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L M Chin

Publications and source records attributed to L M Chin.

At least 19 recordsLinked to original sources
PubMed1992

Three-dimensional dose distribution of total body irradiation by a dual source total body irradiator.

This study describes the three-dimensional dosimetric characteristics of total body irradiation by our dedicated irradiation unit, which consists of two modified 4-MV linear accelerators mounted opposite each other, providing a field size of 220 cm x 80 cm at the midplane. Our dose calculation algorithm considers the three-dimensional contour of the patient to evaluate the primary and scatter doses. The data base for the calculation includes tissue-to-maximum ratios measured for the large fields. The lung dose correction was calculated using the methods of Batho or ratio of TMR. The accuracy of the calculated dose distributions was verified by measurements with ionization chambers in a humanoid phantom. We also describe and verified a technique to achieve desirable midline lung doses using lead shields. The flexibility and the accuracy of the planning system offers the potential in optimizing the therapeutic ratio for total body treatments.

Algorithms↗
PubMed1990

Monte Carlo calculations of scatter to primary ratios for normalisation of primary and scatter dose.

The separation of total absorbed dose into primary and scatter components is a commonly used technique in photon dose calculations. The primary dose component can be characterised by a measured narrow beam attenuation coefficient and a single normalisation value which establishes the relative proportion of the primary to the total dose at some reference depth and field size. The determination of this normalisation value from measured data requires an extrapolation of measured values for finite field sizes to obtain a zero field size value. We have used Monte Carlo simulations to score primary and scatter dose for photon beams of 4, 6, 10, 15 and 24 MV and report values of the scatter to primary ratio at the depth of dose maximum for the circular equivalent of a 10 cm x 10 cm field. These values have an uncertainty of less than 1% and can be used in lieu of extrapolation of measured data to establish the relative magnitude of the primary dose for a wide range of photon beam energies.

Computer Simulation↗
PubMed1989

Three-dimensional photon dose distributions with and without lung corrections for tangential breast intact treatments.

The influence of lung volume and photon energy on the 3-dimensional dose distribution for patients treated by intact breast irradiation is not well established. To investigate this issue, we studied the 3-dimensional dose distributions calculated for an 'average' breast phantom for 60Co, 4 MV, 6 MV, and 8 MV photon beams. For the homogeneous breast, areas of high dose ('hot spots') lie along the periphery of the breast near the posterior plane and near the apex of the breast. The highest dose occurs at the inferior margin of the breast tissue, and this may exceed 125% of the target dose for lower photon energies. The magnitude of these 'hot spots' decreases for higher energy photons. When lung correction is included in the dose calculation, the doses to areas at the left and right margin of the lung volume increase. The magnitude of the increase depends on energy and the patient anatomy. For the 'average' breast phantom (lung density 0.31 g/cm3), the correction factors are between 1.03 to 1.06 depending on the energy used. Higher energy is associated with lower correction factors. Both the ratio-of-TMR and the Batho lung correction methods can predict these corrections within a few percent. The range of depths of the 100% isodose from the skin surface, measured along the perpendicular to the tangent of the skin surface, were also energy dependent. The range was 0.1-0.4 cm for 60Co and 0.5-1.4 cm for 8 MV. We conclude that the use of higher energy photons in the range used here provides lower value of the 'hot spots' compared to lower energy photons, but this needs to be balanced against a possible disadvantage in decreased dose delivered to the skin and superficial portion of the breast.

Breast Neoplasms↗
PubMed1988

Lead-polystyrene transition zone dosimetry in high-energy photon beams.

In order to study the dose enhancement under sheets of lead positioned directly on the skin of patients, parallel-plate ionization chamber measurements in high-energy photon beams (4-15 MV) were performed below a lead-polystyrene interface. The dose in the transition zone can be much higher or lower than in the situation with full buildup of polystyrene. The enhancement of ionization directly beneath the lead-polystyrene interface, compared to the ionization at a reference depth in polystyrene, increases with photon energy and field size. The field size dependence is due to an increase in relative contribution to the energy fluence of low-energy photons scattered in the phantom and for the 4 MV beam also to photons scattered in the head of the accelerator. By adding a thin (100 microns) plastic absorber against the lead, the low-energy and large-angle electrons, which give rise to the enhanced interface dose, can largely be removed. The data indicate that lead as bolus material should only be used with extreme caution.

Humans↗
PubMed1988

Benchmark measurements for lung dose corrections for X-ray beams.

A well defined set of clinically relevant reference measurements for photon dose calculations in the presence of the lung have been provided. These benchmark data were mainly obtained in low-density (rho = 0.31 gcm-3) lunglike material as well as in waterlike plastic for 4 and 15 MV X-ray beams. Some additional measurements were performed with materials having a density of 0.015 gcm-3 and 0.18 gcm-3. Phantom geometries included simple layered geometries, finite lung cross section geometries, simulated mediastinum geometries, and simulated tumor in lung geometries. The data are reported as central axis depth doses. A number of parameters were varied, including the field size, the lung geometry, and the distance in and behind the lung.

Humans↗
PubMed1988

An external beam treatment technique for retinoblastoma.

The main difficulty in the irradiation of retinoblastoma has been to deliver a high uniform dose to the entire retinal surface and spare the lens. Conventional techniques are inadequate to deliver an acceptable dose distribution especially for cases when there are both anterior and posterior lesions. We have developed a procedure to deliver a high dose anteriorly at the ora serrata for a compromise of about 30-35% of the target dose to the lens. The technique consists of 3 pairs of non-coplanar arcs using a 4 MV accelerator. This technique may offer a higher probability of tumor control and cure when gross tumor is present at the ora serrata when compared to the conventional techniques using lateral techniques.

Eye Neoplasms↗
PubMed1987

A coordinate transfer of anatomical information from CT to treatment simulation.

Use of CT in treatment planning requires that the anatomical information obtained from the CT scans can be transferred accurately to the treatment geometry. We have developed a technique which transfers the CT data to the simulation data and vice versa. The method uses skin marks and digital radiographs reconstructed from CT scans to reproduce the patient's position. The technique has been studied with a special phantom. Based on 10 phantom set-ups, CT coordinates can be transferred to simulation coordinates with a standard deviation of 0.25 cm. This method has been applied on a number of clinical cases that involve different disease sites. For these cases, comparisons of set-up films taken at the simulator and the digital radiographs reconstructed from CT data indicate that patient position can be reproduced to within 0.5 cm.

Computer Simulation↗
PubMed1987

A computer-aided treatment planning technique for universal wedges.

Some modern accelerators provide only one wedge built into the head of the treatment machine. A continuous range of wedge angles can be generated using wedged and unwedged beams in different proportions. However, this may pose difficulty in treatment planning since the planners will have to work with twice as many beams to achieve a desirable dose distribution. We have developed a simple algorithm to solve the treatment planning problem involving wedged and open beam combinations. The algorithm is based on the assumption that the user has some knowledge of the dose distribution to be optimized. Some properties of the distributions are then used to set up the system of equations for optimization. Unlike other optimization techniques, it does not require specification of dose value to every interest point selected, and it allows each specified dose value to vary within a clinically acceptable range. Examples are given to illustrate the application of the algorithm for some standard treatment plans.

Algorithms↗
PubMed1985

Progress in 3-D treatment planning for photon beam therapy.

The purpose of this report is to study the feasibility of improving dose distributions using non-coplanar photon beams from a linear accelerator. Non-coplanar beams may enter the patient in any arbitrary configuration. This type of treatment technique requires a three-dimensional (3-D) planning system. Clinical examples are used to illustrate the general problems in 3-D treatment planning, and the potential improvement over coplanar beam treatments. Features of a treatment planning system for 3-D planning are discussed.

Brain Neoplasms↗
PubMed1978

Computer-controlled radiation therapy.

Radiation therapy is often hampered in important body regions by the need to transit sensitive normal tissues which act as dose-limiting barriers. Computer-controlled radiation therapy permits the simultaneous variation of multiple treatment parameters during irradiation of the patient, producing improved dose distributions with the potential for improved local control. Equipment used for this purpose includes a Mevatron XII linear accelerator, redesigned for automatic control, and a PDP 11/45 minicomputer. Dose distributions are shown and potential clinical gains discussed.

Computers↗
PubMedDate not supplied

Wedge-shaped dose distributions by computer-controlled collimator motion.

We have recently installed a linear accelerator, modified to allow computer control of several machine parameters during irradiation of the patient. As an initial feasibility study of computer-controlled radiation therapy, its application to produce wedge-shaped dose distributions by moving the collimator jaws has been evaluated. The required collimator motions have been calculated with an iterative technique. When these routines were used during irradiations of phantoms containing radiographic film, a good correspondence between calculated and measured dose distributions was observed. It is concluded that computer-controlled motion of the collimator jaws to shape the dose distribution is technically feasible. Additionally, this technique has the advantage that the wedge angle can be continuously adjusted and the isodose curves optimized for a particular depth and field size.

Computers↗
PubMedDate not supplied

Two-film brachytherapy reconstruction algorithm.

We have developed a new isocentric two-film reconstruction algorithm for brachytherapy seed and needle implants. The algorithm has no requirements that the two films be orthogonal, symmetric, or even be taken in a transverse plane. In addition, there is no requirement that the two films even have the same number of images. We have found removal of these usual constraints useful for head and neck implants where images are often obscured by patient anatomy. The inherent image matching ambiguities associated with traditional two-film techniques are minimized by considering the image end points, rather than just the image centroids. For two films, the new algorithm, which considers all image combinations at one time, matches all the end-point images on one film with those on the other, and then reconstructs the end-point positions of the seeds. The algorithm minimizes the difference between the actual images and the projected images from the reconstructed seeds. The new two-film image matching problem is shown to be equivalent to the well-known assignment problem. For an implant of N seeds, this equivalence allows the two-film problem to be solved by an algorithm (ACM algorithm 548) that scales with a polynomial power of N, rather than N! as is usually assumed. An implant of N seeds can be matched and reconstructed in approximately (N/20)2s on a VAX 11/780.

Brachytherapy↗
PubMedDate not supplied

Scattered photons produced by beam-modifying filters.

When a beam-modifying filter such as a wedge or a compensator is placed in an x-ray beam, scattered photons are generated in the filter material. The magnitude of the dose contribution from these photons for a 4-MV x-ray beam was measured. At a distance of 30 cm from the filter, a copper sheet of 1-cm thickness produced a dose contribution on the centerline of about 6% of the transmitted primary dose in a 20 X 20 cm2 field. At the edges of the beam, this contribution was only about one-half that on the centerline. The presence of these scattered photons leads to only minor dose errors in clinical applications if simple procedures are followed to account for their contribution.

Humans↗
PubMedDate not supplied

Heterogeneity model for photon beams incorporating electron transport.

A method of calculating photon doses in heterogeneous media incorporating electron transport is studied. The dose is represented as the convolution of kerma with an exponential longitudinal electron spread function which describes the penetration of electrons from one medium to another. At large distances from an interface, the dose approaches an asymptotic value equal to the kerma multiplied by a constant describing the degree of longitudinal and lateral electron equilibrium. For the simple situations studied, this asymptotic dose is adequately described by O'Connor's scaling theorem. The method is compared with both Monte Carlo calculations and measurements for a 15-MV photon beam for various geometries and field sizes. It predicts the dose in regions of electron disequilibrium to within 2% in most cases. In situations of extreme electron disequilibrium, such as within low-density regions at high energies and small field sizes, this represents a significant improvement over many existing techniques.

Electrons↗
PubMedDate not supplied

The influence of ionization chamber and phantom design on the measurement of lung dose in photon beams.

Lung dose correction factors, commonly defined as the ratios of ionization chamber readings in the heterogeneous and homogeneous phantoms, have been compared with those based on accurately determined doses. An analysis of stopping power values, Pwall values, and measurements in lunglike and waterlike materials showed that the wall material and thickness are not very critical in the determination of lung dose correction factors under conditions of electronic equilibrium. When lateral electronic equilibrium is not established due to the extended range of scattered electrons in the low density material, Prepl differs significantly from unity for ionization chambers with thick walls which do not match the lung material in density. An attempt has been made to characterize this effect as a function of photon energy, lung density, field size, and wall thickness.

Humans↗
PubMedDate not supplied

The effect of differences in data base on the determination of absorbed dose in high-energy photon beams using the American Association of Physicists in Medicine protocol.

Exposure rates were adjusted at the National Institute of Standards and Technology (NIST) on January 1, 1986 to take into account more recent values for some physical parameters, mainly in electron stopping power ratios. Exposure calibration factors for 60Co gamma rays Nx will therefore be lowered by 1.1%. Consequently, absorbed dose determinations in high-energy photon beams will be reduced by the same amount if the values for these physical parameters remain unchanged in the American Association of Physicists in Medicine (AAPM) protocol. If the same data base as used at NIST is applied in the AAPM protocol, then Ngas/Nx values, water-air stopping power ratios, and Pwall values will be different. The overall change in absorbed dose determinations using a consistent set of data will be a reduction of 0.8% for 60Co gamma rays and 1.5% for a 20-MV x-ray beam compared to the values before January 1, 1986. Since the net effect is small when different sets of data are applied, the new NIST exposure calibration factors may be used in combination with the AAPM protocol without significant error.

Cobalt Radioisotopes↗