Ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

Sanz, Darı́o E.; Alvarez, Guillermo Daniel; Nelli, Flavio Enrico

doi:10.1088/0031-9155/52/6/007

Cited by 5 publications

(5 citation statements)

References 13 publications

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“…In this paper, an improvement of the telescopic technique for measuring the BSR to a monitor chamber was reported. Compared to the previously published telescopic techniques, our method avoided using bulky lead blocks (as in the method by Kubo), and allowed for the exclusion of room scatter from the measured signal without the need for repeated measurements as in Yu et al (1996 andSanz et al , 2007). Our method also enabled fast measurement setup that, after initial collimator alignment, only required about 20 min.…”

Section: Discussionmentioning

confidence: 99%

“…Modifications to this technique that use smaller collimators have been tested (Duzenli, McClean et al, 1993), but suffered from overestimation of the BSR due to inadvertent inclusion of room scatter into the measured signal (Yu, Sloboda et al, 1996). Yu and Sanz (Yu, Sloboda et al, 1996, Sanz, Alvarez et al, 2007 improved this technique by evaluating and subtracting the scatter component of the signal, though Sanz et al used a small lead slug instead of collimators. Evaluation of the room scatter required doubling the number of measurements.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Monitor backscatter factors for the Varian 21EX and TrueBeam linear accelerators: measurements and Monte Carlo modelling

Zavgorodni¹,

Alhakeem²,

Townson³

2014

Phys. Med. Biol.

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Linac backscattered radiation (BSR) into the monitor chamber affects the chamber's signal and has to be accounted for in radiotherapy dose calculations. In Monte Carlo (MC) calculations, the BSR can be modelled explicitly and accounted for in absolute dose. However, explicit modelling of the BSR becomes impossible if treatment head geometry is not available. In this study, monitor backscatter factors (MBSFs), defined as the ratio of the charge collected in the monitor chamber for a reference field to that of a given field, have been evaluated experimentally and incorporated into MC modelling of linacs with either known or unknown treatment head geometry. A telescopic technique similar to that by Kubo (1989 Med. Phys. 16 295-98) was used. However, instead of lead slits, a 1.8 mm diameter collimator and a small (2 mm diameter) detector positioned at extended source to detector distance were used. This setup provided a field of view to the source of less than 3.1 mm and allowed for MBSF measurements of open fields from 1 × 1 to 40 × 40 cm(2). For the fields with both X and Y dimensions exceeding 15 cm, a diode detector was used. A pinpoint ionization chamber was used for smaller fields. MBSFs were also explicitly modelled in MC calculations using BEAMnrc and DOSXYZnrc codes for 6 and 18 MV beams of a Varian 21EX linac. A method for deriving the D(ch)(forward) values that are used in MC absolute dose calculations was demonstrated. These values were derived from measured MBSFs for two 21EX and four TrueBeam energies. MBSFs were measured for 6 and 18 MV beams from Varian 21EX, and for 6 MV, 10 MV-FFF, 10 MV, and 15 MV beams from Varian TrueBeam linacs. For the open field sizes modelled in this study for the 21EX, the measured MBSFs agreed with MC calculated values within combined statistical (0.4%) and experimental (0.2%) uncertainties. Variation of MBSFs across field sizes was about a factor of two smaller for the TrueBeam compared to 21EX Varian linacs. Measured MBSFs and the derived [Formula: see text] factors allow for the incorporation of the BSR effect into accurate radiotherapy dose calculations without explicit backscatter modelling.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Monitor backscatter factors for the Varian 21EX and TrueBeam linear accelerators: measurements and Monte Carlo modelling

Zavgorodni¹,

Alhakeem²,

Townson³

2014

Phys. Med. Biol.

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“…Backscatter into the beam monitor chambers is known to contribute to the field size dependence of the photon output of medical linear accelerators [1][2][3][4][5][6].…”

Section: Introductionmentioning

confidence: 99%

“…Although most experimental methods for characterizing the backscatter effect are rather cumbersome, the ecliptic method [4] provides an easy experimental set-up that is not invasive to the accelerator's systems.…”

Section: Introductionmentioning

confidence: 99%

An MLC-based version for the ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

Nelli

2015

Australas Phys Eng Sci Med

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A very simple method to measure the effect of the backscatter from secondary collimators into the beam monitor chambers in linear accelerators equipped with multi-leaf collimators (MLC) is presented here. The backscatter to the monitor chambers from the upper jaws of the secondary collimator was measured on three beam-matched linacs by means of three methods: this new methodology, the ecliptic method, and assessing the variation of the beam-on time per monitor unit with dose rate feedback disabled. This new methodology was used to assess the backscatter characteristics of asymmetric over-traveling jaws. Excellent agreement between the backscatter values measured using the new methodology introduced here and the ones obtained using the other two methods was established. The experimental values reported here differ by less than 1% from published data. The sensitivity of this novel technique allowed differences in backscatter due to the same opening of the jaws, when placed at different positions on the beam path, to be resolved. The introduction of the ecliptic method has made the determination of the backscatter to the monitor chambers an easy procedure. The method presented here for machines equipped with MLCs makes the determination of backscatter to the beam monitor chambers even easier, and suitable to characterize linacs equipped with over-traveling asymmetric secondary collimators. This experimental procedure could be simply implemented to fully characterize the backscatter output factor constituent when detailed dosimetric modeling of the machine's head is required. The methodology proved to be uncomplicated, accurate and suitable for clinical or experimental environments.

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“…Experimental evaluation of the effects of backscattered radiation reaching the monitor chamber, from the secondary collimators, remains challenging 2,3,5,7,8,9 . Sanz et al 7 have Figure 1.…”

Section: Introductionmentioning

confidence: 99%

Effects of collimator backscatter in an Elekta linac by Monte Carlo simulation

Kairn

Crowe

Poole

et al. 2009

Australas. Phys. Eng. Sci. Med.

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The effects of radiation backscattered from the secondary collimators into the monitor chamber in an Elekta linac (producing 6 and 10 MV photon beams) are investigated using BEAMnrc Monte Carlo simulations. The degree and effects of this backscattered radiation are assessed by evaluating the changes to the calculated dose in the monitor chamber, and by determining a correction factor for those changes. Additionally, the fluence and energy characteristics of particles entering the monitor chamber from the downstream direction are evaluated by examining BEAMnrc phase-space data. It is shown that the proportion of particles backscattered into the monitor chamber is small (< 0.35%), for all field sizes studied. However, when the backscatter plate is removed from the model linac, these backscattered particles generate a noticeable increase in dose to the monitor chamber (up to approximately 2.4% for the 6 MV beam and up to 4.4% for the 10 MV beam). With its backscatter plate in place, the Elekta linac (operating at 6 and 10 MV) is subject to negligible variation of monitor chamber dose with field size. At these energies, output variations in photon beams produced by the clinical Elekta linear accelerator can be attributed to head scatter alone. Corrections for field-size-dependence of monitor chamber dose are not necessary when running Monte Carlo simulations of the Elekta linac operating at 6 and 10 MV.

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Ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

Cited by 5 publications

References 13 publications

Monitor backscatter factors for the Varian 21EX and TrueBeam linear accelerators: measurements and Monte Carlo modelling

Monitor backscatter factors for the Varian 21EX and TrueBeam linear accelerators: measurements and Monte Carlo modelling

An MLC-based version for the ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

Effects of collimator backscatter in an Elekta linac by Monte Carlo simulation

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