Sd Sharma

Acrylonitrile Butadiene Styrene (ABS) plastic‐based low cost tissue equivalent phantom for verification dosimetry in IMRT

A novel IMRT phantom was designed and fabricated using Acrylonitrile Butadiene Styrene (ABS) plastic. Physical properties of ABS plastic related to radiation interaction and dosimetry were compared with commonly available phantom materials for dose measurements in radiotherapy. The ABS IMRT phantom has provisions to hold various types of detectors such as ion chambers, radiographic/radiochromic films, TLDs, MOSFETs, and gel dosimeters. The measurements related to pretreatment dose verification in IMRT of carcinoma prostate were carried out using ABS and Scanditronix‐Wellhofer RW3 IMRT phantoms for five different cases. Point dose data were acquired using ionization chamber and TLD discs, while Gafchromic EBT and radiographic EDR2 films were used for generating 2D dose distributions. Treatment planning system (TPS) calculated and measured doses in ABS plastic and RW3 IMRT phantom were in agreement within ± 2%. The dose values at a point in a given patient acquired using ABS and RW3 phantoms were found comparable within 1%. Fluence maps and dose distributions of these patients generated by TPS and measured in ABS IMRT phantom were also found comparable both numerically and spatially. This study indicates that ABS plastic IMRT phantom is a tissue‐equivalent phantom and, dosimetrically, it is similar to solid/plastic water IMRT phantoms. Although this material is demonstrated for IMRT dose verification, it can also be used as a tissue‐equivalent phantom material for other dosimetry purposes in radiotherapy. PACS number: 87.53Kn, 87.55Qr, 87.53Bn and 87.55Km

Dosimetry of gamma chamber blood irradiator using Gafchromic EBT film.

Gafchromic films are increasingly being used for dosimetry in medical and industrial applications of ionizing radiation because of their favorable characteristics such as self developing in nature, easy to handle, dose rate independent response, insensitivity to normal room lights, high resolution and insensitivity to variation in the environment conditions. The dosimetry measurements using Gafchromic EBT film, Fricke dosimeter and thermoluminescence (TL) dosimeter were carried out on gamma blood irradiators to establish the suitability of Gafchromic EBT film in such applications. The dose rates determined by Gafchromic EBT film, Fricke dosimeter and TL dosimeter powder are found in good agreement to one another within the uncertainty of measurement. The two-dimensional dose distributions determined using Gafchromic EBT film reflects the expected dose distribution inside the sample compartment of the blood irradiator. The results of this study establish Gafchromic EBT film a suitable dosimeter for routine dosimetry on gamma blood irradiators.

A dose verification method for high-dose-rate brachytherapy treatment plans.

AIM To evolve a fast dose verification method for high-dose-rate (HDR) brachytherapy treatment plans and to demonstrate its applicability in different clinical cases. MATERIALS AND METHODS We developed a software tool in VC++ for the Varisource HDR unit for HDR dosimetry plan verification using TG-43 parameters. HDR treatment dosimetry of a number clinical cases using Varisource was verified by comparison with the treatment planning system (TPS). RESULTS A number of different types of clinical cases treated by Varisource were evaluated. TPS calculated dose values and verification code calculated dose values were found to agree to within 3% for most of the dose calculation points. CONCLUSIONS We have validated with clinical cases a fast and independent dose verification method of the dosimetry at selected points for HDR brachytherapy treatments plan using TG-43 parameters. This can be used for the verification of the TPS calculated dose at various points. The code is written to work with Varisource, but it can conceivably be modified for other sources also by using the fitted constant of the respective source.

A dosimetry method in the transverse plane of HDR Ir-192 brachytherapy source using gafchromic EBT2 film

Radiochromic film dosimetry is increasingly used in brachytherapy applications for its higher resolution ability as compared to other experimental methods. The present study was aimed to assess the accuracy and suitability of use of the improved radiochromic film model, Gafchromic EBT2, to evaluate the dose distribution in the transverse plane of microselectron HDR (192)Ir source. A specially designed and locally fabricated Polymethyl methacrylate (PMMA) phantom was used in this work for the experimental measurement of dose distribution around the source in its transverse plane. The AAPM TG-43U1 recommended radial dose function, g (r), and dose rate constant, Λ, for the source were measured using Gafchromic EBT2 film and thermoluminescent dosimeters (TLD). The EBT2 film measured dosimetric quantities were validated against their values obtained from the TLD measurements and previously published values for the same source available in literature. The dose rate constant and radial dose function for microselectron HDR (192)Ir source obtained from Gafchromic EBT2 film measurements are in agreement with their TLD measured results within 3.9% and 2.8% respectively. They also agree within the accepted range of uncertainty with their experimental and Monte Carlo calculated results reported in literature. This work demonstrates the suitability of using Gafchromic EBT2 film dosimetry in characterization of dose distribution in the transverse plane of HDR Ir-192 source. This is a more efficient method than TLD dosimetry at discrete and distant positions. Relative to TLD dosimetry, it is found to be better reproducible, easy to use and a less expensive method of dosimetry.

Dosimetry during mango irradiation using Gafchromic HD-810 film

The dosimetric characteristics of Gafchromic HD-810 film were evaluated for its possible use as a high-dose dosimeter for routine dosimetry during mango irradiation. The film dosimeter sample of size 2 x 2 cm(2) was used throughout the course of this work. The irradiation of the film dosimeter for characterisation and calibration purposes was carried out in a gamma irradiator. The dose-response of the Gafchromic HD-810 film dosimeter at 550 nm was found to be linear in the dose range 50-1000 Gy, which indicates the feasibility of using this film for dosimetry up to 1000 Gy. The mean inter-dosimeter variation was within 2%, which gives better dose-response consistency of the HD-810 film. The radiation absorbed dose measured by the Gafchromic HD-810 film dosimeter during mango irradiation was compared with that measured by a standard Ceric-cerous dosimeter. This study establishes the Gafchromic HD-810 film as a convenient and technically suitable dosimeter for high-dose dosimetry up to 1.0 kGy during mango irradiation.

Dosimetric verification of a high dose rate brachytherapy treatment planning system in homogeneous and heterogeneous media

OBJECTIVES To verify the dosimetric accuracy of treatment plans in high dose rate (HDR) brachytherapy by using Gafchromic EBT2 film and to demonstrate the adequacy of dose calculations of a commercial treatment planning system (TPS) in a heterogeneous medium. METHODS Absorbed doses at chosen points in anatomically different tissue equivalent phantoms were measured using Gafchromic EBT2 film. In one case, tandem ovoid brachytherapy was performed in a homogeneous cervix phantom, whereas in the other, organ heterogeneities were introduced in a phantom to replicate the upper thorax for esophageal brachytherapy treatment. A commercially available TPS was used to perform treatment planning in each case and the EBT2 films were irradiated with the HDR Ir-192 brachytherapy source. RESULTS Film measurements in the cervix phantom were found to agree with the TPS calculated values within 3% in the clinically relevant volume. In the thorax phantom, the presence of surrounding heterogeneities was not seen to affect the dose distribution in the volume being treated, whereas, a little dose perturbation was observed at the lung surface. Doses to the spinal cord and to the sternum bone were overestimated and underestimated by 14.6% and 16.5% respectively by the TPS relative to the film measurements. At the trachea wall facing the esophagus, a dose reduction of 10% was noticed in the measurements. CONCLUSIONS The dose calculation accuracy of the TPS was confirmed in homogeneous medium, whereas, it was proved inadequate to produce correct dosimetric results in conditions of tissue heterogeneity.

Target foil rupture scenario and provision for handling different models of medical cyclotrons used in India

Medical cyclotron is a particle accelerator used in producing short lived radiotracers such as 18F, 11C, 15O, 13N etc. These radiotracers are labeled with suitable pharmaceuticals for use to gather information related to metabolic activity of the cell using Positron Emission Tomography (PET) scan. Target foil rupture is considered one of the major emergency situations during medical cyclotron operations because there is a potential of over exposure to the working personnel. Radiation protection survey of a self-shielded medical cyclotron installation was carried out during normal and emergency conditions. It is found that the induced activity in the target foil increases with its successive usages. As a case study, we have evaluated the emergency handling procedures of GE PETtrace-6 medical cyclotron. Recommendations have also been made to reduce personal exposure while handling the target foil rupture condition such as the use of L-Bench near the target area and participation of experienced personnel.

Design, implementation and validation of a motorized wedge filter for a telecobalt machine (Bhabhatron-II)

A universal wedge filter of 15W × 20 cm(2) and 60° nominal wedge angle is designed and placed between the collimating jaws and penumbra trimmers inside the treatment head. A pneumatically driven actuating mechanism toggles the wedge between the wedge IN position and wedge OUT position. The effective wedge angles were determined using an analytical formula. An accumulated wedge profile at a depth of 10 cm which was measured using a 2D profiler and dose values at depths of 10 cm and 20 cm for the same experimental setup were used as input parameters in the formula used for determining effective wedge angles. The relationship between the wedge beam weight and effective wedge angle was established. The planned wedge angles were compared with the measured wedge angles and the differences are found to be less than 2° throughout the range of field sizes. Planned doses for various field sizes and wedge angles were measured for verification and the differences were found to be less than 1.8%. This study established that the relationship between the beam weights and effective wedge angles implemented for the motorized wedge filter of medical linacs is not directly applicable for the motorized wedge filter of Telecobalt.

Long term response stability of a well-type ionization chamber used in calibration of high dose rate brachytherapy sources.

Well-type ionization chamber is often used to measure strength of brachytherapy sources. This study aims to check long term response stability of High Dose Rate (HDR)-1000 Plus well-type ionization chamber in terms of reference air kerma rate (RAKR) of a reference 137Cs brachytherapy source and recommend an optimum frequency of recalibration. An HDR-1000 Plus well-type ionization chamber, a reference 137Cs brachytherapy source (CDCSJ5), and a MAX-4000 electrometer were used in this study. The HDR-1000 Plus well-type chamber was calibrated in terms of reference air kerma rate by the Standards Laboratory of the International Atomic Energy Agency (IAEA), Vienna. The response of the chamber was verified at regular intervals over a period of eight years using the reference 137Cs source. All required correction factors were applied in the calculation of the RAKR of the 137Cs source. This study reveals that the response of the HDR-1000 Plus well-type chamber was well within ±0.5% for about three years after calibration/recalibration. However, it shows deviations larger than ±0.5% after three years of calibration/recalibration and the maximum variation in response of the chamber during an eight year period was 1.71%. The optimum frequency of recalibration of a high dose rate well-type chamber should be three years.

Study of structural, optical and electrical properties of gamma irradiated In2O3 thin films for device applications

The influence of gamma irradiation on the structural, optical and electrical properties of indium oxide (