Review of backscatter measurement in kilovoltage radiotherapy using novel detectors and reduction from lack of underlying scattering material

Eaton, David J.; Doolan, Paul

doi:10.1120/jacmp.v14i6.4358

Cited by 15 publications

(11 citation statements)

References 27 publications

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“…The relatively high backscatter can be attributed to a 4% greater effective atomic number in the dense polystyrene (foam) compared to that of water . It has been previously shown that a 5 mm slab of solid water under similar geometries used in this study produced approximately 11% backscatter . This suggests that the backscatter produced by the foam at low kV energies for the thin 5 mm block thickness is comparable to water.…”

Section: Discussionsupporting

confidence: 62%

Characterization of a plastic scintillating detector for the Small Animal Radiation Research Platform (SARRP)

et al. 2018

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Purpose: The purpose of this study was to characterize a small plastic scintillator developed for high resolution, real-time dosimetry of therapy and imaging x-ray beams delivered by an image-guided small animal irradiator. Materials and Methods: A 1 mm diameter, 1 mm long polystyrene BCF-60 scintillating fiber dosimeter was characterized with 220 kVp therapy and 40, 50, 60, 70, and 80 kVp imaging beams on the Small Animal Research Platform (SARRP). Scintillator output, sensitivity (charge per unit dose), linearity, and 0.2-mm resolution beam profile measurements were performed. A validated in-house Monte Carlo (MC) model of the SARRP was used to compute detailed energy spectra at locations of dosimetry, and validated scintillator measurement with MC simulations. Mass energyabsorption coefficients from the National Institute of Standards and Technology (NIST) tables convolved with MC-derived spectra were used in conjunction with Birks ionization quenching factors to correct scintillator output. An air kerma calibration method was employed to correct scintillator output for in-air beam profile measurements with open, 5 9 5, and 3 9 3 mm 2 square field sizes, and compared to MC simulations. Results: Scintillator dose response showed excellent linearity (R 2 ≥ 0.999) for all sensitivity measurements, including output as a function of tube current. Detector sensitivity was 2.41 lC Gy À1 for the 220 kVp therapy beam, and it ranged from 1.21 to 1.32 lC Gy À1 for the 40-80 imaging beams. Percentage difference in sensitivity between the therapy and imaging beams before sensitivity correction and after using the Birks quenching factors were 52.3% and 10.2%, respectively. Percentage differences between the therapy and imaging beam sensitivities after using the air kerma calibration method for in-air measurements was excellent and below 0.3%. In-air beam profile measurements agreed to MC simulations within a mean difference of 2.4% for the 5 9 5 and 3 9 3 mm 2 field sizes, however, the scintillator showed signs of volume averaging at the penumbra edges. Conclusions: A small plastic scintillator was characterized for therapy and imaging energies of a small animal irradiator, with output corrected for using an in-house MC model of the irradiator. The characterization of the scintillator detector system for small fields presents steps toward implementing real-time measurements for quality assurance and small animal treatment and imaging dose verification.

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Section: Discussionsupporting

confidence: 62%

Characterization of a plastic scintillating detector for the Small Animal Radiation Research Platform (SARRP)

et al. 2018

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“…14 In addition, for backscatter factors, a recent review concluded that there is little evidence that established CoP values will not be applicable to clinical units. 15 Uncertainty in measurement can be high, and reported differences in other studies were within this uncertainty, or related to unrepresentative beam spectra. Local values should only be introduced following specific Monte Carlo modelling of the beams Percentage of UK centres using specified tolerance value (x), % or millimetre…”

Section: Kilovoltage Treatment Planning In the Ukmentioning

confidence: 99%

Current status of kilovoltage (kV) radiotherapy in the UK: installed equipment, clinical workload, physics quality control and radiation dosimetry

Palmer

Pearson

Whittard

et al. 2016

The British Journal of Radiology

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Objective: To assess the status and practice of kilovoltage (kV) radiotherapy in the UK. Methods: 96% of the radiotherapy centres in the UK responded to a comprehensive survey. An analysis of the installed equipment base, patient numbers, clinical treatment sites, quality control (QC) testing and radiation dosimetry processes were undertaken. Results: 73% of UK centres have at least one kV treatment unit, with 58 units installed across the UK. Although 35% of units are over 10 years old, 39% units have been installed in the last 5 years. Approximately 6000 patients are treated with kV units in the UK each year, the most common site (44%) being basal cell carcinoma. A benchmark of QC practice in the UK is presented, against which individual centres can compare their procedures, frequency of testing and acceptable tolerance values. We propose the use of internal "notification" and "suspension" levels for analysis. All surveyed centres were using recommended Codes of Practice for kV dosimetry in the UK; approximately the same number using in-air and in-water methodologies for medium energy, with two-thirds of all centres citing "clinical relevance" as the reason for choice of code. 64% of centres had hosted an external dosimetry audit within the last 3 years, with only one centre never being independently audited. The majority of centres use locally measured applicator factors and published backscatter factors for treatments. Monitor unit calculations are performed using software in only 36% of centres. Conclusion: A comprehensive review of current kV practice in the UK is presented. Advances in knowledge: Data and discussion on contemporary kV radiotherapy in the UK, with a particular focus on physics aspects.

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“…The dependence of backscatter build-up on the thickness of underlying material has been investigated previously with ion chamber (Klevenhagen, 1982), radiochromic film (Eaton and Doolan, 2013), and Monte Carlo simulations (Benmakhlouf et al, 2012). While these studies provide a qualitative picture on the dependence, there are several deficiencies for it to be useful for the orthovoltage irradiators investigated in this study.…”

Section: Introductionmentioning

confidence: 99%

“…Both Benmakhlouf et al(2012) and Klevenhagen(1982) only covers the diagnostic x-ray energy (50-150 kVp). While Eaton et al(2013) measured a 220 kVp beam, it had a much harder beam (HVL: 2.2 mm Cu) than the 220/225 kVp irradiators for this study. Secondly, the backscatter factor determined through measurement could suffer from measurement uncertainty.…”

Section: Introductionmentioning

confidence: 99%

Erratum: Impact of backscatter material thickness on the depth dose of orthovoltage irradiators for radiobiology research (2019 Phys. Med. Biol. 64 055001)

et al. 2019

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The orthovoltage x-Ray energy frequently used in radiation research is prone to dosimetry errors due to insufficient backscatter conditions. In many radiobiology studies, especially for cell irradiations, precise dose calculation algorithms such as Convolution-Superposition or Monte Carlo are impractical and as such, less accurate hand calculation methods are used for dose estimation. These dose estimation methods typically assume full backscatter conditions. The purpose of this study is to demonstrate the magnitude of the dose error that results from insufficient backscatter, and to provide lookup tables to account this issue. The beam spectra of several widely used commercial systems (XRAD-225, XRAD-320, SARRP) were used in Monte Carlo (MC) simulations on a series of phantom setups to investigate the impact of varying backscatter conditions on dosimetry. The depth dose curves for different field sizes, water phantom thicknesses and beam qualities were generated. In addition, depth dependent backscatter factors for different field sizes and different beam qualities were calculated. It is demonstrated that as much as a 50% dose difference exists for different backscatter conditions at the beam qualities studied. The choice of cell dish size as well as other changes in the experiment setup can have more than 10% impact on the dose. The impact of backscatter is reduced with a decrease in field size. Further, the thickness needed to provide full backscatter can be approximated as being equal to the field size. It is imperative to ensure full backscatter conditions during system and dosimeter calibration, or to use the look-up table provided in this study.

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Review of backscatter measurement in kilovoltage radiotherapy using novel detectors and reduction from lack of underlying scattering material

Cited by 15 publications

References 27 publications

Characterization of a plastic scintillating detector for the Small Animal Radiation Research Platform (SARRP)

Characterization of a plastic scintillating detector for the Small Animal Radiation Research Platform (SARRP)

Current status of kilovoltage (kV) radiotherapy in the UK: installed equipment, clinical workload, physics quality control and radiation dosimetry

Erratum: Impact of backscatter material thickness on the depth dose of orthovoltage irradiators for radiobiology research (2019 Phys. Med. Biol. 64 055001)

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