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Biomedical subjects

K M Ayyangar

Publications and source records attributed to K M Ayyangar.

At least 37 records · Page 2Linked to original sources
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↗
PubMed1996

Quantitative assessment of beam perturbations caused by silicon diodes used for in vivo dosimetry.

PURPOSE: To measure the effect of silicon diode detectors used for in vivo dosimetry on beam characteristics and determine whether this effect is clinically significant. METHODS AND MATERIALS: Commercially available photon and electron diodes were placed on the central axis of photon and electron beams. The beam characteristics were measured for 6- and 10-MV photon and 6-20-MeV electron energies from a Varian Clinac 1800 medical linear accelerator. Water was used for the medium, and measurements were made for various clinically common field sizes and depths. RESULTS: Beam attenuations along the central axis were 10 and 7.5% for 6- and 10-MV photons, respectively. Electron beam dose reductions were between 13 and 25% for 20-6-MeV electrons. Photon beam flatness varied up to 7% at different depths, but the symmetry was not affected much. Electron beam flatness and symmetry were significantly changed to as much as 18 and 6%, respectively. CONCLUSION: Use of diode detectors on central axis of photon and electron beams for in vivo dosimetry causes significant attenuation and alteration of the beam characteristics. The percentage of the volume affected is significant (e.g., 23% of the volume in a 4 x 4 field gets 10% less dose for a 6-MV photon beam), especially if these diodes are used for in vivo dosimetry on the central axis every day for every treatment, as is done in some clinics. Other beam parameters such as penumbra and skin dose are also affected. It is therefore recommended that the diodes be used only as needed.

Humans↗
PubMed1996

Relationship of wedge isodose angle to percent depth-dose curves.

PURPOSE: An expression that relates the wedge isodose angle to the gradient of the percent depth-dose curves for photon beams has been formulated and verified experimentally. METHODS AND MATERIALS: The expression that relates a wedge angle to another via the gradient of their corresponding percent depth-dose curves was derived. Because the field size affects the percent depth-dose curves, measurement of wedged isodose distributions were carried out for different field sizes to validate the expression. The 6 MV and 23 MV photon beams and a range of field sizes from 5 x 5 cm to 20 x 20 cm were used. For each field size, the wedge angle and percent depth-dose curve slope were measured at the depth of 10 cm. Measured and calculated wedge angles were compared. RESULTS: The variation of the wedge angle caused by the change in field size is about 6 degrees. For field sizes larger than 10 x 10 cm, the variation of wedge angle is less than 3 degrees. The calculated wedge angles agree with the measured wedge angles to within 1 degree. The knowledge of the extent of wedge angle variation due to the change in field size and the availability of an expression to determine an accurate wedge angle would be helpful in the use of the universal wedge equation.

Filtration↗
PubMed1996

A computer method to model the dose distribution of high energy photon grid therapy in three dimensions.

High energy photon grid therapy is the application of multiple circular divergent photon beams to the palliative care of patients. The photon beams are defined by cylindrical holes in a large attenuator placed in the path of an open radiation beam. We have developed a computer method to model the three-dimensional dose distribution of high energy photon grid therapy. The semiempirical method requires the measured data from a single cylindrical hole of the size to be modeled. For a given hole diameter, any arbitrary geometry can be calculated. This information is important when investigating the clinical implications of grid therapy for patient care.

Algorithms↗
PubMed1996

A method of gap and field size determination for abutting a treatment field to a previously treated area.

A simple method of determining the new field size and the skin gap needed when abutting to a previously treated (or simulated) field is presented. The more complex case of abutting a treatment field between two previously treated fields and determining the new field length and the skin gap at each end is also addressed. Both methods utilize a single measurement on the patient's skin and the commonly used gap formula based on similar triangles. Both methods easily lend themselves to implementation through a simple BASIC program which eases any difficulties in calculation encountered during simulation.

Algorithms↗
PubMed1996

Differential dose delivery for cancers of the head and neck.

A technique was developed for the treatment of head and neck cancers that employs differential dose delivery to primary and nodal areas while simultaneously correcting the dose delivered through various thickness of tissue in the field(s). Our technique modulates the intensity of the static lateral fields in three dimensions utilizing data acquired from a pre-treatment CT scan. This technique has been used to treat patients with various cancers of the head and neck. All patients tolerated treatment well with no observed increase in acute normal tissue reactions.

Head and Neck Neoplasms↗
PubMed1995

Dose distribution considerations of medium energy electron beams at extended source-to-surface distance.

PURPOSE: To determine the effects of extended source-to-surface distance (SSD) on dose distributions for a range of medium energy electron beams and cone sizes. METHODS AND MATERIALS: The depth-dose curves and isodose distributions of 6 MeV, 10 MeV, and 14 MeV electron beams from a dual photon and multielectron energies linear accelerator were studied. To examine the influence of cone size, the smallest and the largest cone sizes available were used. Measurements were carried out in a water phantom with the water surface set at three different SSDs from 101 to 116 cm. RESULTS: In the region between the phantom surface and the depth of maximum dose, the depth-dose decreases as the SSD increases for all electron beam energies. The effects of extended SSD in the region beyond the depth of maximum dose are unobservable and, hence, considered minimal. Extended SSD effects are apparent for higher electron beam energy with small cone size causing the depth of maximum dose and the rapid dose fall-off region to shift deeper into the phantom. However, the change in the depth-dose curve is small. On the other hand, the rapid dose fall-off region is essentially unaltered when the large cone is used. The penumbra enlarges and electron beam flatness deteriorates with increasing SSD.

Electrons↗
PubMed1995

Confirmation of target localization and dosimetry for 3D conformal radiotherapy treatment planning by MR imaging of a ferrous sulfate gel head phantom.

A detailed methodology has been developed to verify the three-dimensional (3D) radiation dose mapping of conformal therapy radiation treatment planning by FeMRI dosimetry. A phantom that consisted of a human skull filled with a 1-mM solution of ferrous sulfate and 0.1-N sulfuric acid gelled with 7.5% (by weight) gelatin was employed. With a spherical target volume in the head phantom, five noncoplanar conformal beams were designed through the use of a 3D treatment planning system developed in-house. The phantom was irradiated with a 6-MV linear accelerator to a total dose of 25 Gy delivered to the periphery of the target volume. The phantom and a set of calibration vials were scanned simultaneously in a GE 1.5T MR imager with six different multiscan inversion-recovery pulse sequences. The values of T1 were evaluated on a pixel-by-pixel basis through the use of custom-built software on a UNIX workstation and were converted to dose using calibration data. Comparisons of dose distributions between those measured by FeMRI and those calculated by 3D treatment planning show good agreement.

Biophysical Phenomena↗
PubMed1994

Validity of universal wedge equation over the range of 60Cobalt to 25 MV photon beam energies.

PURPOSE: Evaluating the validity of the universal wedge equation over the range of 60Cobalt (60CO) to 25 MV photon beam energies. METHODS AND MATERIALS: The universal wedge equation relates the beam weight of the wedged field to the effective wedge angle produced by combining the wedged field to the nonwedged field and is expressed as tan (theta) = B tan (theta w), where B is the normalized weight imposed on the wedged field, theta w is the maximum wedge angle of the wedge filter, and theta is the effective wedge angle. The isodose distributions from 60Co to 25 MV photon beam energies were used. For each photon energy, the isodose distributions of wedged and nonwedged fields were combined in different proportions. The effective wedge angle was determined from each resultant isodose distribution. RESULTS: The relationship between the weight of wedged field and the effective wedge angle was found to be nonlinear for wedge filters with maximum wedge angle greater than 30 degrees. The universal wedge equation predicts the effective wedge angle to within 2 degrees compared to the measured value.

Cobalt Radioisotopes↗
PubMed1992

Dual isotope SPECT in the evaluation of recurrent brain tumor.

Dual isotope brain SPECT was performed in a patient with recurrent brain tumor (grade II astrocytoma). In this case, a good coregistration of brain perfusion and tumor images was obtained because Tc-99m HMPAO and the Tl-201 chloride imaging were done simultaneously using a Triad SPECT unit. This method might play an important clinical role in the evaluation of patients with recurrent brain tumors.

Adult↗
PubMed1991

Computer-CT planning of the electron boost in definitive breast irradiation.

Treatment planning of the electron boost in breast irradiation at this institution is performed by using information from CT scanning and from surgical clips used to define the tumor bed. A refinement of this technique, taking into account the surgical axis of approach, using computer-CT planning is now implemented. The location of the scar and the clips are digitized using a computer to define the tumor bed dimensions. With 3-D computer software a line, simulating the surgical axis and the central ray of the electron beam, is drawn between the deepest clip and the surgical scar. This beam's eye view along the surgical axis is projected onto a screen as a starting point. Appropriate gantry angle, treatment table position, beam energy, and precise shaping of the electron field borders with a 2 cm margin around the clips and the scar are determined. Simulation films comparing clinically set-up fields and computer-CT planned fields were reviewed. In only 5 of 17 patients did the clinically set-up field have adequate inclusion of the tumor bed within the treatment volume. Computer-CT planned fields ensured adequate inclusion of the tumor bed in all, including the remaining 12. In 7 patients obvious increased sparing of normal breast tissue was seen with computer-CT planned fields. This technique enables accurate placement of a shaped electron field and further refinement of electron boost treatment planning. This is especially true in situations in which the tumor bed is located at a site distant from the lumpectomy scar rather than directly beneath it and in cases where the tumor bed lies deep within the breast.

Breast Neoplasms↗
PubMed1991

A review of the role of radiation therapy in the treatment of patients with breast cancer.

Irradiation has been used to treat cancer of the breast since 1895. Over the period between 1895 to the present, the therapeutic use of irradiation has evolved from a cauterizing agent to a sophisticated use of theory and practice to cause tumor cell death while at the same time producing maximum normal tissue sparing. The use of radiation has made it possible for the surgeon to use conservative breast sparing surgery to obtain excellent cosmetic results, while at the same time producing a local control equal to that of mastectomy. For the patient, conservative surgery becomes a better option than mastectomy because it not only produces local control equal to that of mastectomy, but gives the wonderful added benefit of a more superior and pleasing cosmetic result.

Antineoplastic Agents↗
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

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↗