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Biomedical subjects

C S Chui

Publications and source records attributed to C S Chui.

At least 19 recordsLinked to original sources
PubMed2004

Comparison of BACTEC MGIT 960 system and BACTEC 460 TB system for growth and detection of Mycobacteria from clinical specimens.

This study was carried out to compare the performance of BACTEC MGIT 960 with the BACTEC 460 TB for growth and detection of Mycobacteria from human clinical specimens. The BACTEC MGIT 960 instrument is a fully automated system that utilizes MGIT tubes containing an oxygen sensor embedded in silicon at the bottom and filled with 7 mL of modified Middlebrook 7H9 broth. Identical samples were inoculated into the two automated systems and incubated for six weeks. Over a period of three months, 279 specimens were decontaminated and processed according to the standard CDC NALC/NaOH method, using the commercial MycoPrep kit. Forty-two specimens (15%) yielded Mycobacterium tuberculosis; 37 (88%) were detected by the fluorescent BACTEC MGIT 960 and 35 (83%) detected by the radiometric BACTEC 460 TB. Fifteen specimens (5%) yielded Mycobacterium Other Than Tuberculosis (MOTT); 10 (66%) were detected by BACTEC MGIT 960 and 11 (73%) detected by BACTEC 460 TB. The average time to detection and contamination rates and the average time to obtain results of antimicrobial susceptibility tests between the two systems were compared. The performance of the BACTEC MGIT 960 was comparable to the BACTEC 460 TB system which has been the "Gold Standard" for automated detection of TB. The former was more rapid, as sensitive and less labour intensive than the BACTEC 460. Our data demonstrates that the BACTEC MGIT 960 system is an accurate, automated and a non-radioactive alternative to the BACTEC 460 TB for the culture and susceptibility testing of M. tuberculosis.

Bacteriological Techniques↗
PubMed2001

Monte Carlo evaluation of tissue inhomogeneity effects in the treatment of the head and neck.

PURPOSE: To use Monte Carlo dose calculation to assess the degree to which tissue inhomogeneities in the head and neck affect static field conformal, computed tomography (CT)-based 6-MV photon treatment plans. METHODS AND MATERIALS: We retrospectively studied the three-dimensional treatment plans that had been used for the treatment of 5 patients with tumors in the nasopharyngeal or paranasal sinus regions. Two patients had large surgical cavities. The plans were designed with a clinical treatment planning system that uses a measurement-based pencil-beam dose-calculation algorithm with an equivalent path-length inhomogeneity correction. Each plan employs conformally-shaped 6-MV photon beams. Patient anatomy and electron densities were obtained from the treatment planning CT images. For each plan, the dose distribution was recalculated with the Monte Carlo method, utilizing the same beam geometry and CT images. The Monte Carlo method accurately accounts for the perturbation effects of local tissue heterogeneities. The Monte Carlo calculated dose distributions were compared with those from the clinical treatment planning system. RESULTS: The degree to which tissue inhomogeneity affects the dose distributions of individual fields varies with the specific anatomic geometry, especially the size and location of air cavities in relation to the beam orientation and field size. Most of the beam apertures completely enclose the air cavities within or adjacent to the gross tumor volume (GTV). Equivalent squares (including blocking) ranged from approximately 5 to 9.5 cm. A common feature observed for individual fields is that the Monte Carlo calculated doses to tissue directly behind and within an air cavity are lower. However, after combining the fields employed in each treatment plan, the overall dose distribution shows only small differences between the two methods. For all 5 patients, the Monte Carlo calculated treatment plans showed a slightly lower dose received by the 95% of target volume (D(95)) than the plans calculated with the pencil-beam algorithm. The average difference in the target volume encompassed by the prescription isodose line was less than 2.2%. The difference between the dose-volume histograms (DVHs) of the GTV was generally small. For the brainstem and chiasm, the DVHs of the two plans were similar. For the spinal cord, differences in the details of the DHV and the dose to 1 cc (D(1cc)) of the structure were observed, with Monte Carlo calculation generally predicting increased dose indices to the spinal cord. However, these changes are not expected to be clinically significant. CONCLUSION: For 6-MV photons, the effects of both normal tissue inhomogeneities and surgical air cavities on the target coverage were adequately accounted for by conventional pencil beam methods for all of the cases studied. Although differences in details of the DVHs of the normal structures were observed, depending on whether Monte Carlo or pencil-beam algorithm was used for calculation, these differences are not expected to be clinically significant. In general, the pencil-beam calculation corrected for primary attenuation by the equivalent pathlength is a sufficiently accurate method for head-and-neck treatment planning using 6-MV photons.

Air↗
PubMed2001

Treatment planning and delivery of intensity-modulated radiation therapy for primary nasopharynx cancer.

PURPOSE: To implement intensity-modulated radiation therapy (IMRT) for primary nasopharynx cancer and to compare this technique with conventional treatment methods. METHODS AND MATERIALS: Between May 1998 and June 2000, 23 patients with primary nasopharynx cancer were treated with IMRT delivered with dynamic multileaf collimation. Treatments were designed using an inverse planning algorithm, which accepts dose and dose-volume constraints for targets and normal structures. The IMRT plan was compared with a traditional plan consisting of phased lateral fields and a three-dimensional (3D) plan consisting of a combination of lateral fields and a 3D conformal plan. RESULTS: Mean planning target volume (PTV) dose increased from 67.9 Gy with the traditional plan, to 74.6 Gy and 77.3 Gy with the 3D and IMRT plans, respectively. PTV coverage improved in the parapharyngeal region, the skull base, and the medial aspects of the nodal volumes using IMRT and doses to all normal structures decreased compared to the other treatment approaches. Average maximum cord dose decreased from 49 Gy with the traditional plan, to 44 Gy with the 3D plan and 34.5 Gy with IMRT. With the IMRT plan, the volume of mandible and temporal lobes receiving more than 60 Gy decreased by 10-15% compared to the traditional and 3D plans. The mean parotid gland dose decreased with IMRT, although it was not low enough to preserve salivary function. CONCLUSION: Lower normal tissue doses and improved target coverage, primarily in the retropharynx, skull base, and nodal regions, were achieved using IMRT. IMRT could potentially improve locoregional control and toxicity at current dose levels or facilitate dose escalation to further enhance locoregional control.

Algorithms↗
PubMed2001

Inverse planning algorithms for external beam radiation therapy.

Intensity-modulated radiation therapy (IMRT) is a new treatment technique that has the potential to produce superior dose distributions to those of conventional techniques. An important step in IMRT is inverse planning, or optimization. This is a process by which the optimum intensity distribution is determined by minimizing (or maximizing) an objective function. For radiation therapy, the objective function is used to describe the clinical goals, which can be expressed in terms of dose and dose/volume requirements, or in terms of biological indices. There are 2 types of search algorithms, stochastic and deterministic. Typical algorithms that are currently in use are presented. For clinical implementations, other issues are also discussed, such as global minimum vs. local minima, dose uniformity in the target and sparing of normal tissues, smoothing of the intensity profile, and skin flash. To illustrate the advantages of IMRT, clinical examples for the treatment of the prostate, nasopharynx, and breast are presented. IMRT is an emerging technique that has shown encouraging results thus far. However, the technique is still in its infancy and more research and improvements are needed. For example, the effects of treatment uncertainties on the planning and delivery of IMRT requires further study. As with any new technology, IMRT should be used with great caution.

Algorithms↗
PubMed2001

Smoothing intensity-modulated beam profiles to improve the efficiency of delivery.

Intensity-modulated beam profiles are generated by an inverse planning or optimization algorithm, a process that, being computationally complex and intensive, is inherently susceptible to noise and numerical artifacts. These artifacts make delivery of the beams more difficult, oftentimes for little, if any, observable improvement in the dose distributions. In this work we examine two approaches for smoothing the beam profiles. The first approach is to smooth the beam profiles subsequent to each iteration in the optimization process (method A). The second approach is to include a term within the objective function that specifies the smoothness of the profiles as an optimization criterion (method B). The two methods were applied to a phantom study as well as three clinical sites: paraspinal, nasopharynx, and prostate. For the paraspinal and nasopharynx cases, which have critical organs with low tolerance doses in close proximity, method B produced sharper dose gradients, better target dose homogeneity, and more critical organ sparing. In the less demanding prostate case, the two methods give similar results. In addition, method B is more efficient during optimization, requiring fewer iterations, but less efficient during DMLC delivery, requiring a longer beam-on time.

Algorithms↗
PubMed2001

Delivery of intensity-modulated radiation therapy with a conventional multileaf collimator: comparison of dynamic and segmental methods.

Intensity-modulated radiation therapy (IMRT) can be delivered with a conventional multileaf collimator (MLC), either in dynamic mode (DMLC) or in segmental mode (SMLC, also known as "step-and-shoot"). The advantage of DMLC is its ability to deliver the desired intensity profile produced by inverse planning with a high degree of fidelity. The SMLC method, on the other hand, resembles treatment with multiple static fields, and can be more easily verified. However, the use of SMLC requires that the desired profile be approximated by discrete levels of intensity, which may lead to degradation in the delivered dose distribution. Clearly, the results of SMLC delivery depend on the number of levels and the spatial resolution of the intensity distribution. In this work, we compare the DMLC method and the SMLC method employing different numbers of levels and different spatial resolutions. Three disease sites were studied: prostate, nasopharynx, and breast, with three cases for each. In general, a 5- to 10-level SMLC plan produced results comparable to that from a DMLC plan. The target coverage is improved by increasing the number of levels while critical organs are better protected with finer spatial resolutions. The beam-on-time (MUs) requirement for SMLC was approximately 20% less than DMLC, but the delivery time (in minutes) was about twice as long. Thus, the choice depends on many factors including machine capability, quality assurance, target coverage, critical organ protection, beam-on-time, delivery time, and other clinical considerations.

Breast Neoplasms↗
PubMed2001

The treatment of large extraskeletal chondrosarcoma of the leg: comparison of IMRT and conformal radiotherapy techniques.

Extraskeletal chondrosarcoma of the leg is a rare, malignant neoplasm with very few cases having been reported in the literature. In this study we investigate the possibility of using intensity modulated radiotherapy (IMRT) for this type of disease and demonstrate its advantages over conventional three-dimensional (3D) conformal treatment. A case was presented of a patient with extraskeletal chondrosarcoma of the lateral compartment of the leg in which the target volume was 50 cm in length and twisted around the surrounding bones. Both the 3D conformal plan and IMRT plan were designed using the Memorial Sloan-Kettering Cancer Center planning system. The IMRT plan produced a superior dose distribution to the patient as compared to the 3D conformal plan both in terms of dose conformity and homogeneity in the target volumes, and reduction of the maximum dose to the bone. The planning time of the IMRT plan was about 3-5 times shorter than that of the 3D conformal plan. It was demonstrated that the IMRT technique can be used not just for small tumors, but also for large and spiral-shaped tumors close to critical organs. The IMRT method requires less planning time, and provides better target coverage with more sparing of critical structures. When planning patients with multiple target volumes receiving different prescribed doses, the IMRT technique can more easily meet this requirement.

Adult↗
PubMed2001

The measurement of three dimensional dose distribution of a ruthenium-106 ophthalmological applicator using magnetic resonance imaging of BANG polymer gels.

The BANG (MGS Research Inc., Guilford, CT) polymer gel has been used as a dosimeter to determine the three-dimensional (3D) dose distribution of a ruthenium-106 (Ru-106) ophthalmologic applicator. An eye phantom made of the BANG gel was irradiated with the Ru-106 source for up to 1 h. The phantom and a set of calibration vials were scanned simultaneously in a GE 1.5 T MR imager using the Hahn spin-echo pulse sequence with a TR of 2000 ms and two TEs of 20 ms and 100 ms. The T(2) values were evaluated on a pixel-by-pixel basis using custom-built software on a DEC alpha workstation and converted to dose using calibration data. Depth doses and isodose lines of the Ru-106 eye-plaque were generated. It is concluded that the BANG gel dosimetry offers the potential for measuring the 3D dose distributions of an ophthalmologic applicator, with high spatial resolution and relatively good accuracy.

Brachytherapy↗
PubMed2000

Relative profile and dose verification of intensity-modulated radiation therapy.

PURPOSE: To develop a quality assurance (QA) procedure to assess the intensity profile and dosimetry for intensity-modulated (IM) treatment fields using electronic portal imaging devices (EPIDs). METHODS AND MATERIALS: A series of rapidly acquired (approximately 1/sec) portal images are summed and converted to dose. For relative intensity QA, the intended profile is subtracted point-by-point from the measured profile forming a series of error values. The standard deviation, sigma, of the errors, a measure of the goodness of the match, is minimized by applying a normalization and uniform scatter subtraction from the measured profile. For dose verification (dose to isocenter), an empirically determined phantom-correction factor is added to incorporate the effect of patient presence on EPID readings. Seventy prostate treatment fields were used in a phantom study to verify these approaches. Sensitivity was studied by creating artificial mismatches. RESULTS: The average sigma for relative profile verification is 3.3% (percentage of average intended intensity) whereas artificial mismatches resulted in sigma values from 5% to 27%. The average isocentric dose calculated from EPID readings is 1.001 relative to the planned dose with a standard deviation of 0.018. CONCLUSIONS: An EPID can be used for profile verification and absolute isocentric dose measurement for IM fields.

Algorithms↗
PubMed1999

Experimental verification of a CT-based Monte Carlo dose-calculation method in heterogeneous phantoms.

To further validate the Monte Carlo dose-calculation method [Med. Phys. 25, 867-878 (1998)] developed at the Memorial Sloan-Kettering Cancer Center, we have performed experimental verification in various inhomogeneous phantoms. The phantom geometries included simple layered slabs, a simulated bone column, a simulated missing-tissue hemisphere, and an anthropomorphic head geometry (Alderson Rando Phantom). The densities of the inhomogeneity range from 0.14 to 1.86 g/cm3, simulating both clinically relevant lunglike and bonelike materials. The data are reported as central axis depth doses, dose profiles, dose values at points of interest, such as points at the interface of two different media and in the "nasopharynx" region of the Rando head. The dosimeters used in the measurement included dosimetry film, TLD chips, and rods. The measured data were compared to that of Monte Carlo calculations for the same geometrical configurations. In the case of the Rando head phantom, a CT scan of the phantom was used to define the calculation geometry and to locate the points of interest. The agreement between the calculation and measurement is generally within 2.5%. This work validates the accuracy of the Monte Carlo method. While Monte Carlo, at present, is still too slow for routine treatment planning, it can be used as a benchmark against which other dose calculation methods can be compared.

Electrons↗
PubMed1998

A gradient inverse planning algorithm with dose-volume constraints.

An inverse planning algorithm for determining the intensity-modulated beams that will most closely generate a desired dose distribution is presented. The algorithm is three-dimensional and does not explicitly depend on beam energies and modalities. It allows a single prescription dose or a window of acceptable doses to be specified for the target, with additional constraints to account for under- or over-dosing. For the protection of organs at risk, it provides maximum-dose and dose-volume constraints. The latter apply to the entire volume of the organ exposed to the corresponding dose levels. Several levels of each type of constraint, with varying penalty weights, may be specified for each organ. The objective function that serves as the measure of the goodness of the solution is of the least-squares type and is minimized using conjugate gradient methods. Typical clinical cases involving 40,000 points and 4000 rays to be determined require about 10 min of CPU time on a DEC AlphaStation. Results are presented for two clinical sites, prostate and lung. The optimization algorithm yielded plans that featured higher target dose homogeneity, compared with the human planner's plan, while selectively sparing more of the normal organs at the desired dose regions.

Aged↗
PubMed1998

A patient-specific Monte Carlo dose-calculation method for photon beams.

A patient-specific, CT-based, Monte Carlo dose-calculation method for photon beams has been developed to correctly account for inhomogeneity in the patient. The method employs the EGS4 system to sample the interaction of radiation in the medium. CT images are used to describe the patient geometry and to determine the density and atomic number in each voxel. The user code (MCPAT) provides the data describing the incident beams, and performs geometry checking and energy scoring in patient CT images. Several variance reduction techniques have been implemented to improve the computation efficiency. The method was verified with measured data and other calculations, both in homogeneous and inhomogeneous media. The method was also applied to a lung treatment, where significant differences in dose distributions, especially in the low-density region, were observed when compared with the results using an equivalent pathlength method. Comparison of the DVHs showed that the Monte Carlo calculated plan predicted an underdose of nearly 20% to the target, while the maximum doses to the cord and the heart were increased by 25% and 33%, respectively. These results suggested that the Monte Carlo method may have an impact on treatment designs, and also that it can be used as a benchmark to assess the accuracy of other dose calculation algorithms. The computation time for the lung case employing five 15-MV wedged beams, with an approximate field size of 13 X 13 cm and the dose grid size of 0.375 cm, was less than 14 h on a 175-MHz computer with a standard deviation of 1.5% in the high-dose region.

Algorithms↗
PubMed1998

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy.

The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapy became a reality at our institution in 1995. Unlike treatment with static fields using a multileaf collimator, there are significant dosimetric issues which must be assessed before dynamic therapy can be implemented. We have performed a series of calculations and measurements to quantify head scatter for small fields, collimator transmission, and the transmission through rounded leaf ends. If not accounted for, these factors affect the delivered dose to the prostate by 5%-20% for a typical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15 MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileaf collimator, achieved during leaf position calibration and maintained during dose delivery, is also discussed.

Biophysical Phenomena↗
PubMed1997

Total body irradiation with an arc and a gravity-oriented compensator.

PURPOSE: To deliver uniform dose distributions for total-body irradiation (TBI) with an arc field and a gravity-oriented compensator. This technique allows the patient to be treated lying on the floor in a small treatment room. METHODS AND MATERIALS: Through the sweeping motion of the gantry, a continuous arc field can deliver a large field to a patient lying on the floor. The dose profile, however, would not be uniform if no compensator were used, due to the effects of inverse square variation of beam intensity with distance as well as the slanted depth in patient. To solve this problem, a gravity-oriented compensator made of cerrobend alloy was designed. This compensator has a cross-section of an inverted isosceles triangle, with the apex always pointing downward, due to gravity. By properly selecting the thickness of the compensator, the width of the base, and the distance between the pivots to the base, the difference in the path length through the compensator can be made just right to compensate the effects of inverse-square and slanted depth, thus producing a uniform dose profile. RESULTS: Arc fields with a gravity-oriented compensator were used for 6, 10, 15, and 18 MV photon beams. The arc field can cover a patient with a height up to 180 cm. The field width was chosen from 32 to 40 cm at the machine isocenter. The optimal thickness of the compensator was found to be 2.5 cm, and its base was 25 cm wide. The distance from the pivot points to the flat surface of the compensator proximal to the beam ranges from 13 to 14 cm for different beam energies. The dose uniformity at a depth of 10 cm is within +/-5% for all beam energies used in this study. CONCLUSIONS: Highly uniform dose profiles for TBI treatments can be delivered with an arc and a gravity-oriented compensator. The proposed technique is simple and versatile. A single compensator can be used for all energies, because the amount of compensation can be adjusted by changing the distance to the pivot and/or the field size.

Equipment Design↗
PubMed1997

Planning, delivery, and quality assurance of intensity-modulated radiotherapy using dynamic multileaf collimator: a strategy for large-scale implementation for the treatment of carcinoma of the prostate.

PURPOSE: To improve the local control of patients with adenocarcinoma of the prostate we have implemented intensity modulated radiation therapy (IMRT) to deliver a prescribed dose of 81 Gy. This method is based on inverse planning and the use of dynamic multileaf collimators (DMLC). Because IMRT is a new modality, a major emphasis was on the quality assurance of each component of the process and on patient safety. In this article we describe in detail our procedures and quality assurance program. METHODS AND MATERIALS: Using an inverse algorithm, we have developed a treatment plan consisting five intensity-modulated (IM) photon fields that are delivered with DMLC. In the planning stage, the planner specifies the number of beams and their directions, and the desired doses for the target, the normal organs and the "overlap" regions. Then, the inverse algorithm designs intensity profiles that best meet the specified criteria. A second algorithm determines the leaf motion that would produce the designed intensity pattern and produces a DMLC file as input to the MLC control computer. Our quality assurance program for the planning and treatment delivery process includes the following components: 1) verification of the DMLC field boundary on localization port film, 2) verification that the leaf motion of the DMLC file produces the planned dose distribution (with an independent calculation), 3) comparison of dose distribution produced by DMLC in a flat phantom with that calculated by the treatment planning computer for the same experimental condition, 4) comparison of the planned leaf motions with that implemented for the treatment (as recorded on the MLC log files), 5) confirmation of the initial and final positions of the MLC for each field by a record-and-verify system, and 6) in vivo dose measurements. RESULTS: Using a five-field IMRT plan we have customized dose distribution to conform to and deliver 81 Gy to the PTV. In addition, in the overlap regions between the PTV and the rectum, and between the PTV and the bladder, the dose is kept within the tolerance of the respective organs. Our QA checks show acceptable agreement between the planned and the implemented leaf motions. Correspondingly, film and TLD dosimetry indicates that doses delivered agrees with the planned dose to within 2%. As of September 15, 1996, we have treated eight patients to 81 Gy with IMRT. CONCLUSION: For complex planning problems where the surrounding normal tissues place severe constraints on the prescription dose, IMRT provides a powerful and efficient solution. Given a comprehensive and rigorous quality-assurance program, the intensity-modulated fields can be efficaciously and accurately delivered using DMLC. IMRT treatment is now ready for routine implementation on a large scale in our clinic.

Adenocarcinoma↗
PubMed1997

Lying-on position of total skin electron therapy.

PURPOSE: A technique for whole-body electron therapy with the patient in a lying position has been developed. This technique allows Total Skin Electron Therapy (TSET) for those patients who were previously unable to be treated in a conventional standing position. METHODS AND MATERIALS: This study was carried out on a Varian 2100C linear accelerator with a 6 MeV high dose rate electron beam. The collimator was open to a width of 36 x 36 cm. There were two main procedures, with six dual-field techniques: 1) two static AP/PA vertical dual fields (VDF): the patient laid on the floor transversely under the collimator when the gantry was in a vertical position. A 0.6 cm acrylic board was placed 15 cm away from the patient, then the gantry was rotated 25 degrees clockwise and counterclockwise to treat the patient in the supine and prone positions, respectively. 2) Four oblique junction fields (OJF): the patient laid on the floor in a prone and supine position parallel to the wave guide at (227 - body thickness x tan 60 degrees) cm away from the vertical axis of the gantry, then the gantry was rotated 60 degrees toward the patient. A 0.6 cm acrylic board was placed 15 cm away from the patient perpendicular to the beam. The patient was move along the field central axis. It allowed the patient's body to be within the 160 cm effective treatment profile. When the patient's body axis move 5 degrees toward the lateral side of the field central axis, we could obtain a better dose distribution in the vertex of the scalp and the soles of the feet. The angle of the VDF was measured by chamber detectors to obtain the effective treatment profile. Likewise, the optimal profile for the OJF was determined by the same procedures. The Rando phantom was used to measure the superficial dose of the body. RESULTS: The dimension of effective treatment profile for the VDF was 188 x 72 cm at 87% dose level For the OJF, we had to move the patient along the field central axis to obtain the effective treatment profile in a 180 x 85 cm dimension at a 87% dose level. The vertex and sole dose measured in this setup was in the range of 80-88%. CONCLUSIONS: The empirical data showed that the lying-on position for TSET was technically feasible. The dose distribution in the body surface was also compatible with the Stanford standing technique. The nonambulatory skin malignancy patient can be treated in a comfortable and reproducible position.

Electrons↗