The PTW microSilicon diode: Performance in small 6 and 15 MV photon fields and utility of density compensation

Georgiou, G.; Würfel, Jan; Gilmore, M.; Underwood, Tracy; Rowbottom, C.; Fenwick, John D.

doi:10.1002/mp.15329

Cited by 6 publications

(7 citation statements)

References 21 publications

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“…The characteristics of the micro-Silicon detector were successfully used in small field measurements as described in publications. [12][13][14][15][16][17] Simulations were performed using the EGSnrc. 18,19 Monte Carlo code system with user code BEAMnrc/DOSXYZnrc 20 for generating the realistic incident beams and dose calculations.…”

Section: Methodsmentioning

confidence: 99%

“…The PTW unshielded diode#60023 has a sensitive structure of 0.75 mm circular radius, 18 μm thick disk with the sensitive volume of about 0.03 mm 3 (PTW, Freiburg, DE). The characteristics of the micro‐Silicon detector were successfully used in small field measurements as described in publications 12–17 . Simulations were performed using the EGSnrc 18,19 .…”

Section: Methodsmentioning

confidence: 99%

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On the field size definition and field output factors in small field dosimetry

Ding

Das

2023

Medical Physics

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There is a major conceptual difference between small-field and large field dosimetry: different definition of the field size in the cardinal axes 𝑥 and 𝑦. In IAEA TRS-483, field size of a small-field is obtained by measuring the field width (full-width half maximum, FWHM) of the cardinal axis of the beam profiles while field size of a conventional field is defined by the beam defining system, MLC/Jaw opening at the isocenter of a machine. This difference has led to two different approaches to equivalent square: TRS-483 introduced the term, S clin = √ FWHM(x) ⋅ FWHM(y) while the commonly used method is S = 4 x ⋅ y∕(2(x + y)). The result of this study shows that the field size defined by the beam defining system is not only valid for small field but also free of user and measurement uncertainties while field width obtained by measuring FWHM is unreliable, especially for an extremely small field.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

On the field size definition and field output factors in small field dosimetry

Ding

Das

2023

Medical Physics

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“…The measurement of field widths for the S clin approach and those for FOFs were performed with an IBA blue water phantom and newly marketed high resolution unshielded microSilicon detector (PTW # 60023) whose characteristics have been favorably published in small fields. [28][29][30][31][32][33] As field sizes were very small, stealth transmission chamber (IBA) was used for reference signal. 34 Using 0.1 mm step size, profiles were collected in both x and y directions to calculate FWHM for both machines.…”

Section: Methods and Measurementsmentioning

confidence: 99%

Validity of equivalent square field concept in small field dosimetry

et al. 2022

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Purpose The equivalent square (ES) concept has been used for traditional radiation fields defined by the machine collimating system. For small fields, the concept Sclin was introduced based on measuring dosimetric field width (full‐width half maximum, FWHM) of the cardinal axis of the beam profiles. The pros and cons of this concept are evaluated in small fields and compared with the traditional ES using area and perimeter (4A/P) method based on geometric field size settings, for example, light field settings. Methods One hundred thirty‐seven square and rectangular fields from 5–50 mm with every possible permutation (keeping one jaw fixed and varying other jaw from 5 to 50 mm) were utilized to measure FWHM for the validation of Sclin. Using a microSilicon detector and a scanning water tank, measurements were performed on an Elekta (Versa) machine with Agility head and a Varian TrueBeam with different MLC/Jaw design to evaluate the Sclin concept and to understand the effect of exchange factor in small fields. Field output factors were also measured for all 137 fields. Results The data fitting for fields ranging from 5–50 mm between the traditional 4A/P method and Sclin shows differences and indicates a linear relationship with distinct separation of slope for Elekta and Varian machines. For Elekta Agility machine ES based on 4A/P < Sclin and for the VarianTrueBeam 4A/P > Sclin for square fields. Our measured data show that both methods are equally valid but does vary by the machine design. The field output factor is dependent on the elongation factor as well as machine design. For fields with sides ≥10 mm, the exchange factor is nearly identical in both machines with magnitude up to 4%, which is close to measurement uncertainty (±3%), but for small fields (< 10 mm), the Elekta machine has higher exchange factors compared to the Varian machine. Conclusion The results demonstrate that the two concepts for defining equivalent field (Sclin and 4A/P) are equivalent and can be directly related through an empirical equation. This study confirms that 4A/P is still valid for small fields except for very small fields (≤10 mm) where source occlusion is a dominating factor. The Sclin method is potentially sensitive to measurement uncertainty due to measurement of FWHM which is machine‐, detector‐ and user‐dependent, while the 4A/P method relies mainly on geometry of the machine and has less dependency on type of machine, detector, and user. The exchange factors are comparable for both types of machines. The conclusion is based on data from an Elekta with Agility head and a Varian TrueBeam machine that may have potential for bias due to light field/collimator set up and alignment. Care should be taken in extrapolating these data to any other machine.

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“…In addition, six different types of detectors (summarized in Table 1) were used to determine the relative output factors including a microSilicon diode, microDiamond detector, W2 plastic scintillator, A16 and A1SL ionization chambers, and alanine dosimeters. [30][31][32][33][34][35][36][37][38][39][40] The PTW 60023 microSilicon diode (PTW Dosimetry, Freiburg, Germany) is an unshielded, p-type silicon diode designed to yield high-dose stability, low dose per pulse dependence, and limited sensitivity to temperature variations (usually ≤ 0.1 %/K). 31 The active volume is 0.03 mm 3 with a radius of 0.75 mm and thickness of 18 µm, whereas total dimensions are 7 mm (diameter) and 45.5 mm (length).…”

Section: Radiation Detectorsmentioning

confidence: 99%

Accurate machine‐specific reference and small‐field dosimetry for a self‐shielded neuro‐radiosurgical system

Jermain,

Muir,

McEwen

et al. 2024

Medical Physics

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BackgroundThe newly available ZAP‐X stereotactic radiosurgical system is designed for the treatment of intracranial lesions, with several unique features that include a self‐shielding, gyroscopic gantry, wheel collimation, non‐orthogonal kV imaging, short source‐axis distance, and low‐energy megavoltage beam. Systematic characterization of its radiation as well as other properties is imperative to ensure its safe and effective clinical application.PurposeTo accurately determine the radiation output of the ZAP‐X with a special focus on the smaller diameter cones and an aim to provide useful recommendations on quantification of small field dosimetry.MethodsSix different types of detectors were used to measure relative output factors at field sizes ranging from 4 to 25 mm, including the PTW microSilicon and microdiamond diodes, Exradin W2 plastic scintillator, Exradin A16 and A1SL ionization chambers, and the alanine dosimeter. The 25 mm cone served as the reference field size. Absolute dose was determined with both TG‐51‐based dosimetry using a calibrated PTW Semiflex ion chamber and measurements using alanine dosimeters.ResultsThe average radiation output factors (maximum deviation from the average) measured with the microDiamond, microSilicon, and W2 detectors were: for the 4 mm cone, 0.741 (1.0%); for the 5 mm cone: 0.817 (1.0%); for the 7.5 mm cone: 0.908 (1.0%); for the 10 mm cone: 0.946 (0.4%); for the 12.5 mm cone: 0.964 (0.2%); for the 15 mm cone: 0.976 (0.1%); for the 20 mm cone: 0.990 (0.1%). For field sizes larger than 10 mm, the A1SL and A16 micro‐chambers also yielded consistent output factors within 1.5% of those obtained using the microSilicon, microdiamond, and W2 detectors. The absolute dose measurement obtained with alanine was within 1.2%, consistent with combined uncertainties, compared to the PTW Semiflex chamber for the 25 mm reference cone.ConclusionFor field sizes less than 10 mm, the microSilicon diode, microDiamond detector, and W2 scintillator are suitable devices for accurate small field dosimetry of the ZAP‐X system. For larger fields, the A1SL and A16 micro‐chambers can also be used. Furthermore, alanine dosimetry can be an accurate verification of reference and absolute dose typically measured with ion chambers. Use of multiple suitable detectors and uncertainty analyses were recommended for reliable determination of small field radiation outputs.

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The PTW microSilicon diode: Performance in small 6 and 15 MV photon fields and utility of density compensation

Cited by 6 publications

References 21 publications

On the field size definition and field output factors in small field dosimetry

On the field size definition and field output factors in small field dosimetry

Validity of equivalent square field concept in small field dosimetry

Accurate machine‐specific reference and small‐field dosimetry for a self‐shielded neuro‐radiosurgical system

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