Biomedical subjects

G H Chan

Publications and source records attributed to G H Chan.

7 recordsLinked to original sources

PubMed2004

Evaluation of the first commercial Monte Carlo dose calculation engine for electron beam treatment planning.

The purpose of this study is to perform a clinical evaluation of the first commercial (MDS Nordion, now Nucletron) treatment planning system for electron beams incorporating Monte Carlo dose calculation module. This software implements Kawrakow's VMC++ voxel-based Monte Carlo calculation algorithm. The accuracy of the dose distribution calculations is evaluated by direct comparisons with extensive sets of measured data in homogeneous and heterogeneous phantoms at different source-to-surface distances (SSDs) and gantry angles. We also verify the accuracy of the Monte Carlo module for monitor unit calculations in comparison with independent hand calculations for homogeneous water phantom at two different SSDs. All electron beams in the range 6-20 MeV are from a Siemens KD-2 linear accelerator. We used 10,000 or 50,000 histories/cm2 in our Monte Carlo calculations, which led to about 2.5% and 1% relative standard error of the mean of the calculated dose. The dose calculation time depends on the number of histories, the number of voxels used to map the patient anatomy, the field size, and the beam energy. The typical run time of the Monte Carlo calculations (10,000 histories/cm2) is 1.02 min on a 2.2 GHz Pentium 4 Xeon computer for a 9 MeV beam, 10 x 10 cm2 field size, incident on the phantom 15 x 15 x 10 cm3 consisting of 31 CT slices and voxels size of 3 x 3 x 3 mm3 (total of 486,720 voxels). We find good agreement (discrepancies smaller than 5%) for most of the tested dose distributions. We also find excellent agreement (discrepancies of 2.5% or less) for the monitor unit calculations relative to the independent manual calculations. The accuracy of monitor unit calculations does not depend on the SSD used, which allows the use of one virtual machine for each beam energy for all arbitrary SSDs. In some cases the test results are found to be sensitive to the voxel size applied such that bigger systematic errors (>5%) occur when large voxel sizes interfere with the extensions of heterogeneities or dose gradients because of differences between the experimental and calculated geometries. Therefore, user control over voxelization is important for high accuracy electron dose calculations.

Algorithms↗

PubMed2000

Dose measurement based on spectral chi-square minimization using GAFchromic MD-55 film.

A new method of measuring dose using GAFchromic MD-55-2 type film is proposed. It involves 2D fitting of optical density, as a function of both wavelength and dose, to a set of measured calibration data. An unknown dose to a film can then be determined by minimizing the chi-square between the measured optical densities and the fitted calibration data. The methodology was tested, using 60Co calibration, by measuring the dose deposited in films irradiated by a 32p source in a semi-infinite geometry. The results are compared with the expected dose obtained From beta counting, taking into account the dose loss correction due to various attenuating materials, including the film substrate itself, which was determined to be 20%. It was found that the root mean relative deviation between the measured and expected doses was approximately 3%. The excellent agreement also demonstrated the possibility of calibration between gamma and beta particles.

Beta Particles↗

PubMed1997

Early correction of severe unilateral infant ptosis with the Mersilene mesh sling.

Ten consecutive cases of severe unilateral congenital ptosis were surgically corrected before 1 year of age (range 3-11 months) in an attempt to achieve early functional and cosmetic improvement. In all cases, a frontalis suspension using Mersilene mesh was performed. With a mean follow-up of 40.3 months (range 33-54 months), all patients achieved normal or near normal eyelid position and all had their chin-up head posture resolved. One patient developed a mild exposure keratitis that was treated successfully with topical antibiotics and lubricants. Our findings suggest that the Mersilene mesh sling has good potential for ptosis management in infants who are too young for fascial harvesting. However, a larger series with a longer follow-up period is required before the eventual safety and efficacy of the Mersilene mesh sling can be properly ascertained.

Age Factors↗

PubMed1991

Congenital cystic eye with multiple ocular and intracranial anomalies.

We describe a newborn with congenital cystic eye, contralateral persistent hyperplastic primary vitreous, and cerebrocutaneous abnormalities. The cerebrocutaneous abnormalities consisted of agenesis of the corpus callosum, midbrain deformity, malformed sphenoid bone, right upper eyelid coloboma, and a left periocular hamartoma. The results of karyotype analysis of the patient and his parents were normal. The association of congenital cystic eye with contralateral persistent hyperplastic primary vitreous has not been previously reported, to our knowledge. Although no unifying diagnosis exists for the collection of anomalies demonstrated in this patient, the term cranial ectodermopathy broadly classifies most of the defects.

Abnormalities, Multiple↗

PubMed1982

Prismatic scanning method for improving visual acuity in patients with low vision.

Frequently, patients with macular lesions and others who are visually handicapped do not receive adequate low vision care by ophthalmologists if a routine trial of low vision aids has no gratifying results. This paper describes for the first time a technique in which functional vision was achieved with the aid of prismatic scanning in patients for whom the usual low vision aids had proved ineffectual. The procedure in which prisms are used to achieve functional vision in patients with macular lesions in uncomplicated, inexpensive, and rapid; indeed, it can be used by an ophthalmologist in routine office practice with existing equipment.

Adult↗

PubMed1975

Post-traumatic ophthalmic vein arterialization.

An unusual case of post-traumatic exophthalmos without pulsation, bruit or thrill, due to missile injury is presented. Arterialization of the ophthalmic vein represents one of the very few situations where an eye injury can lead to exsanguination and dealth unless the diagnosis is made early. Prompt carotid arteriography is advised to document the pathologic process and guide appropriate therapy.

Adolescent↗

PubMedDate not supplied

Juvenile xanthogranuloma.

Juvenile xanthogranuloma (JXG) is a disease of unknown etiology and pathogenesis which was first recognized by dermatologists. The ocular complications provide the primary concern and notably affect the iris, producing spontaneous hemorrhages in the anterior chamber. Secondary glaucoma and blindness are real threats unless the condition is recognized in the early stages and satisfactory treatment instituted. Other ocular manifestations include epibulbar lesions with corneal involvement and proptosis due to orbital lesions of JXG.

Eye Color↗