Absorbed dose perturbation caused by diodes for small field photon dosimetry

Beddar, Sam; Mason, Donna L.; O’Brien, P.

doi:10.1118/1.597350

Cited by 60 publications

(60 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The diode must be used carefully because it can perturb the profiles depending on the direction of its axis with respect to the direction of scanning and on the presence of a metal shield for scattered photons. 66,67 On the other hand, radiographic films are affected by nonuniform spectral sensitivity, 68 and radiochromic films show a statistical uncertainty that is too high to allow small variations of the FWHM to be detected. 7 The profile-matching method is not able to account for small variations of the FWHM because of two types of uncertainty: the detector's characteristics that can influence the measurement of the profile [69][70][71][72] and the statistical uncertainty of the Monte Carlo simulation.…”

Section: Discussionmentioning

confidence: 99%

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

2008

View full text Add to dashboard Cite

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.

show abstract

Section: Discussionmentioning

confidence: 99%

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

2008

View full text Add to dashboard Cite

show abstract

“…The small width of the silicon diode's depletion zone, when it is aligned with its stem perpendicular to the beam, might be thought to make this orientation preferable. However, electron ranges in the material behind the depletion zone are shorter than those in the water in front of it, asymmetrizing the sensitivity of the diode in this perpendicular orientation and leading to asymmetries in measured penumbra widths and shapes as described by Beddar et al 14 For the perpendicular diode orientation, we have found that penumbra widths measured as the diode exits the beam ͑Fig. 3͒ are around 0.2 mm larger than widths measured as the diode enters the beam.…”

Section: Iib2 Measurementsmentioning

confidence: 82%

Monte Carlo modeling of small photon fields: Quantifying the impact of focal spot size on source occlusion and output factors, and exploring miniphantom design for small‐field measurements

2009

View full text Add to dashboard Cite

The accuracy with which Monte Carlo models of photon beams generated by linear accelerators (linacs) can describe small-field dose distributions depends on the modeled width of the electron beam profile incident on the linac target. It is known that the electron focal spot width affects penumbra and cross-field profiles; here, the authors explore the extent to which source occlusion reduces linac output for smaller fields and larger spot sizes. A BEAMnrc Monte Carlo linac model has been used to investigate the variation in penumbra widths and small-field output factors with electron spot size. A formalism is developed separating head scatter factors into source occlusion and flattening filter factors. Differences between head scatter factors defined in terms of in-air energy fluence, collision kerma, and terma are explored using Monte Carlo calculations. Estimates of changes in kerma-based source occlusion and flattening filter factors with field size and focal spot width are obtained by calculating doses deposited in a narrow 2 mm wide virtual "milliphantom" geometry. The impact of focal spot size on phantom scatter is also explored. Modeled electron spot sizes of 0.4-0.7 mm FWHM generate acceptable matches to measured penumbra widths. However the 0.5 cm field output factor is quite sensitive to electron spot width, the measured output only being matched by calculations for a 0.7 mm spot width. Because the spectra of the unscattered primary (psi(pi)) and head-scattered (psi(sigma)) photon energy fluences differ, miniphantom-based collision kerma measurements do not scale precisely with total in-air energy fluence psi = (psi(pi) + psi(sigma) but with (psi(pi)+ 1.2psi(sigma)). For most field sizes, on-axis collision kerma is independent of the focal spot size; but for a 0.5 cm field size and 1.0 mm spot width, it is reduced by around 7% mostly due to source occlusion. The phantom scatter factor of the 0.5 cm field also shows some spot size dependence, decreasing by 6% (relative) as spot size is increased from 0.1 to 1.0 mm. The dependence of small-field source occlusion and output factors on the focal spot size makes this a significant factor in Monte Carlo modeling of small (< 1 cm) fields. Changes in penumbra width with spot size are not sufficiently large to accurately pinpoint spot widths. Consequently, while Monte Carlo models based exclusively on large-field data can quite accurately predict small-field profiles and PDDs, in the absence of experimental methods of determining incident electron beam profiles it will remain necessary to measure small-field output factors, fine-tuning modeled spot sizes to ensure good matching between the Monte Carlo and the measured output factors.

show abstract

“…The diodes should be evaluated for potential dose perturbation based on their respective construction. 22 It is not recommended to use chambers, even microchambers, for output factor measurements below collimator sizes of 20 mm.…”

Section: Iib2 Monthly Accelerator Qamentioning

confidence: 99%

Report of AAPM TG 135: Quality assurance for robotic radiosurgery

et al. 2011

View full text Add to dashboard Cite

The task group (TG) for quality assurance for robotic radiosurgery was formed by the American Association of Physicists in Medicine's Science Council under the direction of the Radiation Therapy Committee and the Quality Assurance (QA) Subcommittee. The task group (TG-135) had three main charges: (1) To make recommendations on a code of practice for Robotic Radiosurgery QA; (2) To make recommendations on quality assurance and dosimetric verification techniques, especially in regard to real-time respiratory motion tracking software; (3) To make recommendations on issues which require further research and development. This report provides a general functional overview of the only clinically implemented robotic radiosurgery device, the CyberKnife V R . This report includes sections on device components and their individual component QA recommendations, followed by a section on the QA requirements for integrated systems. Examples of checklists for daily, monthly, annual, and upgrade QA are given as guidance for medical physicists. Areas in which QA procedures are still under development are discussed.

show abstract

Absorbed dose perturbation caused by diodes for small field photon dosimetry

Cited by 60 publications

References 0 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

Monte Carlo modeling of small photon fields: Quantifying the impact of focal spot size on source occlusion and output factors, and exploring miniphantom design for small‐field measurements

Report of AAPM TG 135: Quality assurance for robotic radiosurgery

Contact Info

Product

Resources

About