The effective point of measurement for depth‐dose measurements in small MV photon beams with different detectors

Wegener, Sonja; Sauer, Otto A.

doi:10.1002/mp.13788

Cited by 9 publications

(9 citation statements)

References 20 publications

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“…Even a small shift of the order of 0.2 mm can lead to a change in response of the order of a few percent very close to the surface. In another work, we obtained individual effective points of measurement for all ionization chamber . These values were used here and they differ from the canonical 0.5 or 0.6 times the cavity radius by a few tens of a millimeter.…”

Section: Discussionmentioning

confidence: 99%

“…In another work, we obtained individual effective points of measurement for all ionization chamber. 30 These values were used here and they differ from the canonical 0.5 or 0.6 times the cavity radius by a few tens of a millimeter. Therefore, to obtain reasonable data in the buildup region, it is necessary to use a chamber type specific effective point of measurement or to shift the recorded curves accordingly.…”

Section: B Influence Of Detector Positioningmentioning

confidence: 99%

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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%

Section: B Influence Of Detector Positioningmentioning

confidence: 99%

Detector response in the buildup region of small MV fields

2020

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“…EPOM is a well‐known concept in the dosimetry of ionization chamber, which is used to assign the measured absorbed dose in the chamber volume to a particular point in the undisturbed phantom. 28 A judicious choice of EPOM could minimize the overall perturbation factors passing from ionization‐to‐air (the dose quantity measured) to dose to water. 29 Since the response of PXCs is homogeneous after the uniformity calibration of MatriXX, 4 , 7 it is reasonable to speculate that the EPOMS of all the PXCs share the same depth.…”

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

A novel angular dependency model for MatriXX response and its application to true composite dose verification for IMRT plans

Zhou¹,

Sang²,

Chen

et al. 2021

J Applied Clin Med Phys

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This paper proposes a model for the angular dependency of MatriXX response and investigates whether MatriXX, with the angular-model-based approach can be applied to true composite dose verification for IMRT plans. Method: This model attributes the angular dependence of MatriXX response to dynamical translation of its effective measurement plane (EMP) due to the change of beam angle. Considering this mechanism, true composite dose verifications for IMRT plans specified in AAPM TG 119 report using both MatriXX and Gafchromic EBT3 films were undertook and compared to validate the applicability of MatriXX for patient specific QA of composite beam IMRT plans. Dose verifications using MatriXX with and without angular-model-based approach were performed. Results: MatriXX with angular-model-based approach achieved gamma passing rates with 3%/3 mm and 3%/2 mm criteria better than 98.3% and 98.1% respectively for true composite dose verification of plans in AAPM TG 119 report.The 3%/3 mm and 3%/2 mm gamma passing rates using MatriXX without angular-model-based approach ranged from 85.8% to 98.2% and from 81.3% to 96.5%, respectively. The p-values from the single sided paired t-test indicated no statistical difference between the passing rates from MatriXX with angularmodel-based approach and from films, and significant difference between the passing rates from uncorrected MatriXX and from films. Conclusion:The proposed model for angular dependent MatriXX response is necessary and effective. Dose verification using MatriXX withThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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“…These represent, for the most part, subtle modelling errors. The model variants correspond to models 4, 5, 7, 8, 9, 10, 11, and 13 described elsewhere 21 …”

Section: Methodsmentioning

confidence: 99%

Sensitivity and specificity of secondary dose calculation for head and neck treatment plans

Wegener,

Abu Rashed,

Sauer

et al. 2023

J Applied Clin Med Phys

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PurposeSecondary dose calculation (SDC) with an independent algorithm is one option to perform plan‐specific quality assurance (QA). While measurement‐based QA can potentially detect errors in plan delivery, the dose values are typically only compared to calculations on homogeneous phantom geometries instead of patient CT data. We analyzed the sensitivity and specificity of an SDC software by purposely introducing different errors and determined thresholds for optimal decisions.MethodsThirty head and neck VMAT plans and 30 modifications of those plans, including errors related to density and beam modelling, were recalculated using RadCalc with a Monte Carlo algorithm. Decision thresholds were obtained by receiver operating characteristics (ROC) analysis. For comparison, measurement‐based QA using the ArcCHECK phantom was carried out and evaluated in the same way.ResultsDespite optimized decision thresholds, none of the systems was able to reliably detect all errors. ArcCHECK analysis using a 2%/2 mm criterion with a threshold of 91.1% had an area under the curve (AUC) of 0.80. Evaluating differences in recalculated and planned DVH parameter of the target structures in RadCalc with a 2% threshold an AUC of 0.86 was achieved. Out‐of‐field deviations could be attributed to weaknesses in the beam model.ConclusionsSecondary dose calculation with RadCalc is an alternative to established measurement‐based phantom QA. Different tools catch different errors; therefore, a combination of approaches should be preferred.

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The effective point of measurement for depth‐dose measurements in small MV photon beams with different detectors

Cited by 9 publications

References 20 publications

Detector response in the buildup region of small MV fields

Detector response in the buildup region of small MV fields

A novel angular dependency model for MatriXX response and its application to true composite dose verification for IMRT plans

Sensitivity and specificity of secondary dose calculation for head and neck treatment plans

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