Convolutions and Deconvolutions in Radiation Dosimetry

Harder, Dietrich; Looe, Hui Khee; Poppe, Björn

doi:10.1016/b978-0-444-53632-7.00913-8

Cited by 12 publications

(22 citation statements)

References 42 publications

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“…The lateral dose–response function of a detector acts as the convolution kernel K(x,y) transforming the dose profile D(x,y) into the measured signal profile M(x,y) according to the convolution model:

M (x, y) = D (x, y) * K (x, y)

The area‐normalized function K(x,y) characterizes the detector’s volume effect, which comprises the geometrical volume averaging effect due to the finite extension of the detector’s dimensions and the perturbation of electron fluence due to the electron density of the detector’s components differing from that of normal water.…”

Section: Methodsmentioning

confidence: 99%

Technical Note: Characterization of the new microSilicon diode detector

Schönfeld

Poppinga²,

Kranzer³

et al. 2019

Medical Physics

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PurposeDosimetric properties of the new microSilicon diode detector (60023) have been studied with focus on application in small‐field dosimetry. The influences of the dimensions of the sensitive volume and the density of the epoxy layer surrounding the silicon chip of microSilicon have been quantified and compared to its predecessor (Diode E 60017) and the microDiamond (60019, all PTW‐Freiburg, Germany).MethodsDose linearity has been studied in the range from 0.01 to 8.55 Gy and dose‐per‐pulse dependence from 0.13 to 0.86 mGy/pulse. The effective point of measurement (EPOM) was determined by comparing measured percentage depth dose curves with a reference curve (Roos chamber). Output ratios were measured for nominal field sizes from 0.5 × 0.5 cm2 to 4 × 4 cm2. The corresponding small‐field output correction factors, k, were derived with a plastic scintillation detector as reference. The lateral dose–response function, K(x), was determined using a slit beam geometry.ResultsMicroSilicon shows linear dose response (R 2 = 1.000) in both low and high dose range up to 8.55 Gy with deviations of only up to 1% within the dose‐per‐pulse values investigated. The EPOM was found to lie (0.7 ± 0.2) mm below the front detector’s surface. The derived k for microSilicon (0.960 at s eff = 0.55 cm) is similar to that of microDiamond (0.956), while Diode E requires larger corrections (0.929). This improved behavior of microSilicon in small‐fields is reflected in the slightly wider K(x) compared to Diode E. Furthermore, the amplitude of the negative values in K(x) at the borders of the sensitive volume has been reduced.ConclusionsCompared to its predecessor, microSilicon shows improved dosimetric behavior with higher sensitivity and smaller dose‐per‐pulse dependence. Profile measurements demonstrated that microSilicon causes less perturbation in off‐axis measurements. It is especially suitable for the applications in small‐field output factors and profile measurements.

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M (x, y) = D (x, y) * K (x, y)

Section: Methodsmentioning

confidence: 99%

Technical Note: Characterization of the new microSilicon diode detector

Schönfeld

Poppinga²,

Kranzer³

et al. 2019

Medical Physics

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“…The most common of these being silicon diodes, or more recently synthetic diamond detector, and commercial scintillation detector . Air‐filled ionization chambers are considered less suitable, which is attributed to (a) the geometrical volume‐averaging effect owing to the physical dimensions of the effective sensitive air cavity and (b) the perturbation of the fluence of secondary electrons due to the low density air cavity and other nonwater equivalent components such as the chamber wall and the inner electrode . Therefore, the output ratios measured with these detectors must be corrected using the appropriate field output correction factors.…”

Section: Introductionmentioning

confidence: 99%

“…A mathematical approach useful to quantify the role of the volume effect in small field dosimetry is the convolution model , represented by the convolution integral

M false(x, y false) = \int_{- \infty}^{+ \infty} \int_{- \infty}^{+ \infty} K false(x - ξ, y - η false) D false(ξ, η false) d ξ d η

which transforms a given lateral dose profile, D ( x , y ), into the distorted lateral signal profile, M ( x , y ), by convolution with the detector's lateral dose–response function, K ( x , y ) . The detector‐specific convolution kernel K ( x , y ) (free from polarity effect) has been shown useful to characterize the physical effects underlying both the small field output correction factors and the distortion of the signal profiles due to the volume effect associated with the detector's construction.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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Purpose The recent developments of compact air‐filled ionization chambers for use in small photon beams have raised the needs to address the associated polarity effect. The polarity effect of five compact ionization chambers has been quantified at small field sizes. The origins of the polarity effect are studied experimentally and through Monte‐Carlo simulations. For this purpose, the one‐dimensional lateral dose–response functions were determined using positive and negative chamber's polarity. Monte‐Carlo simulations were performed to study the underlying mechanism of the polarity effect by quantifying the charge imbalance in the collecting electrode and cable. Methods Five novel compact ionization chamber designs have been studied (PTW‐Freiburg: Semiflex 3D 31021, PinPoint 3D 31022 and PinPoint 31023; IBA Dosimetry: Razor chamber CC01‐G and Razor Nano‐chamber CC003). Output ratios were measured down to a nominal field side length of 3 mm using both polarities to evaluate the polarity effect at different field sizes. The small field output correction factors were derived using a scintillator detector as reference. To identify the origins of the polarity effect, slit beam measurements were performed to obtain their lateral dose–response functions. All measurements were performed using three chamber orientations: axial, radial crossplane, and radial inplane. The chambers were modeled according to the manufacturers' blueprints using the Monte‐Carlo package EGSnrc. The charge imbalance due to electrons entering and leaving the inner electrode and cable was studied using an adapted user‐code. Results The output ratios obtained using all five chambers show field size‐dependent polarity effects at small field sizes in the axial orientation, whereas no significant field size dependence of the polarity effect has been observed in the radial orientations. The chambers' lateral dose–response functions reveal that the radiation‐induced charge imbalance in the inner electrode and cable is the main cause of the observed polarity effect at small field sizes. The effect is weakest for the largest PTW 31021 chamber but intensifies for smaller chambers with decreasing sensitive air volume. Consistent results have been obtained between Monte‐Carlo simulations and measurement data. Conclusions Awareness needs to be raised so that the polarity effect of novel compact ionization chambers is appropriately accounted for in small field dosimetry. The results in this work are useful to identify the magnitude of the polarity effect correction and to assist in the decisions on choosing the appropriate chambers and setups during measurements. The origins of the observed polarity effect have been elucidated using the chambers' lateral dose–response functions. The adapted Monte‐Carlo user‐code has been used to compute the radiation‐induced charge imbalance in the chamber's components. It opens the possibility to study the chamber's design with the aim to minimize its polarity effect. Small field output correction factors computed according to...

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“…In its present state of development, summarized by van't Veld et al (2001) and in the notation proposed by Looe et al (2013), Harder et al (2014), and Looe et al (2015), the 'convolution model' can briefly be summarized as follows: The relationship between the indicated signal profile M(x,y) of the detector and the undisturbed true dose profile D(x,y) in an x,y plane at right angles with the beam axis in a water phantom is described by the 2D convolution integral…”

Section: Introductionmentioning

confidence: 99%

Experimental determination of the lateral dose response functions of detectors to be applied in the measurement of narrow photon-beam dose profiles

Poppinga¹,

Meyners²,

Delfs³

et al. 2015

Phys. Med. Biol.

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This study aims at the experimental determination of the detector-specific 1D lateral dose response function K(x) and of its associated rotational symmetric counterpart K(r) for a set of high-resolution detectors presently used in narrow-beam photon dosimetry. A combination of slit-beam, radiochromic film, and deconvolution techniques served to accomplish this task for four detectors with diameters of their sensitive volumes ranging from 1 to 2.2 mm. The particular aim of the experiment was to examine the existence of significant negative portions of some of these response functions predicted by a recent Monte-Carlo-simulation (Looe et al 2015 Phys. Med. Biol. 60 6585-607). In a 6 MV photon slit beam formed by the Siemens Artiste collimation system and a 0.5 mm wide slit between 10 cm thick lead blocks serving as the tertiary collimator, the true cross-beam dose profile D(x) at 3 cm depth in a large water phantom was measured with radiochromic film EBT3, and the detector-affected cross-beam signal profiles M(x) were recorded with a silicon diode, a synthetic diamond detector, a miniaturized scintillation detector, and a small ionization chamber. For each detector, the deconvolution of the convolution integral M(x) = K(x) ∗ D(x) served to obtain its specific 1D lateral dose response function K(x), and K(r) was calculated from it. Fourier transformations and back transformations were performed using function approximations by weighted sums of Gaussian functions and their analytical transformation. The 1D lateral dose response functions K(x) of the four types of detectors and their associated rotational symmetric counterparts K(r) were obtained. Significant negative curve portions of K(x) and K(r) were observed in the case of the silicon diode and the diamond detector, confirming the Monte-Carlo-based prediction (Looe et al 2015 Phys. Med. Biol. 60 6585-607). They are typical for the perturbation of the secondary electron field by a detector with enhanced electron density compared with the surrounding water. In the cases of the scintillation detector and the small ionization chamber, the negative curve portions of K(x) practically vanish. It is planned to use the measured functions K(x) and K(r) to deconvolve clinical narrow-beam signal profiles and to correct the output factor values obtained with various high-resolution detectors.

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Convolutions and Deconvolutions in Radiation Dosimetry

Cited by 12 publications

References 42 publications

Technical Note: Characterization of the new microSilicon diode detector

Technical Note: Characterization of the new microSilicon diode detector

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

Experimental determination of the lateral dose response functions of detectors to be applied in the measurement of narrow photon-beam dose profiles

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