A comparison of dosimetry techniques in stereotactic radiosurgery

Heydarian, M.; Hoban, Peter; Beddoe, A H

doi:10.1088/0031-9155/41/1/008

Cited by 181 publications

(159 citation statements)

References 31 publications

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“…[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] It is known that the main problems with the detectors are retraceable to their finite size compared to the small size of the beams and to the nonwater equivalence of the materials. Moreover, the dosimetry of small beams is complicated by the lack of lateral electronic equilibrium.…”

Section: Introductionmentioning

confidence: 99%

Total scatter factors of small beams: A multidetector and Monte Carlo study

2008

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The scope of this study was to estimate total scatter factors ͑s c,p ͒ of the three smallest collimators of the Cyberknife radiosurgery system ͑5-10 mm in diameter͒, combining experimental measurements and Monte Carlo simulation. Two microchambers, a diode, and a diamond detector were used to collect experimental data. The treatment head and the detectors were simulated by means of a Monte Carlo code in order to calculate correction factors for the detectors and to estimate total scatter factors by means of a consistency check between measurement and simulation. Results for the three collimators were: s c,p ͑5 mm͒ = 0.677± 0.004, s c,p ͑7.5 mm͒ = 0.820± 0.008, s c,p ͑10 mm͒ = 0.871± 0.008, all relative to the 60 mm collimator at 80 cm source-to-detector distance. The method also allows the full width at half maximum of the electron beam to be estimated; estimations made with different collimators and different detectors were in excellent agreement and gave a value of 2.1 mm. Correction factors to be applied to the detectors for the measurement of s c,p were consistent with a prevalence of volume effect for the microchambers and the diamond and a prevalence of scattering from high-Z material for the diode detector. The proposed method is more sensitive to small variations of the electron beam diameter with respect to the conventional method used to commission Monte Carlo codes, i.e., by comparison with measured percentage depth doses ͑PDD͒ and beam profiles. This is especially important for small fields ͑less than 10 mm diameter͒, for which measurements of PDD and profiles are strongly affected by the type of detector used. Moreover, this method should allow s c,p of Cyberknife systems different from the unit under investigation to be estimated without the need for further Monte Carlo calculation, provided that one of the microchambers or the diode detector of the type used in this study are employed. The results for the diamond are applicable only to the specific detector that was investigated due to excessive variability in manufacturing.

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Section: Introductionmentioning

confidence: 99%

Total scatter factors of small beams: A multidetector and Monte Carlo study

2008

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“…The PDD maximum difference between the FXG and the 0.1 cm 3 IC [10] are 4.2 %, 3.8 % and 2.9 % for 1 x 1 cm 2 , 3 x 3 cm 2 and 5 x 5 cm 2 , respectively. These results can be explained through the followings facts: a) for smaller field sizes the lateral electronic equilibrium does not exist [14][15][16][17][18][19], if the detector dimensions are less than 1/3 of the field size dimension; b) the dosimeter can lose resolution due to the dose gradient [17,20,21] and c) the photons energy spectra can be different of those used in standard laboratory for the IC calibration [15,17,22]. …”

Section: Percentage Depth Dosementioning

confidence: 99%

Ferrous Xylenol Gel measurements for 6 and 10 MV photons in small field sizes

et al. 2007

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The Fricke dosimeter is a ferrous sulfate aqueous solution that, when irradiated, oxidizes the Fe 2+ ions to Fe 3+ . This new concentration, generally determined through spectrophotometry, is directly proportional to the ionizing radiation absorbed energy. The Fricke Xylenol Gel dosimeter (FXG) was developed through the incorporation of swine skin gelatin and xylenol orange. These modifications provided better signal stability and sensitivity for lower absorbed dose measurements, such as those used in radiotherapy. In this work FXG samples were irradiated with absorbed doses of 2 Gy, from 6 MV and 10 MV photons, using small field sizes geometry for dosimetric parameters determination. All the FXG dosimeter readings were accomplished with our specially developed spectrophotometer, using a narrow light beam at the wavelength of 585 nm, where the highest absorbance sensitivity occurs. From our results, we can confirm not only that the FXG dosimetric system (FXG plus a high lateral spatial resolution spectrophotometer) can be used for general dosimetry, but as well for small field size dosimetry of interest in radiosurgery.

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“…Andreo and Brahme found errors of up to 1% may be introduced by the assumption of field size independence. Heydarian et al (8) performed calculations of stopping‐power ratios as a function of depth, based on electron spectra obtained with EGS4, again using a precalculated broad‐beam photon data as input (i.e. collimation devices not modeled).…”

Section: Introductionmentioning

confidence: 99%

The influence of field size on stopping‐power ratios in‐ and out‐of‐field: quantitative data for the BrainLAB m3 micro‐multileaf collimator

Taylor

Kairn²,

Kron

et al. 2012

J Applied Clin Med Phys

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The objective of this work is to quantify the systematic errors introduced by the common assumption of invariant secondary electron spectra with changing field sizes, as relevant to stereotactic radiotherapy and other treatment modes incorporating small beam segments delivered with a linac‐based stereotactic unit. The EGSnrc/BEAMnrc Monte Carlo radiation transport code was used to construct a dosimetrically‐matched model of a Varian 600C linear accelerator with mounted BrainLAB micro‐multileaf collimator. Stopping‐power ratios were calculated for field sizes ranging from 6×6 mm2 up to the maximum (98×98 mm2), and differences between these and the reference field were computed. Quantitative stopping power data for the BrainLAB micro‐multileaf collimator has been compiled. Field size dependent differences to reference conditions increase with decreasing field size and increasing depth, but remain a fraction of a percent for all field sizes studied. However, for dosimetry outside the primary field, errors induced by the assumption of invariant electron spectra can be greater than 1%, increasing with field size. It is also shown that simplification of the Spencer‐Attix formulation by ignoring secondary electrons below the cutoff kinetic energy applied to the integration results in underestimation of stopping‐power ratios of about 0.3% (and is independent of field size and depth). This work is the first to quantify stopping powers from a BrainLAB micro‐multileaf collimator. Many earlier studies model simplified beams, ignoring collimator scatter, which is shown to significantly influence the spectrum. Importantly, we have confirmed that the assumption of unchanging electron spectra with varying field sizes is justifiable when performing (typical) in‐field dosimetry of stereotactic fields. Clinicians and physicists undertaking precise out‐of‐field measurements for the purposes of risk estimation, ought to be aware that the more pronounced spectral variation results in stopping powers (and hence doses) that differ more than for in‐field dosimetry.PACS number: 87.10.RT; 87.53.Ly; 87.56.jf; 87.56.jk

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A comparison of dosimetry techniques in stereotactic radiosurgery

Cited by 181 publications

References 31 publications

Total scatter factors of small beams: A multidetector and Monte Carlo study

Total scatter factors of small beams: A multidetector and Monte Carlo study

Ferrous Xylenol Gel measurements for 6 and 10 MV photons in small field sizes

The influence of field size on stopping‐power ratios in‐ and out‐of‐field: quantitative data for the BrainLAB m3 micro‐multileaf collimator

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