scintillator coupled with an inexpensive commercial 30 frames-per-second (FPS) CCD-video recorder (16.7 ms shutter time) was employed for imaging IMRT delivery. The measurements were performed on a Varian 2100 C/D linear accelerator equipped with a 120-leaf multileaf-collimator (MLC). A characterization of the EPID was performed that included measurements of spatial resolution, linac pulse-rate dependence, linear output response, signal uniformity, and imaging artifacts. The average pixel intensity for fields imaged with the EPID was found to be linear in the delivered monitor units of static non-IMRT fields between 3x3 and 15x15 cm2. A systematic increase of the average pixel intensity was observed with increasing field size, leading to a maximum variation of 8%. Deliveries of a clinical step-and-shoot mode leaf sequence were imaged at 600 MU/min. Measurements from this IMRT delivery were compared with experimentally validated MLC controller log files and were found to agree to within 5%. An analysis of the EPID image data allowed identification of three types of errors: (1) 5 out of 35 segments were undelivered; (2) redistributing all of the delivered segment MUs; and (3) leaf movement during segment delivery. Measurements with the EPID at lower dose rates showed poor agreement with log files due to an aliasing artifact. The study was extended to use a high-speed camera (1-1000 FPS and 10 micros shutter time) with our EPID to image the same delivery to demonstrate the feasibility of imaging without aliasing artifacts. High-speed imaging was shown to be a promising direction toward validating IMRT deliveries with reasonable image resolution and noise.

Electrons↗

PubMed2004

An extensive log-file analysis of step-and-shoot intensity modulated radiation therapy segment delivery errors.

We present a study to evaluate the monitor unit (MU), dosimetric, and leaf-motion errors found in the delivery of 91 step-and-shoot IMRT treatment plans performed at three nominal dose rates using a dual modality high energy Linac (Varian 2100 C/D, Varian Medical Systems Inc., Palo Alto, CA) equipped with a 120-leaf multileaf collimator (MLC). The analysis was performed by studying log files generated by the MLC controller system. Recent studies by our group have validated that the automatically generated MLC log files accurately record the actual system delivery. A total of 635 beams were delivered at three nominal dose rates: 100, 300, and 600 MU/min. The log files were manually retrieved and analysis software was developed to extract the recorded MU delivery and leaf positions for each segment. Our analysis revealed that the magnitude of segment MU errors were independent of the planned segment MUs. Segment MU errors were found to increase with dose rate having maximum errors per segment of +/-1.8 MU at 600 MU/min, +/-0.8 MU at 300 MU/min, and +/-0.5 MU at 100 MU/min. The total absolute MU error in each plan was observed to increase with the number of plan segments, with the trend increasing more rapidly for higher dose rates. Three dimensional dose distributions were recomputed based on the observed segment MU errors for three plans with large cumulative absolute MU errors. Comparison with the original treatment plans indicated no clinically significant consequences due to these errors. In addition, approximately 80% of the total segment deliveries reported at least one collimator leaf moving at least 1 mm (projected at isocenter) during segment delivery. Such errors occur near the end of segment delivery and have been previously observed by our group using a fast video-based electronic portal imaging device. At 600 MU/min, between 5% and 23% of the plan MUs were delivered during leaf motion that had exceeded a 1 mm position tolerance. These leaf motion errors were not included in the treatment plan recalculations performed in this study.

Biophysical Phenomena↗

PubMed2004

Comparison of small photon beams measured using radiochromic and silver-halide films in solid water phantoms.

In this study, we compared the dosimetric properties of four of the most commonly used films for megavoltage photon-beam dosimetry when irradiated under identical conditions by small multileaf-collimator (MLC) defined beamlets. Two silver-halide films (SHFs), Kodak XV2 and EDR2, and two radiochromic films (RCFs), Gafchromic HS and MD55-2, were irradiated by MLC-defined 1 x 1 cm2 beamlets from a Varian 2100 C/D linac equipped with a 120-leaf MLC. The beamlets were delivered with the accelerator gantry set laterally (90 degrees rotation) upon a solid-water compression film phantom at 100 cm source-to-surface distance which was positioned with the films parallel to the beam axis. Beamlets were delivered at central axis, 5.0 cm, and 10.5 cm off-axis for both leaf-end and leaf-side defined beamlets. The film dosimetry was performed using a quantitative optical density (OD) imaging system that was validated in a previous study. No significant differences between SHF and RCF measurements were observed in percentage depth doses, horizontal depth profiles, or two-dimension spatial isodose distributions in both the central axis and off-axis measurements. We found that regardless of the type of film used, RCF or SHF, a consistent data set for small beam dose modeling was generated. Previous validation studies based on the use of RCF and OD imaging system would indicate that all film produce an accurate result for small beam characterization.

Equipment Failure Analysis↗

PubMed2003

Web-based submission, archive, and review of radiotherapy data for clinical quality assurance: a new paradigm.

PURPOSE: To report on the implementation of a web-based system (the Resource Center for Emerging Technologies [RCET] System) that provides immediate access to the patient radiotherapy planning and delivery data for clinical quality assurance (QA) by the experts. MATERIALS AND METHODS: An infrastructure of comprehensive tools required for preparation, submission, auto-archiving, web-based review, and retrieval of diagnostic images, treatment planning images, and radiation therapy objects has been developed. These tools represent approximately 1.1 million lines of computer code development in seven languages (V, C++, Visual Basic, Java, ASP, HTML, and SQL) and consist of a secure auto-anonymizing upload and auto-archiving patient database, a web-based secure object archiving network system, a web-based rapid review tool, a web-based upload/download tool, and a personal computer client data application for data object preparation, visualization, and submission, named NetSys. The RCET system enables users to share radiotherapy data in a secure environment. This paradigm of electronic data exchange makes remote peer review very efficient and convenient. RESULTS: The RCET system can help the radiation therapy community ensure consistent evaluation of its therapies. It will encourage proactive QA. An example of proactive clinical QA would be to provide atlases of target and critical structure definitions, to serve as class solutions, as well as dose prescription, specification, and reporting examples for guidance to the radiation oncologists in the community. The web-based clinical quality assurance is ideally suited for emerging technologies in radiation therapy that generate complex and voluminous multimodality imaging and planning data. CONCLUSIONS: The RCET system enables users to share multimodality imaging data, radiation therapy planning, and delivery data on demand. Our design paradigm will allow rapid peer review of radiotherapy data through a simple personal computer-based web browser.

Humans↗

PubMed2003

A novel linear programming approach to fluence map optimization for intensity modulated radiation therapy treatment planning.

We present a novel linear programming (LP) based approach for efficiently solving the intensity modulated radiation therapy (IMRT) fluence-map optimization (FMO) problem to global optimality. Our model overcomes the apparent limitations of a linear-programming approach by approximating any convex objective function by a piecewise linear convex function. This approach allows us to retain the flexibility offered by general convex objective functions, while allowing us to formulate the FMO problem as a LP problem. In addition, a novel type of partial-volume constraint that bounds the tail averages of the differential dose-volume histograms of structures is imposed while retaining linearity as an alternative approach to improve dose homogeneity in the target volumes, and to attempt to spare as many critical structures as possible. The goal of this work is to develop a very rapid global optimization approach that finds high quality dose distributions. Implementation of this model has demonstrated excellent results. We found globally optimal solutions for eight 7-beam head-and-neck cases in less than 3 min of computational time on a single processor personal computer without the use of partial-volume constraints. Adding such constraints increased the running times by a factor of 2-3, but improved the sparing of critical structures. All cases demonstrated excellent target coverage (> 95%), target homogeneity (< 10% overdosing and < 7% underdosing) and organ sparing using at least one of the two models.

Algorithms↗

PubMed2003

A conceptual model integrating spatial information to assess target volume coverage for IMRT treatment planning.

PURPOSE: We propose a model that integrates the spatial location of each voxel within a clinical target volume (CTV) to differentiate the merit of intensity-modulated radiation therapy (IMRT) plans with similar dose-volume histogram (DVH). This conceptual model is based on the hypothesis that various subregions within a given CTV that may carry different degree of risk in containing microscopic disease. METHODS AND MATERIALS: We hypothesize that a correlation between the probability of microscopic tumor extension and the risk of lymph node metastasis of a particular voxel point within CTV can be inferred based on its distance from the surface of radiographically evident gross disease. A preliminary observation was gathered from existing clinicopathologic data, and, based on these observations, a conceptual model for exponential-decay microscopic-extension probability function around primary tumor and linear function parameters relating the likelihood of lymph node metastasis to the distance from primary tumor was proposed. This model was generated to provide scoring functions to examine the merit of IMRT plans. To test the feasibility of this model, we generated two IMRT plans with similar and clinically acceptable DVH-based CTV coverage. Planning data were transferred to a data analysis software package (Matlab, The Mathworks Inc.). A 3D scoring function was calculated for each voxel inside the CTV. The adequacy of target coverage was evaluated by several novel approaches: 2D dose-volume scoring-function histograms (DVSH), the integral probability of relative residual tumor burden (RRTB), and tumor control probabilities (TCP) employing the scoring function as a pseudo-clonogen density distribution. RESULTS: Incorporating parameters for the risk of containing microscopic disease in each voxel into the scoring function algorithm, 2D DVSHs, RRTBs, integral RRTBs, and TCPs were computed. On each axial image, an RRTB map could locate the regions at greatest risk. These scoring functions were able to differentiate the merit of CTV coverage of clinically different IMRT plans but having very similar DVHs; one with cold spots centrally located over the gross tumor, and the other with more acceptable cold spots on the periphery of the CTV further away from the gross tumor volume. CONCLUSIONS: We demonstrated the feasibility and potential utility of an IMRT scoring method derived from this conceptual modeling approach. These methods are capable of ranking treatment plans with similar DVH profiles but different underdosed regions within the target. We will examine the accuracy of model parameters by performing tumor-specific image-pathologic correlation studies. Upon validation of these parameters, incorporating this scoring function model into plan optimization may have the potential to avoid underdosing subvolumes within CTV that harbor a higher likelihood of microscopic disease.

Humans↗

PubMed2003

Patterns of failure in patients receiving definitive and postoperative IMRT for head-and-neck cancer.

PURPOSE: To analyze the patterns of locoregional failure in patients with head-and-neck cancer treated with inverse planning intensity-modulated radiotherapy (IMRT). METHODS AND MATERIALS: Between February 1997 and December 2000, 165 patients with histologically confirmed head-and-neck cancer were treated using a parotid-sparing inverse planning IMRT protocol. Thirty-nine patients who received either palliative repeat irradiation or IMRT as a boost were excluded from this analysis, leaving 126 patients for this analysis. Of the 126 patients, 30 were women and 96 were men (median age 56 years, range 13-84). Fifty-two patients (41%) received definitive IMRT. Of the 52 patients, 17 were treated with RT alone and 35 with concurrent cisplatin-based chemotherapy regimens. Seventy-four patients (59%) received postoperative IMRT. The median follow-up was 26 months (range 12-55). IMRT was used only in the upper neck for salivary sparing. The lower neck was treated with a conventional AP low-neck port abutted to the inferior IMRT dose distribution border. The radiation dose was prescribed to the two clinical target volumes (CTVs) according to the assumed risk of containing disease. The mean dose for definitive IMRT patients was 72.64 +/- 4.83 Gy to CTV1 and 64.34 +/- 5.15 Gy to CTV2. The mean dose to CTV1 and CTV2 in postoperative cases was 68.53 +/- 4.71 Gy and 60.95 +/- 5.33 Gy, respectively. The locations of failure were analyzed. RESULTS: Seventeen locoregional failures (persistent or recurrent disease) were found. Of these 17 failures, 9 (53%) were inside CTV1. One failure (6%) was marginal to CTV1 but inside CTV2. One failure (6%) occurred outside CTV1 but inside CTV2. Another failure was marginal to CTV2. Of the 17 failures, 5 (28%) were found outside of the IMRT field and in the lower neck. Dose-volume histogram analysis revealed that for all but 1 patient, the recurrent/persistent disease within the CTVs received comparable or superior dose coverage relative to the CTV. The 2-year actuarial locoregional control rate was 85%, and the ultimate locoregional control rate after surgical salvage was 89%. We observed no dermal failure and only one marginal failure in the region adjacent to the spared parotid glands. CONCLUSION: We have shown that the target definition and coverage for patients treated with IMRT for parotid sparing is adequate. The predominant tumor failure within CTV1 may imply the need to identify patients with radioresistant tumor subvolumes (such as hypoxic regions) within the CTV. This information would assist in discriminating a subgroup of tumors for a more aggressive and target-specific therapeutic approach.

Adolescent↗

PubMed2003

Validation of dynamic MLC-controller log files using a two-dimensional diode array.

Intensity-modulated radiation therapy (IMRT) delivered with multi-leaf collimator (MLC) in the step-and-shoot mode uses multiple static MLC segments to achieve intensity modulation. For typical IMRT treatment plans, significant numbers of segments are delivered with monitor units (MUs) of much less than 10. Verification of the ability of the linear accelerator (linac) to deliver small MU segments accurately is an important step in the IMRT commissioning and quality assurance (QA) process. Recent studies have reported large discrepancies between the intended and delivered segment MUs. These discrepancies could potentially cause large errors in the delivered patient dose. We have undertaken a systematic study to evaluate the accuracy of the dynamic MLC log files, which are created automatically by our commercial MLC workstation after each delivery, in recording the fractional MU delivered in the step-and-shoot mode. Two linac models were evaluated with simple-geometry leaf sequences and delivered with different total MUs and different nominal dose rates. A commercial two-dimensional diode array was used for the measurement. Large discrepancies between the intended and delivered segment MUs were found. The discrepancies were larger for small MU segments at higher dose rate, with some small MU segments completely undelivered. The recorded fractional MUs in the log files were found to agree with what was delivered within the limits of our experimental uncertainty. Our results indicate that it is important to verify the delivery accuracy of small MU segments that could potentially occur in a patient treatment and that the log files are useful in checking the integrity of the linac delivery once validated. Thus validated log files can be used as a QA tool for general IMRT delivery and patient-specific plan verification.

Documentation↗

PubMed2003

A method for the reconstruction of four-dimensional synchronized CT scans acquired during free breathing.

Breathing motion is a significant source of error in radiotherapy treatment planning for the thorax and upper abdomen. Accounting for breathing motion has a profound effect on the size of conformal radiation portals employed in these sites. Breathing motion also causes artifacts and distortions in treatment planning computed tomography (CT) scans acquired during free breathing and also causes a breakdown of the assumption of the superposition of radiation portals in intensity-modulated radiation therapy, possibly leading to significant dose delivery errors. Proposed voluntary and involuntary breath-hold techniques have the potential for reducing or eliminating the effects of breathing motion, however, they are limited in practice, by the fact that many lung cancer patients cannot tolerate holding their breath. We present an alternative solution to accounting for breathing motion in radiotherapy treatment planning, where multislice CT scans are collected simultaneously with digital spirometry over many free breathing cycles to create a four-dimensional (4-D) image set, where tidal lung volume is the additional dimension. An analysis of this 4-D data leads to methods for digital-spirometry, based elimination or accounting of breathing motion artifacts in radiotherapy treatment planning for free breathing patients. The 4-D image set is generated by sorting free-breathing multislice CT scans according to user-defined tidal-volume bins. A multislice CT scanner is operated in the ciné mode, acquiring 15 scans per couch position, while the patient undergoes simultaneous digital-spirometry measurements. The spirometry is used to retrospectively sort the CT scans by their correlated tidal lung volume within the patient's normal breathing cycle. This method has been prototyped using data from three lung cancer patients. The actual tidal lung volumes agreed with the specified bin volumes within standard deviations ranging between 22 and 33 cm3. An analysis of sagittal and coronal images demonstrated relatively small (<1 cm) motion artifacts along the diaphragm, even for tidal volumes where the rate of breathing motion is greatest. While still under development, this technology has the potential for revolutionizing the radiotherapy treatment planning for the thorax and upper abdomen.

Adult↗