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K M Ayyangar

Publications and source records attributed to K M Ayyangar.

At least 19 recordsLinked to original sources
PubMed2001

Commissioning of Peacock System for intensity-modulated radiation therapy.

The Peacock System was introduced to perform tomographic intensity-modulated radiation therapy (IMRT). Commissioning of the Peacock System included the alignment of the multileaf intensity-modulating collimator (MIMiC) to the beam axis, the alignment of the RTA device for immobilization, and checking the integrity of the CRANE for indexing the treatment couch. In addition, the secondary jaw settings, couch step size, and transmission through the leaves were determined. The dosimetric data required for the CORVUS planning system were divided into linear accelerator-specific and MIMiC-specific. The linear accelerator-specific dosimetric data were relative output in air, relative output in phantom, percent depth dose for a range of field sizes, and diagonal dose profiles for a large field size. The MIMiC-specific dosimetric data were the in-plane and cross-plane dose profiles of a small and a large field size to derive the penumbra fit. For each treatment unit, the Beam Utility software requires the data be entered into the CORVUS planning system in modular forms. These modules were treatment unit information, angle definition, configuration, gantry and couch angles range, dosimetry, results, and verification plans. After the appropriate machine data were entered, CORVUS created a dose model. The dose model was used to create known simple dose distribution for evaluation using the verification tools of the CORVUS. The planned doses for phantoms were confirmed using an ion chamber for point dose measurement and film for relative dose measurement. The planning system calibration factor was initially set at 1.0 and will be changed after data on clinical cases are acquired. The treatment unit was released for clinical use after the approval icon was checked in the verification plans module.

Humans↗
PubMed2001

Independent dose calculations for the PEACOCK System.

An independent dose calculation method has been developed to validate intensity-modulated radiation therapy (IMRT) plans from the NOMOS PEACOCK System. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle and each arc position. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The independent calculation uses the pencil beam data viz tissue maximum ratio (TMR) and beam profiles for a single 1 x 0.8-cm beamlet formed by the NOMOS multileaf intensity-modulating collimator (MIMiC) leaf. The pencil beam data were measured for the 6-MV photon beam from Siemens PRIMUS linear accelerator using film dosimetry. The dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for its beam intensity. Isodose distributions are generated using the independent dose calculations and compared to the CORVUS plans. Isodose distributions show good agreement with the CORVUS plans for a number of clinical cases. The independent dose calculation algorithm is described in this paper.

Algorithms↗
PubMed2001

Quality assurance procedures for the Peacock system.

The Peacock system is the product of technological innovations that are changing the practice of radiotherapy. It uses dynamic beam modulation technique and inverse planning algorithm, both of which are new methodologies, to perform intensity-modulation radiation therapy (IMRT). The quality assurance (QA) procedure established by Task Group No. 40 did not adequately consider these emerging modalities. A review of literature indicates that published articles on QA procedures concentrate primarily on the verification of dose delivered to phantom during commissioning of the system and dose delivered to phantom before treating patients. Absolute dose measurements using ion chambers and relative dose measurements using film dosimetry have been used to verify delivered doses. QA on equipment performance and equipment safety is limited. This paper will discuss QA on equipment performance, equipment safety, and patient setup reproducibility.

Algorithms↗
PubMed2001

Immobilization devices for intensity-modulated radiation therapy (IMRT).

Three-dimensional conformal radiation therapy (3DCRT) and intensity-modulated radiation therapy (IMRT) plans show radiation dose distribution that is highly conformal to the target volume. The successful clinical implementation of these radiotherapy modalities requires precise positioning of the target to avoid a geographical miss. Effective reduction in target positional inaccuracies can be achieved with the proper use of immobilization devices. This paper reviews some of the immobilization devices that have been used and/or have the potential of being used for IMRT. The immobilization devices being reviewed include stereotactic frame, Talon system, thermoplastic molds, Alpha Cradles, and Vac-Lok system. The implementation of these devices at various anatomical sites is discussed.

Humans↗
PubMed2001

Comparative study between IMRT with NOMOS BEAK and linac-based radiosurgery in the treatment of intracranial lesions.

A comparative study was undertaken to examine intracranial irradiation using intensity-modulation radiation therapy (IMRT) and linear accelerator-based radiosurgery. The IMRT was examined using the Peacock system with a BEAK attachment. A clinical case involving a metastatic brain lesion, treated with 3 radiosurgery isocenters, was planned for IMRT. The radiosurgery was planned using the Leibinger planning system. The IMRT was planned using the CORVUS planning system. The CORVUS planning system uses an inverse planning algorithm, a recent development in radiotherapy. Isodose distributions and dose volume histograms were generated and compared. Analysis of the dosimetry shows that the dose conformity and homogeneity within the target using the RTOG guidelines are superior for IMRT. The advantages of IMRT using inverse planning system include the ease of planning and execution of treatment, especially for cases that involve concave targets that require multiple isocenters using radiosurgery.

Algorithms↗
PubMed2001

Leaf sequencing techniques for MLC-based IMRT.

The nonuniform fields required by intensity-modulation radiation therapy (IMRT) can be delivered using conventional multileaf collimators (MLC) as beam modulators. In MLC-based IMRT, the nonuniform field is initially converted into an intensity map represented as a matrix of beam intensities. The intensity map is then decomposed into a series of subfields or segments of uniform intensities. Although there are many ways of segmenting the beam intensity matrix, a resulting subfield is only deliverable if it satisfies the constraints imposed by the MLC. These constraints exist as a result of the design of the MLC. The simplest constraint of the MLC is that its pairs of leaves can only move in and out in one dimension. Additional constraints include collision of opposing leaves and the need to match the tongue-and-groove to reduce interleaf leakage. The practical aspect of MLC-based IMRT requires that an optimized algorithm decomposes the nonuniform field into the least number of segments and therefore reduces the delivery time. This paper examines the static use and the dynamic use of MLCs to perform MLC-based IMRT.

Radiotherapy, Conformal↗
PubMed2001

Commissioning and quality assurance for MLC-based IMRT.

The commissioning and quality assurance (QA) associated with the implementation of linear accelerator multileaf collimator (MLC)-based intensity-modulated radiation therapy (IMRT) at the University of Nebraska Medical Center are described. Our MLC-based IMRT is implemented using the PRIMUS linear accelerator interface through the IMPAC record and verification system to the CORVUS treatment planning system. The "step-and-shoot" technique is used for this MLC-based IMRT. Commissioning process requires the verification of predefined parameters available on the CORVUS and the collection of some machine data. The machine data required are output factor in air and output factor in phantom, and percent depth dose for a number of field sizes. In addition, inplane and crossplane dose profiles of 4 x 4 cm and 20 x 20 cm field sizes and diagonal dose profiles of a large field size have to be measured. Validation of connectivity and dose model includes the use of uniform intensity bar strips, triangular-shaped nonuniform intensity bar strip, and N-shaped target. QA procedure follows the recommendation of the AAPM Task Group No. 40 report. In addition, the leaf position accuracy and reproducibility of the MLC should be checked at regular intervals. The dose validation is implemented through the hybrid plan where the patient beam parameters are applied to a flat phantom. Independent dose calculation method is used to confirm the dose delivery plan and data input to the CORVUS.

Equipment Design↗
PubMed2001

Independent dose calculations for the corvus MLC IMRT.

Two independent dose calculation methods have been explored to validate MLC-based IMRT plans from the NOMOS CORVUS system. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The 2 independent calculation methods are based on a pencil beam kernel convolution and a Clarkson-type differential scatter summation, respectively. The pencil beam data for a 1 x 1-cm beam, as formed by the multileaf collimator, were measured for the 6-MV photon beam from a Siemens PRIMUS linear accelerator using film dosimetry. In the pencil beam method, the dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for beam intensity. The scatter summation method used the conversion of measured depth dose data into scatter maximum ratios. In this method, the differential scatter from each pencil beam is corrected for the beam intensity. Isodose distributions were generated using the independent dose calculations and compared to the CORVUS plans. Although isodose distributions from both methods show good agreement with the CORVUS plan, our implementation of the differential scatter summation approach seems more favorable. The 2 independent dose calculation algorithms are described in this paper.

Radiotherapy Dosage↗
PubMed2000

Review of dosimetric functions for meterset calculations.

Mathematical expressions used to calculate doses in a patient, based on data measured in a phantom, have to be simple, understandable, and reliable to minimize possible calculational error. In light of this concern, this paper reviews the dosimetric functions used in meterset calculations to determine the treatment times or monitor units for a prescribed dose. The dosimetric functions are the percent depth dose, the tissue-air ratio, the tissue-phantom ratio, and the inverse square law. This review examined the definition of the dosimetric functions, the inter-relationships among the dosimetric functions, and the mathematical expressions used in meterset calculations for nonstandard source-to-surface distances in a phantom.

Humans↗
PubMed2000

IORT apparatus design improvement through the evaluation of electron spectral distributions using Monte Carlo methods.

Clinically used IORT electron beam characteristics may vary with respect to typical external beams due to the decrease of lateral scatter equilibrium and the addition of the IORT apparatus itself. Additionally, chamber size effects may lead to inaccurate measurements of the changes in electron beam characteristics. The causal components of these beam characteristics are often difficult or impossible to measure using experimental techniques. For this reason, and for potential design improvement, the electron beams were modeled using the OMEGA/BEAM Monte Carlo software for radiation transport. The IORT electron beam characteristics of the Varian Clinac 1800 were studied for 6, 12, and 20 MeV electrons and 1-4 in. diameter flat-end applicators. The characteristics studied include electron energy spectra, percentage depth dose, and cross-plane profiles. It was found that by increasing the thickness of the aluminum base plate of the main attachment, the dose at d(max) outside the primary field could be reduced from approximately 9% to 1% of maximum.

Biophysical Phenomena↗
PubMed1999

Accuracy of magnetic resonance imaging stereotactic coordinates with the cosman-roberts-wells frame.

Quality assessment on the accuracy of a Cosman-Roberts-Wells (CRW) magnetic resonance imaging (MRI) stereotactic ring which had nonferrous stainless steel screws and positioning posts and a localizer with petroleum jelly in the fiducials, purchased in 1994, revealed errors of greater than 4 mm with targets in phantoms. Image fusion of objects within the phantom indicated the central area was accurately depicted by CT or MRI. We then tested a newer CRW- MRI ring (MRIA-IHR with titanium screws and posts) and localizer (MRIA-2-LF with fiducials filled with copper sulfate) and found that the MRI stereotactically calculated target coordinates matched both the known position of these targets in the phantom as well as the CT stereotactically calculated coordinates within approximately 1 mm. We also describe excellent superimposition of CT and MRI stereotactically determined surfaces in a recent clinical case using the new hardware. This shows that recent modifications to the CRW-MRI stereotactic system can make it accurate for small targets, but we emphasize that all systems need to undergo ongoing local quality assessment to ensure acceptable accuracy in practice.

Brain↗
PubMed1998

Do we need Monte Carlo treatment planning for linac based radiosurgery? A case study.

The accuracy of conventional empirical and semi-empirical dose calculation algorithms for radiation therapy treatment planning is limited. The main problem is that these algorithms fail to adequately consider the lateral transport of radiation. Most conventional algorithms use measured dose distribution data as input. These data induce an added inaccuracy to stereotactic radiosurgery dose calculations due to the difficulty of acquiring accurate dosimetric data for very small beams; however, since multiple arcs of large solid angles are usually used in stereotactic radiosurgery, the errors introduced by conventional dose algorithms are quite likely to be diluted. The use of Monte Carlo treatment planning for stereotactic radiosurgery has been investigated and described in the present paper. The OMEGA Monte Carlo code system is used as the dose engine in an in-house developed radiosurgery treatment planning system. The Monte Carlo treatment plans are done for two typical clinical cases. In one case, the collimator of 20 mm diameter is used and the lesion is located in the peripheral part of the brain. In the other case, the collimator diameter is 30 mm and the lesion is in the central part of the brain. The resultant dose distributions are compared with those calculated with a conventional dose algorithm which is based on the standard Tissue Maximum Ratio (TMR)/Off Axis Ratio (OAR) formalism. Without the inhomogeneity correction, the conventional algorithm yields accurate relative dose distributions for both cases compared with the Monte Carlo calculations. The absolute dose at the isocenter may be overestimated by the conventional algorithm by 1.5% for the first case and 2.6% for the second case; however, using the method of ratio of TMRs for inhomogeneity correction, the overestimation can be greatly reduced for both cases. The inclusion of the inhomogeneity correction into the conventional dose algorithm does not alter the relative dose distributions. Based on the clinical cases studied, it may be concluded that the conventional dose algorithm is sufficient for radiosurgery treatment planning and the Monte Carlo based radiosurgery treatment planning is unwarranted.

Aged↗
PubMed1998

Dose perturbation caused by high-density inhomogeneities in small beams in stereotactic radiosurgery.

The influence of high-density tissue heterogeneities in small-diameter beams used in stereotactic radiosurgery has been investigated. Dose perturbation immediately behind aluminium sheets, used to simulate a high-density tissue inhomogeneity such as bone, was studied in a solid water phantom. Dose reduction factors (DRFs), which are the ratios of the dose in the presence of the inhomogeneity to dose in a uniform density solid water phantom, were measured with a diamond detector for three thicknesses of aluminium. DRFs exhibit dependence on both the inhomogeneity thickness and the beam diameter. The DRF decreases with inhomogeneity thickness. The DRF initially decreases with increase in the beam diameter from 12.5 to 25 mm. For fields greater than 25 mm, the DRFs are nearly constant. The commonly used algorithms such as the TAR ratio method underestimate the magnitude of the measured effect. A good agreement between these measurements and Monte Carlo calculations is obtained. The influence of the high-density inhomogeneity on the tissue maximum ratio (TMR) was also measured with the inhomogeneity at a fixed depth dmax from the entrance surface. The TMR is reduced for all detector-inhomogeneity distances investigated. The dose build-up phenomenon observed in the presence of low-density air inhomogeneity is absent in the presence of a high-density inhomogeneity. The beam width (defined by 50% dose points) immediately beyond the inhomogeneity is unaffected by the high-density inhomogeneity. However, the 90%-10% and 80%-20% dose penumbra widths and the dose outside the beam edge (beyond the 50% dose point) are reduced. This reduction in dose outside the beam edge is caused by the reduced range of the secondary radiation (photons and electrons) in the high-density medium.

Data Interpretation, Statistical↗
PubMed1998

On compensator design for photon beam intensity-modulated conformal therapy.

Recently the compensator has been shown to be an in expensive and reliable dose delivery device for photon beam intensity-modulated radiation therapy (IMRT). The goal of IMRT compensator design is to produce an optimized primary fluence profile at the patient's surface obtained from the optimization procedure. In this paper some of the problems associated with IMRT compensator design, specifically the beam perturbations caused by the compensator, are discussed. A simple formula is derived to calculate the optimal compensator thickness profile from an optimized primary fluence profile. The change of characteristics of a 6 MV beam caused by the introduction of cerrobend compensators in the beam is investigated using OMEGA Monte Carlo codes. It is found that the compensator significantly changes the energy spectrum and the mean energy of the primary photons at the patient's surface. However, beam hardening does not have as significant an effect on the percent depth dose as it does on the energy spectrum. We conclude that in most situations the beam hardening effect can be ignored during compensator design and dose calculation. The influence of the compensator on the contaminant electron buildup dose is found to be small and independent of the compensator thickness of interest. Therefore, it can be ignored in the compensator design and included as a correction into the final dose distribution. The scattered photons from the compensator are found to have no effect on the surface dose. These photons produce a uniform low fluence distribution at the patient's surface, which is independent of compensator shape. This is also true for very large fields and extremely asymmetric and nonuniform compensator thickness profiles. Compared to the primary photons, the scattered photons have much larger angular spread and similar energy spectrum at the patient's surface. These characteristics allow the compensator thickness profile and the dose distribution to be calculated from the optimized fluence profile of primary photons, without considering the scattered photons.

Calibration↗
PubMed1997

A fast numerical algorithm for electron mean energy calculation in radiation therapy.

A numerical algorithm for calculating the mean energy of radiotherapy electron beams has been developed. This algorithm is fast and accurate which makes it suitable for routine clinical use. First, a Gaussian distribution of the electron energy spectrum is derived from the linear Boltzmann equation. Based on this Gaussian spectrum, and after introducing a correction to the CSDA mean energy, a recursive-iterative algorithm for the mean energy calculation is developed. The multiple-scattering correction is taken into account using Yang's pathlength distribution theory. Numerical results of the present algorithm are compared with the results obtained through Monte Carlo simulation as well as Harder's formula. Good agreement with Monte Carlo simulation is achieved. Also the new algorithm is much more accurate than the commonly-used empirical formula of Harder.

Algorithms↗
PubMed1997

Clinical fusion of three-dimensional images using Bremsstrahlung SPECT and CT.

UNLABELLED: Infusional brachytherapy for treatment of neoplasms, with colloidal 32P has been used to treat various tumors in the pancreas, liver, brain, lung, and head and neck. In performing such treatments, anatomical verification of the location of the administered 32P from the image obtained by Bremsstrahlung SPECT alone is not possible due to the lack of internal landmarks, since the radionuclide is distributed only in the tumor and does not usually accumulate in the normal organs. The purpose of this study was to provide a practical three-dimensional approach for image fusion between Bremsstrahlung SPECT and CT. METHODS: The tumors in four cancer patients were injected directly with 32P under CT guidance. A Bremsstrahlung SPECT study using 99mTc backscatter sources to obtain the body contour was then performed. SPECT images were used to generate the skin contours using a threshold detection method. A three-dimensional surface was generated from these contours using a tiling program and fused with a corresponding CT surface generated from a CT scan in the same patient through an iterative surface-fitting algorithm. The three-dimensional surface of the region of high-activity, corresponding to the infused tumor, was then generated using the Bremsstrahlung SPECT data by mapping the iso-count surfaces through a computer program. The three-dimensional image of the organ then was fused with the registered CT-SPECT datasets. RESULTS: The accuracy of fit measured as the mean distance between the SPECT and CT surfaces was in the range of 3-4 mm. CONCLUSION: The anatomical co-registration of Bremsstrahlung SPECT with CT images using the outer surface-fitting algorithm is a reliable tool. This correlation permits direct anatomic confirmation of the region of the 32P activity distribution with the anatomic site selected for injection.

Algorithms↗