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Biomedical subjects

F M Khan

Publications and source records attributed to F M Khan.

101 records · Page 6Linked to original sources
PubMedDate not supplied

Dose buildup for obliquely incident photon beams.

For obliquely incident photon beams, the buildup of dose with depth is markedly different from normally incident beams. However, relatively little data on this topic exists for high-energy photon beams of energy greater than 6 MV. Measurements of dose in the buildup region were made using a plane-parallel ionization chamber in a polystyrene phantom with obliquely incident 6-, 10-, 18-, and 24-MV x-ray beams angled 0 degrees to 84 degrees. Buildup curves at these angles were plotted and from these an obliquity factor, defined as the ratio of ionization charge collected at a point for a particular angle of incidence to that collected at the same point at normal incidence, was determined. For each energy, the obliquity factor as a function of depth, field size, and source-chamber distance was studied. Results indicate that the obliquity factor is highly dependent on the beam energy, angle of incidence, the collimator opening, and the source-skin distance. A mathematical expression has been developed to predict the dose in the buildup region of high-energy photon beams for various angles of beam incidence, field size, and chamber distance.

Humans↗
PubMedDate not supplied

The polarity effect for commercially available plane-parallel ionization chambers.

The polarity effect was investigated for three different commercially available plane-parallel ionization chambers: the Memorial Pipe chamber, the Victoreen/Nuclear Associates model 30-329 chamber manufactured by PTW, Frieburg, and the Capintec PS-033 thin-window ionization chamber. The primary study was the polarity effect versus depth below the phantom surface for 6-, 10-, 18-, and 24-MV x-ray beams, and 9- and 22-MeV electron beams. The polarity effect in the region of nonelectronic equilibrium that exists at the interface of two dissimilar materials, polystyrene and aluminum, was investigated as well as the effects of field size. For the group of plane-parallel ionization chambers that we studied, we found a polarity effect of only 1%-2% for electron beams at the depth of dmax. At depths greater than dmax, the polarity effect for electrons increased and was as high as 4.5% for some chambers. When used in the buildup region of high-energy photon beams, these same chambers exhibited up to a 30% difference in collected charge between one polarity and the other. This effect and its relationship to physical chamber characteristics is discussed.

Radiometry↗
PubMedDate not supplied

Interface dose perturbation as a measure of megavoltage photon beam energy.

The description of the quality of a photon beam has usually been characterized by a single value such as the half-value layer, the effective attenuation coefficient, the percent depth dose, and most recently by the ionization ratio (IR). Although the IR is simple and easy to measure, it lacks sensitivity at photon energies above 10 MV. This paper describes a method based on dose perturbation at an interface and defines the forward dose perturbation factor (FDPF) as a measure of beam quality. Comparisons between the two methods are given for photon energies ranging from 60Co to 24 MV. The results show that the FDPF method is more sensitive to spectral changes at photon energies above 10 MV than the IR.

Cobalt Radioisotopes↗
PubMedDate not supplied

Determination of surface dose rate from a 90Sr ophthalmic applicator.

Surface dose rates from two 90Sr ophthalmic applicators were measured using thin thermoluminescent dosimeter (TLD) chips. The TLD's were calibrated against the 90Sr source itself. The calibration technique is described. The results were compared to those obtained by three other techniques. The calibration in terms of roentgen equivalent betas per second provided by the manufacturer was found to be approximately 30% lower when compared to our measured dose rates. The surface dose rates obtained with the TLD's calibrated against 6-MV x rays were 18% higher. One of the two sources was sent to both the National Institute of Standards and Technology (NIST) and Amersham International for calibration. NIST and Amersham use extrapolation chambers as a Bragg-Gray cavity to determine the surface dose rate. NIST results gave values higher than ours by approximately 5%. The Amersham results were approximately 35% lower than ours.

Humans↗
PubMedDate not supplied

Measurement of dose in the buildup region using fixed-separation plane-parallel ionization chambers.

Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed-separation plane-parallel ionization chambers are most commonly used for this purpose. Recent papers have re-emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed-separation plane-parallel ionization chambers. Data for Co-60, 6-, 10-, 18-, and 24-MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane-parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of greater than 19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.

Humans↗
PubMedDate not supplied

Validity of transition-zone dosimetry at high atomic number interfaces in megavoltage photon beams.

Measurement of dose or dose perturbation factors at high atomic number interfaces are usually performed with a thin-window parallel-plate ion chamber. In a transition region, under nonequilibrium conditions, accuracy of ion chamber readings for the dose measurements has often been questioned. This paper critically analyzes the factors (stopping power ratio and charge collection) for the dose measurements at interfaces. Monte Carlo simulations were performed to investigate the secondary electron spectrum produced by photon beams and to calculate the stopping power ratios at the point of measurement. The validity of dose measurements was studied for the photon beams in the range of Co-60 gamma rays to 24-MV x rays at bone and lead interfaces with polystyrene, using thermoluminescent dosimeters, extrapolation chamber and several types of commercially available parallel-plate ion chambers. It is observed that for energies greater than 10 MV most parallel-plate chambers can be used to measure dose accurately. At lower energies, however significant differences between measured doses with different detectors were noticed. It is suggested that at high-Z interfaces and lower energies, the dose measurements should be performed with ultrathin-window parallel-plate ion chambers or extrapolation chambers.

Bone and Bones↗
PubMedDate not supplied

Replacement correction (P(repl)) for ion chamber dosimetry.

The use of replacement correction when depth dose distribution is measured with a cylindrical ion chamber is discussed. A review of relevant data shows that the displacement of the effective point of measurement is an appropriate method to make replacement correction for the entire depth ionization curve.

Calibration↗
PubMedDate not supplied

Dosimetry of wedged fields with asymmetric collimation.

Shaping of wedged fields with asymmetric collimators (independent jaws) produces dosimetric effects which need to be taken into account when calculating isodose distributions and accelerator monitor units. These effects were studied using ion chamber dosimetry in a water phantom. Wedged fields of 30 degrees to 60 degrees wedge angles were shaped by an asymmetric collimator. A system of dose calculation was developed to take into account the changes in dose in the open portion of the field. The formalism utilizes regular symmetric field parameters (e.g., Sc, Sp, and TPR). Calculated point doses and profiles agreed well with the measured data.

Biophysical Phenomena↗
PubMedDate not supplied

Molecular characterisation of red cell glucose-6-phosphate dehydrogenase deficiency in north-west Pakistan.

233 Pushtoons (129 males and 104 females), 51 Punjabi Muslims (29 males and 22 females) and 21 Afghans (15 males and 6 females) were screened for the presence of red cell glucose-6-phosphate dehydrogenase (G6PD) variants by a dye decolouration screening test and starch gel electrophoresis. The overall frequency of G6PD deficiency in males was found to be about 10%. 17 male G6PD-deficient samples were further investigated for the C-->T substitution at nucleotide (nt) 563 (the Mediterranean mutation) and the C-->T substitution at nt 1311 (the 'silent' allele) of the G6PD gene by PCR amplification followed by digestion with appropriate restriction enzymes. 10 of the 13 Pushtoon, 2 Punjabi and 1 Afghan males had the 563 mutations. Only 1 (Punjabi) out of 13 G6PD-deficient males with the 563 mutation had the silent mutation at nt 1311. The frequency of the silent mutation was found to be about 0.20 in the 60 Pushtoon and 19 Punjabi non-deficient males.

Base Sequence↗
PubMedDate not supplied

Smoking prevalence, knowledge and attitudes among medical students in Karachi, Pakistan.

A survey of smoking prevalence and attitudes was made among medical students randomly selected from classes at the Aga Khan University, Karachi, Pakistan. Of 271 respondents, 14.4% were current smokers (22.0% male and 3.8% females) and 3.3% ex-smokers. A majority of students recognized the dangers associated with active as well as passive smoking although only 55% of current smokers planned to quit in the near future. Most smokers (96%) believed that they as well as other health professionals needed training on smoking cessation and 95% of all students believed that doctors should play a role model in smoking cessation by not smoking themselves. Specific training and counselling should be a part of the required curriculum at medical schools.

Adolescent↗