Effects of ionization chamber construction on dose measurements in a heterogeneity

Mauceri, Thomas; Kase, Kenneth R.

doi:10.1118/1.596034

Cited by 10 publications

(8 citation statements)

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“…Mauceri and Kase (1987) investigated the effect of mismatch of ionization chamber wall material to phantom material and demonstrated that the matching of ionization chamber wall to measuring media can be ignored provided that a small, approximately tissue-equivalent, thin-walled (≈0.1 g/cm 2 ) ion chamber is used for measuring the correction factor. [19]…”

Section: Methodsmentioning

confidence: 99%

Effect of low-density heterogeneities in telecobalt therapy and validation of dose calculation algorithm of a treatment planning system

Kumar¹,

Sharma²,

Arya³

et al. 2011

J Med Phys

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Telecobalt machines are still prominently used for the treatment of a variety of cancer cases in developing countries. The human body is a heterogeneous composition of variety of tissues and cavities which vary widely in their physical and radiological properties. The presence of heterogeneities in the path of telecobalt beam presents an altered dose distribution in the region of clinical interests. A computerized treatment planning system (TPS) is generally used for calculating the dose distribution in the patient. Experimental measurements were carried out in a telecobalt beam with the objectives to study the effects of low-density heterogeneities and to verify the ability of the ASHA radiotherapy TPS in predicting the altered dose distribution along the central axis and off-axis of the beam. Locally available kailwood was tested for its lung equivalence and measurements were carried out in a polymethyl methacrylate phantom by introducing lung equivalent and air gap heterogeneities. A comparison of experimentally measured and TPS calculated dose values indicates that the TPS overestimates the dose by 11.6% in lung equivalent (kailwood) heterogeneity along the central axis. Similarly, it was found that the TPS overestimates the dose by 3.9% and 5.9%, respectively, with air heterogeneity of 1.0 and 2.0 cm. While testing the adequacy of TPS in off-axis region, it was found that the TPS calculation does not indicate the widening of the beam profile in the low-density heterogeneity region. This study suggests that the effective path length based algorithm of the ASHA radiotherapy TPS is unable to achieve the recommended 3% accuracy of clinical dose calculation in heterogeneous media.

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Section: Methodsmentioning

confidence: 99%

Effect of low-density heterogeneities in telecobalt therapy and validation of dose calculation algorithm of a treatment planning system

Kumar¹,

Sharma²,

Arya³

et al. 2011

J Med Phys

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“…ICFs are roughly determined as ratios of measured ionizations between heterogeneous and water-like media. This is possible for the lung dose correction under conditions of lateral electron equilibrium (Carrasco et al 2004, Fragoso et al 2010, Mauceri and Kase 1987, Rice et al 1988a. However, ICFs should be considered for lung under conditions of lateral electron disequilibrium (Aspradakis et al 2006, Krieger and Sauer 2005, Pisaturo et al 2012, Rice et al 1988a, Ding et al 2007a and for heterogeneous media such as bone with composition different from water (Papanikolaou et al 2004 and Boyer 1990).…”

Section: Introductionmentioning

confidence: 99%

Monte Carlo-based correction factors for ion chamber dosimetry in heterogeneous phantoms for megavoltage photon beams

Araki

2012

Phys. Med. Biol.

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The purpose of this study was to investigate the perturbation correction factors and inhomogeneity correction factors (ICFs) for a thin-walled cylindrical ion chamber in a heterogeneous phantom including solid water, lung and bone plastic materials. The perturbation factors due to the replacement of the air cavity, non-water equivalence of the wall and the stem, non-air equivalence of the central electrode and the overall perturbation factor, P(Q), for a cylindrical chamber, in the heterogeneous phantom were calculated with the EGSnrc/Cavity Monte Carlo code for 6 and 15 MV photon beams. The PTW31010 (0.125 cm(3)) chamber was modeled with Monte Carlo simulations, and was used for measurements and calculations of percentage depth ionization (PDI) or percentage depth dose (PDD). ICFs were calculated from the ratio of the product of the stopping power ratios (SPRs) and P(Q) of lung or bone to solid water. Finally, the measured PDIs were converted to PDDs by using ICFs and were compared with those calculated by the Monte Carlo method. The perturbation effect for the ion chamber in lung material is insignificant at 5 × 5 and 10 × 10 cm(2) fields, but the effect needs to be considered under conditions of lateral electron disequilibrium with a 3 × 3 cm(2) field. ICFs in lung varied up to 2% and 4% depending on the field size for 6 and 15 MV, respectively. For bone material, the perturbation effects due to the chamber wall and the stem were more significant at up to 3.5% and 1.6% for 6 MV, respectively. ICFs for bone material were approximately 0.945 and 0.940 for 6 and 15 MV, respectively. The converted PDDs by using ICFs were in good agreement with Monte Carlo calculated PDDs. The chamber perturbation correction and SPRs should strictly be considered for ion chamber dosimetry in heterogeneous media. This is more important for small field dosimetry in lung and bone materials.

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“…Several authors have investigated this effect and experimental procedures were developed in order to calculate correction factors. Mauceri and Kase (1987) experimentally calculated replacement correction factors using an ionization chamber made of lung-equivalent material. Rice et al (1988) used extrapolation to obtain the reading of an ionization chamber with an infinitesimally small volume of water by fitting measurements performed for different detector volumes.…”

Section: Introductionmentioning

confidence: 99%

Calculation of correction factors for ionization chamber measurements with small fields in low-density media

Pisaturo¹,

Pachoud²,

Bochud³

et al. 2012

Phys. Med. Biol.

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The quantity of interest for high-energy photon beam therapy recommended by most dosimetric protocols is the absorbed dose to water. Thus, ionization chambers are calibrated in absorbed dose to water, which is the same quantity as what is calculated by most treatment planning systems (TPS). However, when measurements are performed in a low-density medium, the presence of the ionization chamber generates a perturbation at the level of the secondary particle range. Therefore, the measured quantity is close to the absorbed dose to a volume of water equivalent to the chamber volume. This quantity is not equivalent to the dose calculated by a TPS, which is the absorbed dose to an infinitesimally small volume of water. This phenomenon can lead to an overestimation of the absorbed dose measured with an ionization chamber of up to 40% in extreme cases. In this paper, we propose a method to calculate correction factors based on the Monte Carlo simulations. These correction factors are obtained by the ratio of the absorbed dose to water in a low-density medium □D(w,Q,V1)(low) averaged over a scoring volume V₁ for a geometry where V₁ is filled with the low-density medium and the absorbed dose to water □D(w,QV2)(low) averaged over a volume V₂ for a geometry where V₂ is filled with water. In the Monte Carlo simulations, □D(w,QV2)(low) is obtained by replacing the volume of the ionization chamber by an equivalent volume of water, according to the definition of the absorbed dose to water. The method is validated in two different configurations which allowed us to study the behavior of this correction factor as a function of depth in phantom, photon beam energy, phantom density and field size.

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Effects of ionization chamber construction on dose measurements in a heterogeneity

Cited by 10 publications

References 0 publications

Effect of low-density heterogeneities in telecobalt therapy and validation of dose calculation algorithm of a treatment planning system

Effect of low-density heterogeneities in telecobalt therapy and validation of dose calculation algorithm of a treatment planning system

Monte Carlo-based correction factors for ion chamber dosimetry in heterogeneous phantoms for megavoltage photon beams

Calculation of correction factors for ionization chamber measurements with small fields in low-density media

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