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Biomedical subjects

N Suntharalingam

Publications and source records attributed to N Suntharalingam.

At least 19 recordsLinked to original sources
PubMed1997

Efficient CT simulation of the four-field technique for conformal radiotherapy of prostate carcinoma.

PURPOSE: Conformal radiotherapy of prostate carcinoma relies on contouring of individual CT slices for target and normal tissue localization. This process can be very time consuming. In the present report, we describe a method to more efficiently localize pelvic anatomy directly from digital reconstructed radiographs (DRRs). MATERIALS AND METHODS: Ten patients with prostate carcinoma underwent CT simulation (the spiral mode at 3 mm separation) for conformal four-field "box" radiotherapy. The bulbous urethra and bladder were opacified with iodinated contrast media. On lateral and anteroposterior DRRs, the volume of interest (VOI) was restricted to 1.0-1.5 cm tissue thickness to optimize digital radiograph reconstruction of the prostate and seminal vesicles. By removing unessential voxel elements, this method provided direct visualization of those structures. For comparison, the targets of each patient were also obtained by contouring CT axial slices. RESULTS: The method was successfully performed if the target structures were readily visualized and geometrically corresponded to those generated by contouring axial images. The targets in 9 of 10 patients were reliable representations of the CT-contoured volumes. One patient had 18 mm variation due to the lack of bladder opacification. Using VOIs to generate thin tissue DRRs, the time required for target and normal tissue localization was on the average less than 5 min. CONCLUSION: In CT simulation of the four-field irradiation technique for prostate carcinoma, thin-tissue DRRs allowed for efficient and accurate target localization without requiring individual axial image contouring. This method may facilitate positioning of the beam isocenter and provide reliable conformal radiotherapy.

Humans↗
PubMed1997

Differential dose delivery using a nondocking applicator for intraoperative radiation therapy.

PURPOSE: Although treatment of a field within a field to deliver a boost dose is quite common with external photon beam radiation therapy, the same is not always true with electron beam radiation or in intraoperative radiation therapy (IORT). The purpose of this work is to report the results and details of a new technique developed to treat a field within a field in intraoperative radiation therapy. METHODS AND MATERIALS: This technique makes use of the nondocking IORT system currently used at our institution. Treatment is given in two segments: the large field is first treated by using standard circular lucite cones; the second dose segment is delivered using a new circular brass cone designed to fit concentrically within the large lucite cone. RESULTS: Central axis depth dose, surface dose, output factors, and two-dimensional beam profiles have been measured for a 7 cm inner diameter (i.d.) flat lucite cone and 3.8 and 5 cm i.d. flat brass cones for electron beam energies ranging from 4-22 MeV. For different clinical target volumes, summed dose distributions differentially weighted in both energy and dose are presented. CONCLUSIONS: A simple technique for delivering differential dose in intraoperative radiation therapy is presented. The technique provides a method for escalating dose to higher values for a defined target volume.

Equipment Design↗
PubMed1997

A dosimetry system for 192IR interstitial breast implants performed at the time of lumpectomy.

PURPOSE: 192Ir interstitial breast implants performed at the time of lumpectomy present a unique problem because they cannot be preplanned, and yet they are expected to produce a treatment dose rate (TDR) from 0.3 to 0.5 Gy/h using sources already procured. The purpose of this work is to describe a system of dosimetry that works within these constraints and has been used to perform more than 600 such implants. METHODS AND MATERIALS: The underlying principle is to fix the ribbon spacing, the interplaner separation, and the linear activity (1 mCi/cm) so that the TDR will depend only on the area (L x W) implanted. The ribbons are spaced 1.5 cm and 2.0 cm apart in single plane and double implants, respectively. Idealized implants were used to study the TDR as a function of the implant dimensions, and to study the effects of varying the ribbon spacing and interplanar separation. Volume-dose histograms were generated to study the homogeneity of dose. RESULTS: The TDRs of single plane implants range from 0.3 Gy/h for small 4 x 4 cm2 implants to 0.4 Gy/h for large 10 x 10 cm2 implants. The TDRs for double plane implants are similar for the same range of dimensions. CONCLUSIONS: Implants with a TDR between 0.3 and 0.5 Gy/h can be performed for a wide range of geometries without preplanning using fixed ribbons spacings of 1.5 and 2.0 cm for single and double plane implants, respectively, and a linear activity of 1 mCi/cm.

Brachytherapy↗
PubMed1997

Experimental determination of fluence correction factors at depths beyond dmax for a Farmer type cylindrical ionization chamber in clinical electron beams.

Recently, it has been recommended that electron beam calibrations be performed at a new reference depth [Burns et al., Med. Phys. 23, 383 (1996)] given by dref = 0.6R50-0.1 cm, where R50 is the depth of 50% depth dose. In order to calibrate electron beams at dref with a Farmer type cylindrical ionization chamber, the values of the perturbation correction factors Pwall and Pfl at dref are required. Using a parallel plate Holt chamber as a reference chamber, the product PwallPfl has been determined for a 6.1-mm-diameter PTW cylindrical ionization chamber at dref as a function of R50 of clinical electron beams (6 < or = nominal energy E < or = 22 MeV). Assuming that Pwall for the PTW chamber is unity in electron beams, the measured Pfl values ranged from 0.96 to 0.98 as the energy is increased. These results are in close agreement with recently reported calculated values. Determination of dref requires the knowledge of R50. A relation between I50 and R50 is given in the IAEA Protocol [TRS No. 277 (IAEA, Vieńna, 1987), pp. 1-98] for broad beams at SSD = 100 cm. It has been shown experimentally that the equation R50 = 1.029 x I50-0.063 cm, derived by Ding et al. [Med. Phys. 22, 489 (1995)] from Monte Carlo simulations of realistic clinical electron beams, can be used satisfactorily to obtain R50 from I50, where I50 is the depth of 50% ionization. The largest difference between the measured value of R50 and that calculated by using the above equation has been found to be about 1 mm at 22 MeV.

Biometry↗
PubMed1997

Anisotropy of an 192iridium high dose rate source measured with a miniature ionization chamber.

The anisotropy of a high dose rate (HDR) 192Ir source was measured in air and in water using a miniature (0.147 cm3) ionization chamber. Measurements were made at a distance of 5 cm from the source center at polar angles from 10 degrees-170 degrees. The anisotropy was found to be less pronounced in water, and the anisotropy is asymmetric about the transverse axis. The results agree with previous ionization chamber and TLD measurements to within +/- 4%. Mean anisotropy factors were determined at each angle from all existing data at 5 cm distance, and compared to published Monte Carlo calculations, and to the values used in the microSelectron HDR brachytherapy planning system (BPS). The Monte Carlo photon transport code appears to systematically underestimate the anisotropy factor by up to 4% in the forward direction and overestimate it by up to 3% in the backward direction. The mean anisotropy factors also indicate that the BPS systematically underestimates the anisotropy factor by up to 3% in the forward direction, and overestimates it by up to 15% in the backward direction. However, the 15% difference occurs at 180 degrees where it is not likely to be clinically significant.

Air↗
PubMed1996

Limitations of the minimum peripheral dose as a parameter for dose specification in permanent 125I prostate implants.

PURPOSE: The objective of this work is to investigate whether the minimum peripheral dose is a practical parameter for dose specification in permanent 125I implants of the prostate. METHODS AND MATERIALS: The investigation was carried out by use of a computer model of ellipsoidal 125I implants in which the average dimension and elongation factor were varied to provide a wide range of geometries. Both ideal and nonideal implants were investigated. The 125I seeds were confined to the target volume except for a portion of the study in which the effect of placing seeds outside the target volume was investigated. RESULTS: The minimum peripheral dose was found to be very sensitive to the seed placement. The irregularities in the seed spacing that inevitably occur in actual implants tend to lower the minimum peripheral dose. As a result, the minimum peripheral dose is generally significantly less than planned by an amount that is unpredictable, and often exceeds 25%. However, the percentage of the target volume that receives a dose less that the prescribed minimum peripheral dose is generally less than 10%. Implanting seeds outside the target volume improves the dose uniformity, but does not appear to offer any advantage in dose coverage, and increases the volume of normal tissue irradiated. CONCLUSION: If a minimum peripheral dose is prescribed for a permanent 125I prostate implant, and the implant is planned using an idealized implant having precisely spaced seeds, the prescribed dose will rarely, if ever, be achieved. Reasonable agreement with the prescribed dose can be achieved only if the requirement for coverage is relaxed from 100 to 90%, or if the total source strength is increased by 20% to compensate for the anticipated imperfections in seed placement.

Brachytherapy↗
PubMed1995

Dosimetric properties of megavoltage grid therapy.

PURPOSE: Grid therapy is a technique used to deliver a high dose of radiation (15-20 Gy) in a single fraction to many small volumes within a large treatment field. This treatment modality is used for the palliative treatment of large, deeply seated tumors, which have either been treated to tolerance with conventional radiation, or, due to massive tumor bulk, would most likely not benefit from a conventional course of radiation therapy. As the dose distribution from megavoltage grid therapy differs significantly from that of conventional radiation therapy (i.e., many large dose gradients exist within the tumor volume), we have measured various dosimetric properties inherent in this unique treatment modality. METHODS AND MATERIALS: The grid is a 16 x 16 array of 1-cm diameter holes in a 7-cm thick piece of custom blocking material. The ratio of shielded to open surface area is 1:1. Depth dose, valley-to-peak ratios, and output factors for this square array grid were measured in a water phantom for several field sizes, as well as for a 1-cm diameter narrow beam using 6 MV and 25 MV photon beams. RESULTS: The depth dose curves for the grid fields lie between those for an open portal and a narrow beam. For the 6-MV beam at dmax, the ratios of the doses delivered to the center of the shielded regions to that under the center of the holes, expressed as valley-to-peak ratios, range from 15 to 40%. At 10 cm, the ratios increase to between 25 and 45%. At 25 MV at both dmax and 10 cm, the valley-to-peak ratios are between 40 and 60%. The output factors, 0.89 for 6 MV and 0.77 for 25 MV, do not depend on field size. CONCLUSION: Megavoltage grid therapy is a unique treatment modality where the dose is delivered differentially to a large volume in one fraction. Characterization of the dosimetric properties has allowed clinical implementation of the grid.

Radiation Protection↗
PubMed1995

Dosimetric characteristics of a commercial multileaf collimator.

The dosimetric characteristics of a multileaf collimator (MLC) retrofitted to a SL25 linear accelerator have been investigated. Central-axis depth dose, surface dose, penumbra, beam flatness and symmetry, field size factors, beam transmission through leaves and/or diaphragms, and leakage between the leaves were measured. Quantitative measurements of all beam parameters show good agreement with the design specifications of the manufacturer. No changes were observed in flatness, symmetry, penumbra, and penetration for both 6- and 25-MV photon beams when compared to the values for the standard collimator. No significant differences were observed in the penumbra as a function of leaf position. Transmission measurements in areas shielded by either X diaphragms or leaves plus diaphragms are less than 1% of dose within open field. The average leakage between leaves is about 2.5% for 6-MV and 3.5% for 25-MV photon beams. The peak value of the leakage at any point between leaves is less than 5%. The dosimetric features of shaped fields using the MLC are comparable to those of alloy shaped fields with the standard SL25 collimator.

Humans↗
PubMed1993

Concept of dose nonuniformity in interstitial brachytherapy.

PURPOSE: Evaluation of the 3-dimensional dose distributions of interstitial implants using the dose uniformity ratio. METHODS AND MATERIALS: Single source, two sources, three and four sources arranged both linearly and in the form of a triangle or a square, ribbons with different seed spacings, a single-plane and double-plane implants were evaluated. The evaluations involved the use of differential dose volume histograms and the dose nonuniformity ratio defined as the ratio of the high dose volume to the reference volume. RESULTS: For a single source, the dose nonuniformity is the same regardless which dose rate is selected as the treatment dose rate. For any multi-source implant, the dose nonuniformity is altered depending on the selection of the reference dose rate. In addition, the dose nonuniformity curve exhibited three characteristics zones. CONCLUSION: The dose nonuniformity ratio can be a useful tool in assessing and optimizing interstitial implants.

Brachytherapy↗
PubMed1991

Quantitative assessment of interstitial implants.

Quantitative assessment of interstitial implants is proposed using volume versus dose curves and four well-defined dosimetric parameters. The volume versus dose curves, both differential and cumulative, provide quantitative data on the volumes of tissues irradiated to different doses. They also offer a qualitative assessment of the variations in dose delivery. The dose nonuniformity ratio (DNR) quantitatively determines the degree of dose nonuniformity specific to the implant configuration. The dose rate at which the DNR shows a minimum value, if selected as the treatment dose rate, gives an optimized dose distribution. The three volumetric irradiation indices are formulated with respect to a well-defined target volume. They offer quantitative data on the extent to which the implant delivers the prescribed dose to the target volume. These dosimetric parameters determine the degree of coverage of the target volume, dose homogeneity within the target volume, and irradiation of tissues outside the target volume. The method of quantitative assessment is demonstrated using, as examples, an ideal Ir-192 double-plane implant and an actual clinical Ir-192 double-plane breast implant.

Brachytherapy↗
PubMed1989

Dosimetric considerations of stereotactic brain implants.

Dose distributions of stereotactic brain implants performed by four institutions were analyzed. In these implants 192Ir or 125I sources were used. The analyses involved an evaluation of the isodose distributions in two orthogonal planes, the dose gradient outside, and the dose homogeneity within the target volume. Quantitative evaluation of the dose homogeneity was performed using three volumetric irradiation indices. The dose homogeneity was observed to improve as the number of catheters increased. However, the number of catheters used is influenced by neurosurgical considerations. Thus, it is necessary to make a compromise between dose homogeneity and the maximum number of catheters to be used. The dose gradient, a centimeter outside the target volume, was found to depend on the geometry of the implant and at distances beyond, it was found to depend on the type of radioisotopes used.

Brachytherapy↗
PubMed1989

A non-docking intraoperative electron beam applicator system.

A non-docking intraoperative radiation therapy electron beam applicator system for a linear accelerator has been designed to minimize the mechanical, electrical, and tumor visualization problems associated with a docking system. A number of technical innovations have been used in the design of this system. These include: (a) a new intraoperative radiation therapy cone design that gives a better dose uniformity in the treatment volume at all depths; (b) a collimation system which reduces the leakage radiation dose to tissues outside the intraoperative radiation therapy cone; (c) a non-docking system with a translational accuracy of 2 mm and a rotational accuracy of 0.5 degrees; and (d) a rigid clamping system for the cones. A comprehensive set of dosimetric characteristics of the intraoperative radiation therapy applicator system is presented.

Combined Modality Therapy↗
PubMed1989

Asymmetric field arc rotations.

Optimal treatment planning of target volume that surrounds a vital critical structure is often very difficult. Treatment techniques using moving beam therapy with fields asymmetric with respect to rotational axis of the collimator head allow treatment of such target volumes with minimal dose to critical structures. The availability of independent motion of the collimator jaws on new medical accelerators allows easy setting up of asymmetric treatment portals. Therefore, treatment techniques utilizing asymmetric field arc rotations with acceptable dose distributions have been possible.

Bone Neoplasms↗
PubMed1988

Dosimetric characteristics of a 6 MV photon beam from a linear accelerator with asymmetric collimator jaws.

Dosimetric measurements have been made of a 6 MV photon beam from a linear accelerator equipped with asymmetric jaws. The field size factors for asymmetrically set fields are compared to those for symmetrically set fields. The change of beam quality has been measured as a function of off-axis position of the asymmetric fields to assess its effect on depth dose. Additional measurements include beam penumbra and shape of isodose curves for open and wedge fields as the field opening is moved asymmetrically from the central ray.

Particle Accelerators↗
PubMed1985

Radiation oncology. Programs for the present and future.

Radiation oncology in 1984 continues to make major advances in the multidisciplinary clinical programs. This has been possible by virtue of the radiation oncologist, who is an active participant in these clinical programs. The changing role for the radiation oncologist has dictated a greater participation in the primary management of the patient's disease process and also participation in multidisciplinary research programs.

Breast Neoplasms↗
PubMed1984

Quality assurance in radiation therapy: future plans in physics.

Modern day radiation therapy has seen the impact of high technology resulting in more sophisticated computer augmented treatment delivery systems, treatment planning procedures and diagnostic imaging techniques. Much work has already been reported in the area of physics efforts related to quality assurance in radiation therapy. Future efforts in physics will have to address the new developments in each component of the whole radiation treatment process. Certain new developments, using both computer and imaging technologies, show promise in providing tools to verify the accuracy of the delivered radiation treatment. Areas receiving careful attention are: integration and registration of information from multiple sources of diagnostic studies; validation of the accuracy of treatment planning systems; assessment of relative merits of alternate dose distributions; improvement of portal and verification film image quality; real time monitoring using light emitting screens and coupled with TV systems; monitoring of treatment and machine parameters using "record and verify" computer systems. The medical physics community, primarily through the American Association of Physicists in Medicine (AAPM), will continue the development of methodologies for technology transfer in the area of quality assurance. Committees and task groups within the AAPM will address the new developments impacting on quality assurance and prepare appropriate protocols and documents to assist the practicing physicist. By necessity, the national Radiological Physics Center (RPC) and the regional Centers for Radiological Physics (CRP) will have to take a major role in the development of new quality assurance programs.

Humans↗