Farhad Moradi | University of Malaya, Malaysia (original) (raw)

Papers by Farhad Moradi

With high-dose applications lacking the benefit of an economic yet versatile dosimeter that provi... more With high-dose applications lacking the benefit of an economic yet versatile dosimeter that provides for a wide dynamic dose range, ongoing research is seeking to introduce suitable thermoluminescent
(TL) material for such needs. Acknowledging the high potential of silica fibres, as developed by members of this group over the past few years, in present work evaluation has been made of 13 types of fibre, differing in dopant, dopant concentration and diameter including P-, Al-, Er-, Ge- and Al-Tm-doped fibres, ultra-high numerical aperture and borosilicate fibre, and two non-doped fibres, quartz and suprasil F300. Evaluation is made in terms of TL response to photon and electron irradiations with the objective of determining a TL material that can offer sensitive yet extended dose capability, saturating only above the few tens of kGy range. The various silica fibres that have been investigated were found to show saturation levels from 5 kGy for Ge-doped fibre (4 mol %) to 80 kGy for 2 mol % Al-doped silica fibre. Borosilicate fibres demonstrated the greatest potential for high dose dosimetry, maintaining a highly-linear response, any tendency towards saturation only being indicated
to beyond receipt of doses of 100 kGy. For this fibre type detailed TL characterizations were conducted, including glow curve analysis, reproducibility and fading tests. The results suggest borosilicate fibre to be suitable for high dose TL dosimetry, providing sufficient sensitivity and appropriate dosimetric characteristics.

The relatively new treatment modality electronic intraoperative radiotherapy (IORT) is gaining po... more The relatively new treatment modality electronic intraoperative radiotherapy (IORT) is gaining popularity, irradiation being obtained within a surgically produced cavity being delivered via a low-energy X-ray source and spherical applicators, primarily for early stage breast cancer. Due to the spatially dramatic dose-rate fall off with radial distance from the source and effects related to changes in the beam quality of the low keV photon spectra, dosimetric account of the Intrabeam system is rather complex. Skin dose monitoring in IORT is of importance due to the high dose prescription per treatment fraction. In this study, modeling of the X-ray source and related applicators were performed using the
Monte Carlo N-Particle (MCNP) transport code. The dosimetric characteristics of the model were validated against measured data obtained using an ionization chamber and EBT3 film as dosimeters.
By using a simulated breast phantom, absorbed doses to the skin for different combinations of applicator size (1.5 to 5 cm) and treatment depth (0.5 to 3 cm) were calculated. Simulation results showed
overdosing of the skin (> 30% of prescribed dose) at a treatment depth of 0.5 cm using applicator sizes larger than 1.5 cm. Skin doses were significantly increased with applicator size, insofar as delivering
12 Gy (60% of the prescribed dose) to skin for the largest sized applicator (5 cm diameter) and treatment depth of 0.5 cm. It is concluded that the recommended 0.5 to 1 cm distance between the skin and applicator surface does not guarantee skin safety and skin dose is generally more significant in case of the larger applicators.

Prior investigation of the suitability of optical fibres as thermoluminescent dosimeters for diag... more Prior investigation of the suitability of optical fibres as thermoluminescent dosimeters for diagnostic and therapeutic radiation beams has not included detailed study of the effect of beam angulation. Present study of such response has made use of optical fibre of cylindrical shape, exposed to 30 kVp photons from an X-ray tube and a 6 MV photon beam from a linear accelerator. The effect of the irradiation medium was also studied, comparing response free-in-air against on-surface and in-depth irradiations through use of solid-water™ phantom. Standard optical fibre (ø =125 µm) shows non-uniform response to beams delivered at different incident angles. Monte Carlo simulation provided support for the experimental results, also obtaining absorbed dose in the fibres. The results of free-in-air condition simulated with mono-energy beam show angle-independent response for photons within the energy range 100–500 keV, while dependency has been observed for beam energies of < 100 keV and > 500 keV. Experimentally, the angular dependency up to 35% is observed in 30 kVp free-in-air, while in 6 MeV beam, this is reduced to 20%, 10%, and 3% in free-in-air, on phantom surface, and in-depth conditions, respectively. The observations have been justified by considering the range of secondary electrons in the dosimeter and the effect of scattered radiation.

In recent years doped silica fibre thermoluminescent dosimeters (TLD) have been demonstrated to h... more In recent years doped silica fibre thermoluminescent dosimeters (TLD) have been demonstrated to have considerable potential for irradiation applications, benefitting from the available sensitivity, spatial resolution and dynamic dose range, with primary focus being on the needs of medical dosimetry. Present study concerns the dose distribution inside a cylindrically shaped gamma-ray irradiator cavity, with irradiator facilities such as the familiar 60 Co versions being popularly used in industrial applications. Quality assurance of the radiation dose distribution inside the irradiation cell of such a device is of central importance in respect of the delivered dose to the irradiated material. Silica fibre TLD dose-rates obtained within a Gammacell-220 irradiator cavity show the existence of non-negligible dose distribution heterogeneity, by up to 20 % and 26 % in the radial and axial directions respectively, Monte Carlo simulations and available literature providing some support for present findings. In practice, it is evident that there is need to consider making corrections to nominal dose-rates in order to avoid the potential for under-dosing.

Nowadays, silica based optical fibres show enough potential to be used as TL dosimeters in differ... more Nowadays, silica based optical fibres show enough potential to be used as TL dosimeters in different applications. Reuse of optical fibre as a practical dosimeter demands to complete removal of accumulated doses via previous irradiations. This work investigates the existence and/or effect of remnant doses in fibre dosimeter from the previous irradiations, and proposes a method to control this artifact. A single mode Ge-doped optical fibre is used as TL radiation sensor, while a well calibrated Gammacell with 60 Co source is used for irradiations. The effect of irradiation history on the TL response of optical fibres is surveyed extensively for doses ranged from 1 to 1000 Gy. The results show that the absorbed dose history in a fibre affects its response in the next irradiation cycles. It is shown that a dose history of around 100 Gy can increase the response of optical fibre by a factor of 1.72. The effect of annealing at higher temperatures on stabilizing the fibre response is also examined and results revealed that another alteration in the structure of trapping states occurs in glass medium which can change the sensitivity of fibres. Preservation of the sensitivity during successive irradiation cycles can be achieved by a proper annealing procedure accompanied by a pre-dose treatment.

Study has been made of the thermoluminescence (TL) response of silica-based Ge-doped cylindrical,... more Study has been made of the thermoluminescence (TL) response of silica-based Ge-doped cylindrical, flat and photonic crystal fibres (referred to herein as PCF-collapsed) to electron (6, 12 and 20 MeV) and photon (6, 10 MV) irradiation and 1.25 MeV γ-rays, for doses from 0.1 Gy to 100 Gy. The electron and photon irradiations were delivered through use of a Var-ian Model 2100C linear accelerator located at the University of Malaya Medical Centre and γ-rays delivered from a 60 Co irradiator located at the Secondary Standard Dosimetry Laboratory (SSDL), Malaysian Nuclear Agency. Tailor-made to be of various dimensions and dopant concentrations (6–10% Ge), the fibres were observed to provide TL yield linear with radiation dose, reproducibility being within 1–5%, with insensitivity to energy and angular variation. The sensitivity dependency of both detectors with respect to field size follows the dependency of the output factors. For flat fibres exposed to 6 MV X-rays, the 6% Ge-doped fibre provided the greatest TL yield while PCF-collapsed showed a response 2.4 times greater than that of the 6% Ge-doped flat fibres. The response of cylindrical fibres increased with core size. The fibres offer uniform response, high spatial resolution and sensitivity, providing the basis of promising TL systems for radiotherapy applications.

In this study the commissioning of a dose calculation algorithm in a currently used treatment pla... more In this study the commissioning of a dose calculation algorithm in a currently used treatment planning system was performed and the calculation accuracy of two available methods in the treatment planning system i.e., collapsed cone convolution (CCC) and equivalent tissue air ratio (ETAR) was verified in tissue heterogeneities. For this purpose an inhomogeneous phantom (IMRT thorax phantom) was used and dose curves obtained by the TPS (treatment planning system) were compared with experimental measurements and Monte Carlo (MCNP code) simulation. Dose measurements were performed by using EDR2 radiographic films within the phantom. Dose difference (DD) between experimental results and two calculation methods was obtained. Results indicate maximum difference of 12% in the lung and 3% in the bone tissue of the phantom between two methods and the CCC algorithm shows more accurate depth dose curves in tissue heterogeneities. Simulation results show the accurate dose estimation by MCNP4C in soft tissue region of the phantom and also better results than ETAR method in bone and lung tissues.