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Cheng B Saw

Publications and source records attributed to Cheng B Saw.

9 recordsLinked to original sources
PubMed2006

Therapeutic radiation physics primer.

Therapeutic radiological physics is the branch of physics as applied to radiation therapy. Therapeutic radiological physics involves the understanding of the radiation sources, types, and characteristics of radiation, interaction of radiation with matter, and thereafter the deposition of energy in matter. In clinical practice, therapeutic radiological physics deals with the technical tasks of preparing a patient to undergo radiation therapy. These tasks include simulation, patient data acquisition, individualized planning, verification, and dose delivery. The role of a therapeutic radiological physicist is to manage the technical aspects of patient care: providing technical expertise to the development of the institution, recommending and introducing new treatment techniques, and ensuring that all patients undergoing radiation therapy receive the best standard of care.

Health Physics↗
PubMed2005

A dose verification method using a monitor unit matrix for dynamic IMRT on Varian linear accelerators.

Dosimetry verification is an important step during intensity modulated radiotherapy treatment (IMRT). The verification is usually conducted with measurements and independent dose calculations. However, currently available independent dose calculation methods were developed for step-and-shoot beam delivery methods, and their uses for dynamic multi-leaf collimator (MLC) delivery methods are not efficient. In this study, a dose calculation method was developed to perform independent dose verifications for a dynamic MLC-based IMRT technique for Varian linear accelerators. This method extracts the machine delivery parameters from the dynamic MLC (DMLC) files generated by the IMRT treatment planning system. Based on the parameters a monitor unit (MU) matrix was separately calculated as two terms: direct exposure from the open MLC field and leakage contributions, where the leaf-end leakage contribution becomes more important in higher dose gradient regions. The MU matrix was used to compute the primary dose and the scattered dose with a modified Clarkson technique. The doses computed using the method were compared with both measurement and treatment planning for 14 and 25 plans respectively. An average of less than 2% agreement was observed and the standard deviation was about 1.9%.

Algorithms↗
PubMed2005

Determination of CT-to-density conversion relationship for image-based treatment planning systems.

The implementation of tissue inhomogeneity correction in image-based treatment planning will improve the accuracy of radiation dose calculations for patients undergoing external-beam radiotherapy. Before the tissue inhomogeneity correction can be applied, the relationship between the computed tomography (CT) value and density must be established. This tissue characterization relationship allows the conversion of CT value in each voxel of the CT images into density for use in the dose calculations. This paper describes the proper procedure of establishing the CT value to density conversion relationship. A tissue characterization phantom with 17 inserts made of different materials was scanned using a GE Lightspeed Plus CT scanner (120 kVp). These images were then downloaded into the Eclipse and Pinnacle treatment planning systems. At the treatment planning workstation, the axial images were retrieved to determine the CT value of the inserts. A region of interest was drawn on the central portion of the insert and the mean CT value and its standard deviation were determined. The mean CT value was plotted against the density of the tissue inserts and fitted with bilinear equations. A new set of CT values vs. densities was generated from the bilinear equations and then entered into the treatment planning systems. The need to obtain CT values through the treatment planning system is very clear. The 2 treatment planning systems use different CT value ranges, one from -1024 to 3071 and the other from 0 to 4096. If the range is correct, it would result in inappropriate use of the conversion curve. In addition to the difference in the range of CT values, one treatment planning system uses physical density, while the other uses relative electron density.

Densitometry↗
PubMed2003

Independent calculations to validate monitor units from ADAC treatment planning system.

Current standards of practice are based on the use of an independent calculation to validate the monitor units (MUs) derived from a treatment planning system. The ADAC PINNACLE treatment planning system has shown discrepancies of 10% or more compared to simple independent calculations for highly contoured areas such as tangential breast and chest wall irradiation. The ADAC treatment planning system generally requires more MUs to deliver the same prescribed dose. Independent MU calculation methods are based on full phantom conditions. On the other hand, the MUs from the ADAC treatment planning system are derived using realistic phantom scatter. As such, differences exist in TMR factors, off-axis wedge factors, and the phantom scatter factor. To systematically study the discrepancies due to phantom conditions, experimental measurements were performed with various percentages of tissue missing. The agreement between the experimental measurements and ADAC calculations was found to be within 2%. Using breast field geometry, a relationship between missing tissue and the dosimetric parameters used by ADAC was developed. This relationship, when applied, yielded independent MU calculations whose values closely matched those from the ADAC treatment planning system.

Algorithms↗
PubMed2003

Dose linearity and uniformity of a linear accelerator designed for implementation of multileaf collimation system-based intensity modulated radiation therapy.

The dose linearity and uniformity of a linear accelerator designed for multileaf collimation system-(MLC) based IMRT was studied as a part of commissioning and also in response to recently published data. The linear accelerator is equipped with a PRIMEVIEW, a graphical interface and a SIMTEC IM-MAXX, which is an enhanced autofield sequencer. The SIMTEC IM-MAXX sequencer permits the radiation beam to be " ON" continuously while delivering intensity modulated radiation therapy subfields at a defined gantry angle. The dose delivery is inhibited when the electron beam in the linear accelerator is forced out of phase with the microwave power while the MLC configures the field shape of a subfield. This beam switching mechanism reduces the overhead time and hence shortens the patient treatment time. The dose linearity, reproducibility, and uniformity were assessed for this type of dose delivery mechanism. The subfields with monitor units ranged from 1 MU to 100 MU were delivered using 6 MV and 23 MV photon beams. The doses were computed and converted to dose per monitor unit. The dose linearity was found to vary within 2% for both 6 MV and 23 MV photon beam using high dose rate setting (300 MU/min) except below 2 MU. The dose uniformity was assessed by delivering 4 subfields to a Kodak X-OMAT TL film using identical low monitor units. The optical density was converted to dose and found to show small variation within 3%. Our results indicate that this linear accelerator with SIMTEC IM-MAXX sequencer has better dose linearity, reproducibility, and uniformity than had been reported.

Particle Accelerators↗
PubMed2002

Clinical implementation of intensity-modulated radiation therapy.

The clinical implementation of intensity-modulated radiation therapy (IMRT) is a complex process because of the introduction of new treatment planning algorithms and beam delivery systems compared to conventional 3-dimensional conformal radiation therapy (3D-CRT) and the lack of established national performance protocols. IMRT uses an inverse-planning algorithm to create nonuniform fields that are only deliverable through a newly designed beam-modulating delivery system. The intent of this paper is to describe our experience and to elucidate the new clinical procedures that must be executed to have a successful IMRT program. Patients who undergo IMRT at our institution are immobilized and simulated before proceeding to computed tomography scan for patient data acquisition. Treatment planning involves the use of different prescription dose formats and different planning techniques compared to 3D-CRT. The desired dose goals for the target and sensitive structures must be specified before initiating the planning process, which is computer intensive. After the plan is completed, the delivery instructions are transferred to the delivery system via either a floppy disk for MIMiC-based IMRT or through the network for MLC-based IMRT. Target localizations are carried out using orthogonal radiographs. Ultrasound imaging system (BAT) is used to localize the prostate. Dose validation is performed using films, ion chambers or dose-calculation-based techniques.

Antineoplastic Protocols↗