The polarity effect of compact ionization chambers used for small field dosimetry

Looe, Hui Khee; Büsing, Isabel; Tekin, Tuba; Brant, Andre; Delfs, Björn; Poppinga, Daniela; Poppe, Björn

doi:10.1002/mp.13227

Cited by 24 publications

(40 citation statements)

References 46 publications

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“…The two smallest studied chambers CC01 and CC003 showed the largest polarity effects in the buildup region, while the larger CC04 showed hardly any differences between the polarities. Irradiation of the central electrode and cables is known to influence the polarity effect . Different materials are used for the inner electrodes of those detectors, Shonka for the CC04, steel for the CC01 and graphite for the CC003.…”

Section: Discussionmentioning

confidence: 99%

Detector response in the buildup region of small MV fields

2020

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Purpose The model used to calculate dose distributions in a radiotherapy treatment plan relies on the data entered during beam commissioning. The quality of these data heavily depends on the detector choice made, especially in small fields and in the buildup region. Therefore, it is necessary to identify suitable detectors for measurements in the buildup region of small fields. To aid the understanding of a detector's limitations, several factors that influence the detector signal are to be analyzed, for example, the volume effect due to the detector size, the response to electron contamination, the signal dependence on the polarity used, and the effective point of measurement chosen. Methods We tested the suitability of different small field detectors for measurements of depth dose curves with a special focus on the surface‐near area of dose buildup for fields sized between 10 × 10 and 0.6 × 0.6 cm2. Depth dose curves were measured with 14 different detectors including plane‐parallel chambers, thimble chambers of different types and sizes, shielded and unshielded diodes as well as a diamond detector. Those curves were compared with depth dose curves acquired on Gafchromic film. Additionally, the magnitude of geometric volume corrections was estimated from film profiles in different depths. Furthermore, a lead foil was inserted into the beam to reduce contaminating electrons and to study the resulting changes of the detector response. The role of the effective point of measurement was investigated by quantifying the changes occurring when shifting depth dose curves. Last, measurements for the small ionization chambers taken at opposing biasing voltages were compared to study polarity effects. Results Depth‐dependent correction factors for relative depth dose curves with different detectors were derived. Film, the Farmer chamber FC23, a 0.13 cm3 scanning chamber CC13 and a plane‐parallel chamber PPC05 agree very well in fields sized 4 × 4 and 10 × 10 cm2. For most detectors and in smaller fields, depth dose curves differ from the film. In general, shielded diodes require larger corrections than unshielded diodes. Neither the geometric volume effect nor the electron contamination can account for the detector differences. The biggest uncertainty arises from the positioning of a detector with respect to the water surface and from the choice of the detector's effective point of measurement. Depth dose curves acquired with small ionization chambers differ by over 15% in the buildup region depending on sign of the biasing voltage used. Conclusions A scanning chamber or a PPC40 chamber is suitable for fields larger than 4 × 4 cm2. Below that field size, the microDiamond or small ionization chambers perform best requiring the smallest corrections at depth as well as in the buildup region. Diode response changes considerably between the different types of detectors. The position of the effective point of measurement has a huge effect on the resulting curves, therefore detector specific rather than general shifts of half th...

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Section: Discussionmentioning

confidence: 99%

Detector response in the buildup region of small MV fields

2020

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“…Recently, the radiation‐induced charge imbalance within conducting components that are connected between the detector's sensitive volume and the electrometer, for example, collecting electrode, contacts or cable, has been shown to contribute significantly to the so‐called polarity effect . When these components are being exposed to ionizing radiation, electrons released in these components can escape from them.…”

Section: Methodsmentioning

confidence: 99%

“…An adapted user‐code described in a previous work was used to calculate the quantities Q in and Q out in the metal contacts and cable of the mD detector according to Eq. .…”

Section: Methodsmentioning

confidence: 99%

“…The quantity | Q EDEP | was computed as described in Section 2.C. More details of the user‐code can be read in the previous work …”

Section: Methodsmentioning

confidence: 99%

“…This aspect has not been studied so far as a possible contributor to the observed over‐response of mD detector at small field sizes. In a recent study, radiation‐induced charge imbalance in the inner electrode and cable has been identified as the main contributor to the observed field size‐dependent polarity effect of compact air‐filled ionization chambers. At small field sizes, the polarity effect of these microchambers is shown to increase with decreasing field size when they are oriented axially, that is, with their symmetrical axes oriented parallel to the beam's axis.…”

Section: Introductionmentioning

confidence: 99%

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The role of radiation‐induced charge imbalance on the dose‐response of a commercial synthetic diamond detector in small field dosimetry

Looe

Poppinga²,

Kranzer³

et al. 2019

Medical Physics

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Purpose Discrepancy between experimental and Monte Carlo simulated dose–response of the microDiamond (mD) detector (type 60019, PTW Freiburg, Germany) at small field sizes has been reported. In this work, the radiation‐induced charge imbalance in the structural components of the detector has been investigated as the possible cause of this discrepancy. Materials and methods Output ratio (OR) measurements have been performed using standard and modified versions of the mD detector at nominal field sizes from 6 mm × 6 mm to 40 mm × 40 mm. In the first modified mD detector (mD_reversed), the type of charge carriers collected is reversed by connecting the opposite contact to the electrometer. In the second modified mD detector (mD_shortened), the detector's contacts have been shortened. The third modified mD detector (mD_noChip) is the same as the standard version but the diamond chip with the sensitive volume has been removed. Output correction factors were calculated from the measured OR and simulated using the EGSnrc package. An adapted Monte Carlo user‐code has been used to study the underlying mechanisms of the field size‐dependent charge imbalance and to identify the detector's structural components contributing to this effect. Results At the smallest field size investigated, the OR measured using the standard mD detector is >3% higher than the OR obtained using the modified mD detector with reversed contact (mD_reversed). Combining the results obtained with the different versions of the detector, the OR have been corrected for the effect of radiation imbalance. The OR obtained using the modified mD detector with shortened contacts (mD_shortened) agree with the corrected OR, all showing an over‐response of less than 2% at the field sizes investigated. The discrepancy between the experimental and simulated output correction factors has been eliminated after accounting for the effect of charge imbalance. Discussions and conclusions The role of radiation‐induced charge imbalance on the dose–response of mD detector in small field dosimetry has been studied and quantified. It has been demonstrated that the effect is significant at small field sizes. Multiple methods were used to quantify the effect of charge imbalance with good agreement between Monte Carlo simulations and experimental results obtained with modified detectors. When this correction is applied to the Monte Carlo data, the discrepancy from experimental data is eliminated. Based on the detailed component analysis using an adapted Monte Carlo user‐code, it has been demonstrated that the effect of charge imbalance can be minimized by modifying the design of the detector's contacts.

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Evaluation of the RUBY modular QA phantom for planar and non‐coplanar VMAT and stereotactic radiations

Poppinga¹,

Kretschmer

Brodbek

et al. 2020

J Applied Clin Med Phys

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Purpose This study evaluates the clinical use of the RUBY modular QA phantom for linac QA to validate the integrity of IGRT workflows including the congruence of machine isocenter, imaging isocenter, and room lasers. The results have been benchmarked against those obtained with widely used systems. Additionally, the RUBY phantom has been implemented to perform system QA (End‐to‐End testing) from imaging to radiation for IGRT‐based VMAT and stereotactic radiations at an Elekta Synergy linac. Material and Methods The daily check of IGRT workflow was performed using the RUBY phantom, the Penta‐Guide, and the STEEV phantom. Furthermore, Winston–Lutz tests was carried out with the RUBY phantom and a ball‐bearing phantom to determine the offsets and the diameters of the isospheres of gantry, collimator, and couch rotations, with respect to the room lasers and kV‐imaging isocenter. System QA was performed with the RUBY phantom and STEEV phantom for eight VMAT treatment plans. Additionally, the visibility of the embedded objects within these phantoms in the images and the results of CT and MR image fusions were evaluated. Results All systems used for daily QA of IGRT workflows show comparable results. Calculated shifts based on CBCT imaging agree within 1 mm to the expected values. The results of the Winston–Lutz test based on kV imaging (2D planar and CBCT) or room lasers are consistent regardless of the system tested. The point dose values in the RUBY phantom agree to the expected values calculated using algorithms in Masterplan and Monte Carlo engine in Monaco within 3% of the clinical acceptance criteria. Conclusion All the systems evaluated in this study yielded comparable results in terms of linac QA and system QA procedures. A system QA protocol has been derived using the RUBY phantom to check the IGRT‐based VMAT and stereotactic radiations workflow at an Elekta Synergy linac.

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The polarity effect of compact ionization chambers used for small field dosimetry

Cited by 24 publications

References 46 publications

Detector response in the buildup region of small MV fields

Detector response in the buildup region of small MV fields

The role of radiation‐induced charge imbalance on the dose‐response of a commercial synthetic diamond detector in small field dosimetry

Evaluation of the RUBY modular QA phantom for planar and non‐coplanar VMAT and stereotactic radiations

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