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Showing 6 results for Ionization Chamber
A. Solimanian, Dr. M. Ghafoori,
Volume 8, Issue 3 (12-2010)
Abstract
Background: Absolute dosimetry of external
beam radiotherapy is carried out by the use of ionization
chambers. These chambers must be calibrated
at a standard dosimetry laboratory before any use in
clinical dosimetry. The secondary standard dosimetry
laboratory of Iran (SSDL) has the duty of calibrating
the ionization chambers used in radiotherapy centers
in Iran. Materials and Methods: The present work has
described traceability of SSDL radiation measurement
standards to relevant international standards,
and calibration of therapy level ionization chambers
in terms of air kerma and absorbed dose to water
against 60Co gamma radiation, as well as uncertainty
evaluation of calibration coefficients. Results: The
expanded uncertainties in the determination of air
kerma and absorbed dose to water are estimated to
be 2% and 2.3% at approximately 95% confidence
level, respectively. Conclusion: In order to maintain
the requirement of ±5% accuracy in the dose delivery,
the combined standard uncertainty of the other
factors in the dose delivery i.e., dose measurement
set-up, dose calculations, treatment planning, patient
set-up, etc, should be less than 2.3%. Iran. J. Radiat.
Res., 2010 8 (3): 195-199
Dr. A. Iftikhar,
Volume 10, Issue 2 (9-2012)
Abstract
Background: The aim of this work was to study
the output factors for Linac using different ionization
chambers and build up caps. Materials and Methods:
Output factors were measured for open square fields
(3 × 3 cm to 40 × 40 cm) defined by collimator jaws
for 6 and 15 MV photon beams from a Varian Clinac
2100C accelerator were measured. The measurements
in air were performed using Compact Cylindrical
and Farmer type ionization chamber fitted with
acrylic and brass build up caps. All measurements
were taken with the detector set with its central axis
perpendicular to the beam central axis and isocentrically
positioned at the reference depth in empty water
phantom. Results and Discussion: It was observed
that out put factors increased with field size for both
6 and 15 MV photon beams. The increase in output
factor is less prominent for brass build-up caps than
acrylic build up caps. Up to 1.53% and 0.97 %
difference were observed between 6 and 15 MV
energies for acrylic and brass build up caps respectively.
For acrylic build up cap, no significant
difference was found for both ionization chambers.
However, measurements for brass and acrylic build
up caps with same ion chamber differ by up to 4.4 %
for 15 MV energy. Conclusions: The measurement of
output factors Shwith cylindrical build-up cap made of
high Z material distorts Shand should be avoided. The
use of buildup caps with Zeff close to that of water
such as acrylic is a good choice. Ionization chamber is
the best choice for Sh measurements for large field
sizes. Iran. J. Radiat. Res., 2012 10(2): 95‐98
M.a. Gadhi, Dr. S.a. Buzdar, M. Afzal, Sh. Fatmi, M.s. Akhtar, A.h. Nizamani,
Volume 11, Issue 3 (7-2013)
Abstract
Background: The effectiveness and safety of brachytherapy treatment is mainly concerned with the calibration of sources and their traceability to internationally accepted Standards. Secondary Standard Dosimetry Laboratory (SSDL) does not offer calibration of ionization chambers with gamma-ray spectrum of high dose rate source 192Ir . This work has been carried out to calibrate the high dose rate (HDR) 192Ir sources. Materials and Methods: An interpolation procedure, using calibrations above 1.25 MeV (60Co), and below 135 kV X-rays (61.1 keV), the exposure-weighted average energy 397 keV of 192Ir is employed. Using Farmer ionization chamber HDR 192Ir source has been calibrated by free in-air measurement technique and then the calibrated source has been used to calibrate well-type ionization chamber. Results: Difference between in-air measurement and that of manufacturer’s remained within ± 3%, and the difference between in-air measurement and well-type chamber remained within ± 1%. Comparison between well-type measurements and manufacturer values shows differences less than ± 2%. All these differences are within the acceptable tolerance limits. Conclusion: The method presented shows good degree of accuracy so that the traceability of calibration is assured. Installation of a new brachytherapy source needs to be calibrated by in-air measurement technique and then this calibrated source should be used for well chamber calibration. Brachytherapy community is suggested to calibrate their sources with this method.
S. Abdollahi, Dr. M. Mohammadi, A. Ebrahimzadeh Pezeshki, Z. Alizadeh,
Volume 13, Issue 2 (4-2015)
Abstract
Background: In order to deliver the precise dose to the target in radiotherapy, absorbed dose to water at the reference point should be assessed. When the calibration procedure is performed for a reference dosimeter in the 60Co beam of a Secondary Standard Dosimetry Laboratory (SSDL), the total uncertainty in absorbed dose to water (Dw) is estimated to be approximately 1.5%. This study attempts to re-measure the ND,W factors for all available field chambers at Reza Radiotherapy & Oncology Centre (RROC). Materials and Methods: Consistency and linearity checks were performed for a range of available chambers using a check source. The ND,W factors were also measured for the ionization chambers. All cylindrical chambers have been cross calibrated at 6 MV photon beam using a Siemens Primus Plus Linac. The Plane Parallel Chamber has been cross calibrated at the highest available electron beam and the ND,W factor has been measured. Results: The tolerance of consistency and linearity checks has been reported to be within 0.3%. The ND,W value for field Farmer chamber was found in agreement with certificate within 1%. In contrast for small active volume chambers, the deviation from the SSDL reports was 2.3%. For the plane parallel chamber, the difference between SSDL and Home measurements are found to be 12%. Conclusion: Although the calibration of reference chambers used for absolute dosimetry through a Primary Standard Dosimetry Laboratory (PSDL) or SSDL is recommended, for field chambers this can be done at home department as a Tertiary Standard Dosimetry Laboratory (TSDL).
H. Keivan, Dr. D. Shahbazi-Gahrouei, A. Shanei,
Volume 16, Issue 3 (7-2018)
Abstract
Background: Modern radiation therapy such as intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) has resulted in using small therapeutic photon beams. The scope of this work is to investigate the variation in efficiency of different ionization chambers and semiconductor diodes in small multi-leaf collimator (MLC) defined fields of ARTISTE linear accelerator. Materials and Methods: Dose distributions measurements was done in field sizes ranging from 0.5×0.5 to 10×10 cm2 combining with Monte Carlo (MC) simulation. The treatment head of linac and the detectors were simulated by means of BEAMnrc/DOSXYZnrc of EGSnrc MC. The parameters such as output ratio (OR), penumbral width, dosimetric field size and the percentage surface dose in small radiation fields was evaluated using ionization chambers and semiconductor dosimeters. Results: ORs and beam profiles resulting from various measurements showed significant difference between ionization chambers and diodes in small fields. Derivation of less than 2% was observed between EDGE and Diode P. ORs vary by more than 30% for 1×1 cm2 field size but, in larger field sizes differences was less than 1%. Penumbra underestimation was seen in Semiflex relative to pinpoint ionization chamber. No difference was seen in the pattern of surface dose among the applied detectors. Conclusion: Dosimetric characteristics of different detectors showed significant differences in small photon beams. Profiles and ORs analysis with different dosimeters showed that not only water equivalency of detectors, but also dosimeter active volume is important factors for determination of dosimetric behavior in small photon beams.
H. Keivan, R. Maskani, Dr. D. Shahbazi-Gahrouei, A. Shanei, S. Pandesh, E. Tarighati Sereshke,
Volume 20, Issue 1 (1-2022)
Abstract
Background: Small photon beams are increasingly used in modern radiotherapy modalities. In small photon fields, the dosimetric field size will deviate from the nominal field size. An effective field size (FSeff) for use in small field dosimetry has been defined to overcome this issue. The present study aims to investigate the suitability of two ionization chambers and two semiconductor diodes in the measurement of 6MV photon beam profiles and to analyze the variations of FSeff in smaller fields. Materials and Methods: Measurements were made at 6 MV photon beams of a Siemens Artiste linear accelerator and transverse profiles were acquired for nominal square field sizes of side 1×1 to 10×10 cm2 via the irradiation of detectors and radiochromic film. Full width at half maximum (FWHM) at the 50% isodose level was used to calculate FSeff. Results: The uncertainty of the FWHM values derived from the in-plane and cross-plane profiles (ΔFWHM%) were below 6% for all the detectors were below 6% except for Semiflex in the 1×1 field size. In small field sizes (less than 3 × 3 cm2), larger differences occurred between the dosimetric and nominal field sizes in all detectors. No significant differences between nominal and effective field sizes were observed in a field rage of 4×4 - 10×10 cm2. Conclusion: In the acquisition of small field profiles, selection of an appropriate detector is influential in accurate measurements. The findings of present study support the argument that both the size and composition of detectors affect the small field profile measurements.
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