Abstract:The purpose of this study was the investigation of perturbation factors for microionization chambers in small field dosimetry and the influence of penumbra for different spot sizes. To this purpose, correlated sampling was implemented in the EGSnrc Monte Carlo (MC) user code cavity: CScavity. CScavity was first benchmarked against results in the literature for an NE2571 chamber. An efficiency increase of 17 was attained for the calculation of a realistic chamber perturbation factor in a water phantom. Calculat… Show more
“…In order to obtain a consistent conversion of absorbed dose in the chamber cavity to absorbed dose in water ͑i.e., the product of all perturbation factors must be equal to the ratio of absorbed dose in water to absorbed dose in the chamber͒, a series of scoring volumes is defined starting at the fully modeled chamber and ending at a small volume of water placed at the centroid of the chamber. Similar to previous studies, [23][24][25] a series of cavity doses is defined as follows: with liquid water ͑density of 1.000 g / cm 3 ͒, and ͑7͒ D w,point : absorbed dose in a 1 mm radius sphere of water placed at the centroid of the chamber and representing absorbed dose at a point in water at the location of measurement.…”
Section: Iib Monte Carlo Methodsmentioning
confidence: 99%
“…The convention adopted here corresponds to the definitions used in previous publications. [23][24][25] The Exradin A12 and A14 cylindrical ionization chambers ͑Standard Imaging, WI, USA͒ are modeled based on drawings kindly provided by the manufacturer. A series of scoring volumes is created, with the chamber model modified for different, correlated calculations as described above.…”
Section: Iib Monte Carlo Methodsmentioning
confidence: 99%
“…For ionization chambers, this perturbation is caused by several factors, which can be classified as follows: ͑1͒ The presence of structural and electronic components, ͑2͒ a detection material with a different atomic composition than water, ͑3͒ a detection material with different density than water, and ͑4͒ a detector volume that is finite. As formalized in reference dosimetry protocols, 11-14 and more recent publications, [23][24][25] ionization chamber perturbation factors represent each of these effects individually ͑although in practice, they are correlated͒ when converting absorbed dose to the chamber to absorbed dose to water. The presence of the chamber stem, central electrode, and wall will cause the charged-particle fluence to change in terms of quality and quantity due to attenuation and scatter in these components.…”
Section: Iia Formalism Of Absorbed Dose At a Pointmentioning
Purpose: For the purpose of nonstandard beam reference dosimetry, the current concept of reporting absorbed dose at a point in water located at a representative position in the chamber volume is investigated in detail. As new nonstandard beam reference dosimetry protocols are under development, an evaluation of the role played by the definition of point of measurement could lead to conceptual improvements prior to establishing measurement procedures. Methods: The present study uses the current definition of reporting absorbed dose to calculate ionization chamber perturbation factors for two cylindrical chamber models ͑Exradin A12 and A14͒ using the Monte Carlo method. The EGSnrc based user-code EGSគchamber is used to calculate chamber dose responses of 14 IMRT beams chosen to cause considerable dose gradients over the chamber volume as previously used by Bouchard and Seuntjens ͓"Ionization chamber-based reference dosimetry of intensity modulated radiation beams," Med. Phys. 31͑9͒, 2454-5465 ͑2004͔͒. Results: The study shows conclusively the relative importance of each physical effect involved in the nonstandard beam correction factors of 14 IMRT beams. Of all correction factors involved in the dosimetry of the beams studied, the gradient perturbation correction factor has the highest magnitude, on average, 11% higher compared to reference conditions for the Exradin A12 chamber and about 5% higher for the Extradin A14 chamber. Other perturbation correction factors ͑i.e., P wall , P stem , and P cel ͒ are, on average, less than 0.8% different from reference conditions for the chambers and beams studied. The current approach of reporting measured absorbed dose at a point in water coinciding with the location of the centroid of the chamber is the main factor responsible for large correction factors in nonstandard beam deliveries ͑e.g., intensity modulated radiation therapy͒ reported in literature. Conclusions: To reduce or eliminate the magnitude of perturbation correction factors in nonstandard beam reference dosimetry, two possible ways to report absorbed dose are suggested: ͑1͒ Reporting average dose to the sensitive volume of the chamber filled with water, combined with removing the reference field implicit gradient effect when measuring output factors, and ͑2͒ reporting average dose to the chamber itself during output factor verifications. The first option could be adopted if clinical beam correction factors are negligible. The second option could simplify quality assurance procedures when correction factors are not negligible and have to be calculated using Monte Carlo simulations.
“…In order to obtain a consistent conversion of absorbed dose in the chamber cavity to absorbed dose in water ͑i.e., the product of all perturbation factors must be equal to the ratio of absorbed dose in water to absorbed dose in the chamber͒, a series of scoring volumes is defined starting at the fully modeled chamber and ending at a small volume of water placed at the centroid of the chamber. Similar to previous studies, [23][24][25] a series of cavity doses is defined as follows: with liquid water ͑density of 1.000 g / cm 3 ͒, and ͑7͒ D w,point : absorbed dose in a 1 mm radius sphere of water placed at the centroid of the chamber and representing absorbed dose at a point in water at the location of measurement.…”
Section: Iib Monte Carlo Methodsmentioning
confidence: 99%
“…The convention adopted here corresponds to the definitions used in previous publications. [23][24][25] The Exradin A12 and A14 cylindrical ionization chambers ͑Standard Imaging, WI, USA͒ are modeled based on drawings kindly provided by the manufacturer. A series of scoring volumes is created, with the chamber model modified for different, correlated calculations as described above.…”
Section: Iib Monte Carlo Methodsmentioning
confidence: 99%
“…For ionization chambers, this perturbation is caused by several factors, which can be classified as follows: ͑1͒ The presence of structural and electronic components, ͑2͒ a detection material with a different atomic composition than water, ͑3͒ a detection material with different density than water, and ͑4͒ a detector volume that is finite. As formalized in reference dosimetry protocols, 11-14 and more recent publications, [23][24][25] ionization chamber perturbation factors represent each of these effects individually ͑although in practice, they are correlated͒ when converting absorbed dose to the chamber to absorbed dose to water. The presence of the chamber stem, central electrode, and wall will cause the charged-particle fluence to change in terms of quality and quantity due to attenuation and scatter in these components.…”
Section: Iia Formalism Of Absorbed Dose At a Pointmentioning
Purpose: For the purpose of nonstandard beam reference dosimetry, the current concept of reporting absorbed dose at a point in water located at a representative position in the chamber volume is investigated in detail. As new nonstandard beam reference dosimetry protocols are under development, an evaluation of the role played by the definition of point of measurement could lead to conceptual improvements prior to establishing measurement procedures. Methods: The present study uses the current definition of reporting absorbed dose to calculate ionization chamber perturbation factors for two cylindrical chamber models ͑Exradin A12 and A14͒ using the Monte Carlo method. The EGSnrc based user-code EGSគchamber is used to calculate chamber dose responses of 14 IMRT beams chosen to cause considerable dose gradients over the chamber volume as previously used by Bouchard and Seuntjens ͓"Ionization chamber-based reference dosimetry of intensity modulated radiation beams," Med. Phys. 31͑9͒, 2454-5465 ͑2004͔͒. Results: The study shows conclusively the relative importance of each physical effect involved in the nonstandard beam correction factors of 14 IMRT beams. Of all correction factors involved in the dosimetry of the beams studied, the gradient perturbation correction factor has the highest magnitude, on average, 11% higher compared to reference conditions for the Exradin A12 chamber and about 5% higher for the Extradin A14 chamber. Other perturbation correction factors ͑i.e., P wall , P stem , and P cel ͒ are, on average, less than 0.8% different from reference conditions for the chambers and beams studied. The current approach of reporting measured absorbed dose at a point in water coinciding with the location of the centroid of the chamber is the main factor responsible for large correction factors in nonstandard beam deliveries ͑e.g., intensity modulated radiation therapy͒ reported in literature. Conclusions: To reduce or eliminate the magnitude of perturbation correction factors in nonstandard beam reference dosimetry, two possible ways to report absorbed dose are suggested: ͑1͒ Reporting average dose to the sensitive volume of the chamber filled with water, combined with removing the reference field implicit gradient effect when measuring output factors, and ͑2͒ reporting average dose to the chamber itself during output factor verifications. The first option could be adopted if clinical beam correction factors are negligible. The second option could simplify quality assurance procedures when correction factors are not negligible and have to be calculated using Monte Carlo simulations.
“…Over the past decades, Monte Carlo methods have been improved in accuracy such that they are more and more used for the calculation of the dose conversion factor, in particular for the dosimetry under reference conditions using air‐filled ionization chambers. Furthermore, the Monte Carlo‐based decomposition of the dose conversion factor into perturbation factors as described by Bouchard et al . and others turned out to be an effective method for the dosimetry also under nonreference conditions.…”
The fluence-based decomposition of the dose conversion factor leads to a fluence-based formulation of perturbation factors, referred to as volume, medium, and extracameral perturbation factor. These factors offer useful explanations for the behavior of detectors in nonreference conditions. An example was given for cylindrical detectors at dose profile measurements.
“…Ionization chambers that are commonly used in CD fields are not suitable in small and nonstandard radiation fields because of a lack of spatial resolution and accuracy in the absorbed dose measurements caused by the fluence perturbation. Besides this, a recent study that quantified perturbation factors for small ionization chambers in small field dosimetry has revealed that even 0.016 cm 3 volume ionization chambers are not suitable to be used in a 0.8×0.8 cm 2 field [11]. Another disadvantage of the small ionization chamber is the amount of charge collected within the radiation field which can be comparable to the leakage of the dosimetry system itself.…”
This work investigated the suitability of passive dosimeters for reference dosimetry in small fields with acceptable accuracy. Absorbed dose to water rate was determined in nine small radiation fields with diameters between 4 and 35 mm in a Leksell Gamma Knife (LGK) and a modified linear accelerator (linac) for stereotactic radiosurgery treatments. Measurements were made using Gafchromic film (MD-V2-55), alanine and thermoluminescent (TLD-100) dosimeters and compared with conventional dosimetry systems. Detectors were calibrated in terms of absorbed dose to water in 60Co gamma-ray and 6 MV x-ray reference (10×10 cm2) fields using an ionization chamber calibrated at a standards laboratory. Absorbed dose to water rate computed with MD-V2-55 was higher than that obtained with the others dosimeters, possibly due to a smaller volume averaging effect. Ratio between the dose-rates determined with each dosimeter and those obtained with the film was evaluated for both treatment modalities. For the LGK, the ratio decreased as the dosimeter size increased and remained constant for collimator diameters larger than 8 mm. The same behaviour was observed for the linac and the ratio increased with field size, independent of the dosimeter used. These behaviours could be explained as an averaging volume effect due to dose gradient and lack of electronic equilibrium. Evaluation of the output factors for the LGK collimators indicated that, even when agreement was observed between Monte Carlo simulation and measurements with different dosimeters, this does not warrant that the absorbed dose to water rate in the field was properly known and thus, investigation of the reference dosimetry should be an important issue. These results indicated that alanine dosimeter provides a high degree of accuracy but cannot be used in fields smaller than 20 mm diameter. Gafchromic film can be considered as a suitable methodology for reference dosimetry. TLD dosimeters are not appropriate in fields smaller than 10 mm diameters.
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