Biomedical subjects

Jonathan G Li

Publications and source records attributed to Jonathan G Li.

10 recordsLinked to original sources

PubMed2004

An immobilization system for claustrophobic patients in head-and-neck intensity-modulated radiation therapy.

PURPOSE: To evaluate the effectiveness of an immobilization treatment system used for claustrophobic patients in head-and-neck intensity-modulated radiation therapy (IMRT). METHODS AND MATERIALS: Instead of the thermoplastic facemask, the Vac Fix (S & S Par Scientific, Odense, Denmark) mold is used for immobilization of claustrophobic patients at the University of Florida in head-and-neck IMRT. The immobilization procedure combines the use of commercial stereotactic infrared (IR) ExacTrac camera system (BrainLAB, Inc., Westchester, IL) for patient setup and monitoring. The Vac Fix mold is placed on the headrest and folded up as needed to provide support before the mold is hardened. For the camera system, a frame referred to as a "tattoo-free immobilization accessory" is fabricated, on which the IR markers can be placed. A patient-specific dental impression is made with the bite tray. The movement of the markers, connected through the dental impression of the patient, accurately represents the overall patient motion. Patient movement is continuously monitored and repositioning is performed whenever patient movement exceeds the predefined tolerance limit. Monitored patient movements are recorded at a certain frequency. Recorded data are analyzed and compared with those of patients immobilized with the thermoplastic facemask plus the camera system that is the standard immobilization system in our clinic. RESULTS: For three patients treated with the Vac Fix mold plus the camera system, on average, the histogram-based uncertainties, U(95)(5), U(95)(20), and mean displacement, R(mean) (mm) were 1.03, 1.08, and 0.60, respectively. These values are close to those obtained with the mask plus the camera system. The Vac Fix mold plus the camera system often requires more beam interruptions because of repositioning than the mask plus the camera system (on average, the Vac Fix mold plus the camera system required repositioning 7.7 times and the mask plus the camera system required repositioning 1.8 times during 20 treatments). CONCLUSION: The Vac Fix mold immobilization procedure plus the camera monitoring system has been set up for patients who are claustrophobic or cannot tolerate a mask during head-and-neck IMRT. Although this system causes more frequent beam delivery interruptions, it is as effective as the mask plus the camera system in immobilizing patients within the tolerance limit.

Equipment Design↗

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

Unnecessary laryngeal irradiation in the IMRT era.

PURPOSE: The purpose of this review is to address the issue of unnecessary irradiation of the normal larynx. METHODS: We performed a literature review and dosimetry evaluation of intensity modulated radiotheraphy (IMRT) techniques. RESULTS: Data suggest that matching conventional parallel opposed photon fields through adenopathy is a reasonable alternative to the common practice of unnecessarily irradiating the normal larynx to high doses. The dosimetry evaluation demonstrates that the laryngeal dose is much higher when the entire neck is included in the IMRT field as opposed to using a standard anterior low-neck field below the level of the thyroid notch. CONCLUSIONS: Lateral opposed fields should not be extended to include the larynx to avoid matching fields through adenopathy. IMRT fields should not include the normal larynx when the laryngeal dose would be substantially lower with a technique that shields the larynx in an anterior low-neck field.

Head and Neck Neoplasms↗

PubMed2004

Irradiating the groin nodes without breaking a leg: a comparison of techniques for groin node irradiation.

The purpose of this study was to determine the optimal technique for delivering postoperative radiotherapy for vulvar cancer and other tumors requiring treatment of the inguinal nodes. This project compared tumor coverage and normal tissue sparing for the 5 main radiotherapy techniques that are used to treat vulvar cancer. The intensity-modulated radiation therapy (IMRT) plan was undesirable because it resulted in an excessive dose to portions of the central pelvic structures. The photon thunderbird with skin match was unacceptable because it underdosed a portion of the groin region. The electron thunderbird was ideal for thin patients but was not applicable for most patients because of excessive dose to the skin and subcutaneous tissues. The photon through-and-through and the photon thunderbird with deep match were acceptable in most situations. In thin patients, where the depth of the inguinal vessels is less than 3 cm, the electron thunderbird is the technique of choice. In the average-sized patient, both the photon through-and-through and the photon thunderbird with deep match are reasonable options. The available literature suggests that the risk of femoral neck fracture or necrosis of the femoral head is approximately 11% at 5 years using the photon through-and-through technique. In our opinion, this is an acceptable price to pay for reliable node coverage, setup simplicity, and zero risk of overdose at field junctions.

Female↗

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

Evaluation of intrafraction patient movement for CNS and head & neck IMRT.

Intrafraction patient motion is much more likely in intensity-modulated radiation therapy (IMRT) than in conventional radiotherapy primarily due to longer beam delivery times in IMRT treatment. In this study, we evaluated the uncertainty of intrafraction patient displacement in CNS and head and neck IMRT patients. Immobilization is performed in three steps: (1) the patient is immobilized with thermoplastic facemask, (2) the patient displacement is monitored using a commercial stereotactic infrared IR camera (ExacTrac, BrainLab) during treatment, and (3) repositioning is carried out as needed. The displacement data were recorded during beam-on time for the entire treatment duration for 5 patients using the camera system. We used the concept of cumulative time versus patient position uncertainty, referred to as an uncertainty time histogram (UTH), to analyze the data. UTH is a plot of the accumulated time during which a patient stays within the corresponding movement uncertainty. The University of Florida immobilization procedure showed an effective immobilization capability for CNS and head and neck IMRT patients by keeping the patient displacement less than 1.5 mm for 95% of treatment time (1.43 mm for 1, and 1.02 mm for 1, and less than 1.0 mm for 3 patients). The maximum displacement was 2.0 mm.

Central Nervous System↗

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

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

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↗