A practical and theoretical definition of very small field size for radiotherapy output factor measurements

Charles, Paul; Cranmer-Sargison, Gavin; Thwaites, David; Crowe, Scott; Kairn, Tanya; Knight, Richard; Kenny, John; Langton, Christian; Trapp, Jamie

doi:10.1118/1.4868461

Cited by 77 publications

(73 citation statements)

References 46 publications

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“…6 (a-f) indicate that these small field size differences have effects on delivered dose that are substantial when the fields are smaller than or equal to 1 9 1 cm 2 and negligible when the fields are larger than or equal to 2 9 2 cm 2 . This difference in the effect of field size uncertainty matches Charles et al's 1.5 cm threshold for a ''very small'' field, defined as the field size below which a 1 mm difference in field size leads to a greater than 1 % difference in dose output [6]. For very small fields, the specification of appropriate field sizes in the radiotherapy treatment planning system, when inputting field output factors and other beam configuration data, is particularly important.…”

Section: Discussionmentioning

confidence: 57%

“…As treatment fields become smaller, due to the broadening implementation of modulated radiotherapy techniques and the increasing adoption of stereotactic techniques, the importance of collimator positioning reproducibility increases. For fields smaller than 1.5 cm across, field size changes of 1 mm can lead to central axis dose differences greater than 1 % [6]; and for fields 1 cm across, sub-millimetre changes in field size can lead to dose uncertainties of several percent [6,[15][16][17].…”

Section: Introductionmentioning

confidence: 99%

“…As the achievable accuracy of small field dosimetry has increased over the last decade [2,4], so has recognition of the effect that small changes in collimator positioning can have on that dosimetry [5,6], to the point that evaluation of the radiation field size is becoming a recommended step in small field output measurement [3,6]. This study aims to investigate the practical challenge of radiation field size consistency, by quantifying the potential extent and effects of the variability in field collimation that may exist between otherwise matched linacs.…”

Section: Introductionmentioning

confidence: 99%

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Field size consistency of nominally matched linacs

Kairn

Asena

Charles

et al. 2015

Australas Phys Eng Sci Med

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Given that there is increasing recognition of the effect that sub-millimetre changes in collimator position can have on radiotherapy beam dosimetry, this study aimed to evaluate the potential variability in small field collimation that may exist between otherwise matched linacs. Field sizes and field output factors were measured using radiochromic film and an electron diode, for jaw- and MLC-collimated fields produced by eight dosimetrically matched Varian iX linacs (Varian Medical Systems, Palo Alto, USA). This study used nominal sizes from 0.6 × 0.6 to 10 × 10 cm(2), for jaw-collimated fields, and from 1 × 1 to 10 × 10 cm(2) for MLC-collimated fields, delivered from a zero (head up, beam directed vertically downward) gantry angle. Differences between the field sizes measured for the eight linacs exceeded the uncertainty of the film measurements and the repositioning uncertainty of the jaws and MLCs on one linac. The dimensions of fields defined by MLC leaves were more consistent between linacs, while also differing more from their nominal values than fields defined by orthogonal jaws. The field output factors measured for the different linacs generally increased with increasing measured field size for the nominal 0.6 × 0.6 to 1 × 1 cm(2) fields, and became consistent between linacs for nominal field sizes of 2 × 2 cm(2) and larger. The inclusion in radiotherapy treatment planning system beam data of small field output factors acquired in fields collimated by jaws (rather than the more-reproducible MLCs), associated with either the nominal or the measured field sizes, should be viewed with caution. The size and reproducibility of the fields (especially the small fields) used to acquire treatment planning data should be investigated thoroughly as part of the linac or planning system commissioning process. Further investigation of these issues, using different linac models, collimation systems and beam orientations, is recommended.

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Section: Discussionmentioning

confidence: 57%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Field size consistency of nominally matched linacs

Kairn

Asena

Charles

et al. 2015

Australas Phys Eng Sci Med

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“…Many recent studies have applied Monte Carlo simulations to examine the output factors of small field sizes used for radiosurgery such as CyberKnife, 6 , 7 Gamma Knife, 8 , 9 and Brainlab (10) . In addition, numerous studies have used Monte Carlo simulation to examine the accuracy of diode detectors measurements for small field dosimetry 11 , 12 , 13 , 14 …”

Section: Introductionmentioning

confidence: 99%

Output factor comparison of Monte Carlo and measurement for Varian TrueBeam 6 MV and 10 MV flattening filter‐free stereotactic radiosurgery system

Cheng

Ning

Arora

et al. 2016

J Applied Clin Med Phys

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The dose measurements of the small field sizes, such as conical collimators used in stereotactic radiosurgery (SRS), are a significant challenge due to many factors including source occlusion, detector size limitation, and lack of lateral electronic equilibrium. One useful tool in dealing with the small field effect is Monte Carlo (MC) simulation. In this study, we report a comparison of Monte Carlo simulations and measurements of output factors for the Varian SRS system with conical collimators for energies of 6 MV flattening filter‐free (6 MV) and 10 MV flattening filter‐free (10 MV) on the TrueBeam accelerator. Monte Carlo simulations of Varian's SRS system for 6 MV and 10 MV photon energies with cones sizes of 17.5 mm, 15.0 mm, 12.5 mm, 10.0 mm, 7.5 mm, 5.0 mm, and 4.0 mm were performed using EGSnrc (release V4 2.4.0) codes. Varian's version‐2 phase‐space files for 6 MV and 10 MV of TrueBeam accelerator were utilized in the Monte Carlo simulations. Two small diode detectors Edge (Sun Nuclear) and Small Field Detector (SFD) (IBA Dosimetry) were applied to measure the output factors. Significant errors may result if detector correction factors are not applied to small field dosimetric measurements. Although it lacked the machine‐specific kQitalicclin,Qitalicmsrfitalicclin,fitalicmsr correction factors for diode detectors in this study, correction factors were applied utilizing published studies conducted under similar conditions. For cone diameters greater than or equal to 12.5 mm, the differences between output factors for the Edge detector, SFD detector, and MC simulations are within 3.0% for both energies. For cone diameters below 12.5 mm, output factors differences exhibit greater variations.PACS number(s): 87.55.k, 87.55.Qr

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“…Output factors are determined when normalizing the central axis dose in water (maximum dose) from a specific field size to the central axis dose from a 10 x 10 cm 2 field size (Charles et al, 2014). Then, smaller the field, the output factor suffers reduction and affect the uncertainties in dosimetry measurements.…”

Section: Output Factormentioning

confidence: 99%

Campos pequenos de radiação e materiais alternativos em dosimetria com espectroscopia de ressonância magnética eletrônica

Rech¹

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Rech AB. Small radiation field and alternative materials in dosimetry with electron magnetic resonance spectroscopy [thesis]. Ribeirão Preto: Universidade de São Paulo, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto; 2017.To keep up with advances in radiotherapy, dosimetric systems need improvement to meet the standards established for treatment accuracy; simultaneously, radioactive exposure goes beyond clinical use, and the detection in unforeseen scenarios is of interest too. Electron magnetic resonance spectroscopy (EMR) can detect paramagnetic centers created in materials exposed to radiation, relating spectral response with absorbed dose, thus performing nondestructively dosimetry, by keeping the information after the readout. After standardization of alanine as a high dose detector and the applicability in clinical studies, difficulties allowed the investigation of other EMR dosimetry materials; on the other hand, classifying compounds that are present daily and with the possibility of tissue equivalence is another stimulus for the expansion of alternative materials analysis. The development of this thesis is between two distinct but interconnected topics; first minidosimeters are presented for small field dosimetry, with a clinical approach, and the investigation of alternative materials, both topics applied in EMR dosimetry. Concerning the minidosimeters, some pellets' aspects are studied, a new concept of detector, called EPResize®; and a stereotactic radiosurgery end-to-end audit are presented; about alternative materials research, more than 20 compounds were studied, based on ammonium, lithium, potassium and sodium. The results showed that the difficulties in determining the dose with small fields in a clinical dose range is an issue that still needs much attention and adequacy of the dosimetric systems, in order to extract the greatest possible sensitivity, with the need to employ parameters and methods of analysis besides the daily used; several materials were adequate for EMR dosimetry, such as ammonium sulfate, sodium formate, sodium dithionate, sodium citrate and different sulfites, which, even when not satisfying clinical aspects, are alternatives for control and determination of doses in other scenarios. The ability to perform clinical dosimetry with RME and the standardization of this system allow the improvement of treatments accuracy, and the availability of a greater variety of EMR dosimetry materials facilitates the need for dose mapping unforeseen cases.

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A practical and theoretical definition of very small field size for radiotherapy output factor measurements

Cited by 77 publications

References 46 publications

Field size consistency of nominally matched linacs

Field size consistency of nominally matched linacs

Output factor comparison of Monte Carlo and measurement for Varian TrueBeam 6 MV and 10 MV flattening filter‐free stereotactic radiosurgery system

Campos pequenos de radiação e materiais alternativos em dosimetria com espectroscopia de ressonância magnética eletrônica

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