S. D. Sharma

Measurement of Dielectric Constant and Loss Factor of the Dielectric Material at Microwave Frequencies

A new technique to evaluate the dielectric constant and loss factor of a homogeneous dielectric material using rectangular shaped perturb cavity has been developed. The values of S-parameters are measured experimentally by placing the sample in the center of the cavity resonator. Sample under test is fabricated in the form of a cylinder. The real and imaginary part of the permittivity can be then calculated from the shift in the resonance frequency and Q-factor. The results of a Teflon sample are also tabulated.

Decreased plasma glutathione in cancer of the uterine cervix

Plasma total glutathione (GSH) content (reduced plus oxidized) was estimated in varying grades of cervical intraepithelial neoplasia (CIN) and in invasive cervical cancer. The values were compared with age-matched control women. The results show significantly lower level of plasma GSH in CIN III and invasive cancer compared to controls (0.724 versus 1.082 and 0.622 versus 1.082 mumol/ml of plasma, P < 0.05). Further, the odds ratio analysis showed high plasma GSH content was found to be protective against the development of cervical cancer. The results suggest a plausible association of plasma GSH with cervical carcinogenesis. The quantitative changes occurring in plasma total glutathione during cervical carcinogenesis is a useful finding and might represent a systemic biochemical marker for precancerous and cancerous lesions of the uterine cervix.

Performance evaluation of a dedicated computed tomography scanner used for virtual simulation using in-house fabricated CT phantoms

Comprehensive tests on single slice CT scanner was carried out using in-house fabricated phantoms/test tools following AAPM recommended methods to independently validate the auto-performance test (APT) results. Test results of all the electromechanical parameters were found within the specified limits. Radiation and sensitivity profile widths were within ± 0.05 cm of the set slice thickness. Effective energy corresponding to nominal kVp of 80, 110 and 130 were 49.99, 55.08 and 59.48 keV, respectively. Percentage noise obtained by APT was 1.32% while the independently measured value was 0.38%. Observed contrast resolutions by independent method at 0.78% and 12% contrast difference were 4 mm and 1.25 mm (= 4 lp/cm) respectively. However, high contrast resolution (limiting spatial resolution) by APT at 50, 10 and 2% MTF levels were 9, 12.5 and 14.1 lp/cm respectively. Difference in calculated and measured CT numbers of water, air, teflon, acrylic, polystyrene and polypropylene were in the range of 0 to 24 HU, while this difference was 46 and 94 HU in case of nylon and bakelite respectively. The contrast scale determined using CT linearity phantom was 1.998×10−4 cm−1/CT number. CT dose index (CTDI) and weighted CTDI (CTDIw) measured at different kVp for standard head and body phantoms were smaller than manufacturer-specified and system-calculated values and were found within the manufacturer-specified limit of ± 20%. Measured CTDIs on surface (head: 3.6 cGy and body: 2.6 cGy) and at the center (3.3 cGy, head; and 1.2 cGy, body) were comparable to reported values of other similar CT scanners and were also within the industry-quoted CTDI range. Comprehensive QA and independent validation of APT results are necessary to obtain baseline data for CT virtual simulation.

Characteristics of mobile MOSFET dosimetry system for megavoltage photon beams

The characteristics of a mobile metal oxide semiconductor field effect transistor (mobile MOSFET) detector for standard bias were investigated for megavoltage photon beams. This study was performed with a brass alloy build-up cap for three energies namely Co-60, 6 and 15 MV photon beams. The MOSFETs were calibrated and the performance characteristics were analyzed with respect to dose rate dependence, energy dependence, field size dependence, linearity, build-up factor, and angular dependence for all the three energies. A linear dose-response curve was noted for Co-60, 6 MV, and 15 MV photons. The calibration factors were found to be 1.03, 1, and 0.79 cGy/mV for Co-60, 6 MV, and 15 MV photon energies, respectively. The calibration graph has been obtained to the dose up to 600 cGy, and the dose-response curve was found to be linear. The MOSFETs were found to be energy independent both for measurements performed at depth as well as on the surface with build-up. However, field size dependence was also analyzed for variable field sizes and found to be field size independent. Angular dependence was analyzed by keeping the MOSFET dosimeter in parallel and perpendicular orientation to the angle of incidence of the radiation with and without build-up on the surface of the phantom. The maximum variation for the three energies was found to be within ± 2% for the gantry angles 90° and 270°, the deviations without the build-up for the same gantry angles were found to be 6%, 25%, and 60%, respectively. The MOSFET response was found to be independent of dose rate for all three energies. The dosimetric characteristics of the MOSFET detector make it a suitable in vivo dosimeter for megavoltage photon beams.

A Survey on the Quality Assurance Procedures Used in Intensity Modulated Radiation Therapy (IMRT) at Indian Hospitals

A national survey to obtain information about the Quality Assurance (QA) procedures and methods being followed at Indian radiotherapy centers for intensity modulated radiation therapy (IMRT) was conducted. A questionnaire containing parameters relevant to IMRT QA was evolved to collect the information pertaining to the QA of IMRT delivery system, QA of IMRT treatment planning system, and patient specific IMRT QA. The questionnaire was circulated to 40 hospitals in the country and responses of 31 centers were received. Survey results showed that 71% centers are having adequate machine specifi c IMRT QA programme, 19% centers have inadequate machine specific IMRT QA programme and 9% centers have irrelevant machine specific IMRT QA programme. No specific answer for question of QA tests of TPS specific to IMRT were received from the user. Almost all the centers have programme of setup verification of the patient by means of EPID/DRR/OBI. However, 91% of centers could not provide any information about the QA methodology of the devices used for setup verification. For patient specific dosimetric QA, almost all the hospitals have the program of pre-treatment dose verification using calibrated ionization chambers of sensitive volumes in the range of 0.01 to 0.65 cc. Dosimetric verification is performed by combining dose from all gantry angles to a single gantry angle. Two dimensional (2D) dosimetry systems such as radiographic and radiochromic films, 2D array of ionization chambers/ semiconductor diodes and EPID are also used in patient specific dosimetry verifications. Majority of the centers (about 48%) accept the plan with 3% dose difference and 3 mm dose to distance agreement criteria with gamma index less than unity. However, a number of other acceptance criteria specific to institution and tumor site are being also followed. This survey reveals that a variety of IMRT QA program is being followed at the Indian hospitals. This study has brought into focus the need to evolve a national protocol for IMRT QA so that treatment outcomes of all the IMRT centers of country can be compared.

Development of a Spherical 125I-Brachytherapy Seed for Its Application in the Treatment of Eye and Prostate Cancer

Palladium-coated silver beads of 0.5 mm (phi) were used to adsorb (125)I, encapsulated inside a titanium capsule by an Nd:YAG laser, for use as a brachytherapy source. Experimental conditions, such as feed activity, carrier concentration, and reaction time, were optimized for the maximum adsorption of (125)I. Uniform distribution of radioactivity on the source core was ascertained by the autoradiography technique. Leachability of (125)I was found to be <0.01%. The dose-rate constant of the new source was estimated to be 1.045 cGyh(-1)U(-1). This newly developed source could be an alternative to other (125)I sources.

Interferometric focal length measurement of positive and negative lenses using a lateral-shearing cyclic path optical configuration setup and polarization phase-shifting interferometry

We present a simple interferometric focal length measurement technique for measuring the focal length of positive and negative lenses using a lateral-shearing cyclic path optical configuration (CPOC) setup and polarization phase-shifting interferometry (PPSI). The technique requires an auxiliary lens, whose focal length need not be known accurately, for generating a focused point source at its focal plane. The focal point of the test lens is adjusted to be in the vicinity of the focus of the auxiliary lens. The radii of the resulting spherical wave fronts emerging from the test lens, due to the defocus, for two different longitudinal shifted positions of the test lens are measured using the lateral-shearing CPOC setup and PPSI. Focal length of the test lens is determined from the known longitudinal shift and from the measured radii of the spherical wave fronts. Results obtained for 200.0xa0mm convex and 100.0xa0mm concave test lenses are presented.

Estimation of Eye Lens Dose During Brain Scans Using Gafchromic Xr-QA2 Film in Various Multidetector CT Scanners

The purpose of this study was to estimate eye lens dose during brain scans in 16-, 64-, 128- and 256-slice multidetector computed tomography (CT) scanners in helical acquisition mode and to test the feasibility of using radiochromic film as eye lens dosemeter during CT scanning. Eye lens dose measurements were performed using Gafchromic XR-QA2 film on a polystyrene head phantom designed with outer dimensions equivalent to the head size of a reference Indian man. The response accuracy of XR-QA2 film was validated by using thermoluminescence dosemeters. The eye lens dose measured using XR-QA2 film on head phantom for plain brain scanning in helical mode ranged from 43.8 to 45.8xa0mGy. The XR-QA2 film measured dose values were in agreement with TLD measured dose values within a maximum variation of 8.9%. The good correlation between the two data sets confirms the viability of using XR-QA2 film for eye lens dosimetry.

Tsallis nonextensive entropy and the multiplicity distributions in high energy leptonic collisions

The nonextensive behavior of entropy is exploited to explain the regularity in multiplicity distributions in e+e− collisions at high energies. The experimental data are analyzed by using Tsallis q-statistics. We propose a new approach of applying Tsallis q-statistics, wherein the multiplicity distribution is divided into two components; two-jet and multijet components. A convoluted Tsallis distribution is fitted to the data. It is shown that this method gives the best fits which are several orders better than the conventional fit of Tsallis distribution.

Dosimetric Characteristics of a PIN Diode for Radiotherapy Application

The PIN diode developed by Bhabha Atomic Research Centre (BARC) was modified for its use as a dosimeter in radiation therapy. For this purpose the diode was mounted on a printed circuit board (PCB) and provided with necessary connections so that its response against irradiation can be recorded by a standard radiotherapy electrometer. The dosimetric characteristics of the diode were studied in Co-60 gamma rays as well as high energy X-rays. The measured sensitivity of this PIN diode is 4 nC/cGy which is about ten times higher than some commercial diode dosimeters. The leakage current from the diode is 0.04 nA. The response of the PIN diode is linear in the range of 20–1000 cGy which covers the full range of radiation dose encountered in radiotherapy treatments. The non-linearity of the diode response is 3.5% at 20 cGy and it is less than 1.5% at higher dose values. Its repeatability is within 0.5%. The angular response variation is about 5.6% within ± 60° with respect to normal beam incidence. The response of the PIN diode at 6 and 18 MV X-rays varies within 2% with respect to its response at Co-60 gamma rays. The source to surface distance (SSD) dependence of the PIN diode was studied for Co-60 beam. It was found that the response of the diode decreases almost linearly relative to given dose for beams with constant collimator setting but increasing SSD (decreasing dose-rate). Within this study the diode response varied by about 2.5% between the maximum and minimum SSD. The dose-rate dependence of the PIN diode for 6 and 15 MV-rays was studied. The variation in response of diode for both energies in the studied dose range is less than 1%. The field size dependence of the PIN diode response is within 1% with respect to the response of ionisation chamber. These studies indicate that the characteristics of the PIN diode are suitable for use in radiotherapy dosimetry.