A Monte Carlo study of accelerator head scatter

Chaney, Edward L.; Cullip, T; Gabriel, T. A.

doi:10.1118/1.597194

Cited by 126 publications

(101 citation statements)

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“…The filter is a major contributor of scattered radiation in the treatment head (Chaney et al 1994, Fix et al 2001 and can lead to higher doses outside the treatment field , Kry et al 2010, O'Brien et al 1991.. Due to the non-uniform shape of the flattening filter, the beam energy distribution will vary with off-axis distance, creating challenges for dose calculation algorithms. One way of removing these unfavourable properties of conventional clinical radiation beams would be to remove the flattening filter.…”

Section: Introductionmentioning

confidence: 99%

A Monte Carlo study of a flattening filter-free linear accelerator verified with measurements

et al. 2010

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Abstract. A Monte Carlo model of an Elekta Precise linear accelerator has been built and verified by measured data for a 6 MV and 10 MV photon beam running with and without a flattening filter in the beam line. In this study the flattening filter was replaced with a 6 mm thick copper plate, provided by the linac vendor, in order to stabilize the beam. Several studies have shown that removal of the filter improves some properties of the photon beam, which could be beneficial for radiotherapy treatments. The investigated characteristics of this new beam included output, spectra, mean energy, half value layer and the origin of scattered photons. The results showed an increased dose output per initial electron at the central axis of 1.76 and 2.66 for the 6 MV and 10 MV beams, respectively. The number of scattered photons from the accelerator head was reduced by (31.70.03) % (1 SD) for the 6 MV beam and (47.60.02) % for the 10 MV beam. The photon energy spectrum of the unflattened beam was softer compared to a conventional beam and did not vary significantly with off-axis distance, even for the largest field size (0-20 cm off-axis).

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Section: Introductionmentioning

confidence: 99%

A Monte Carlo study of a flattening filter-free linear accelerator verified with measurements

et al. 2010

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“…All the simulations were performed at the open field geometry conditions, which corresponds to a 5ϫ40 cm 2 field at an 85 cm surface to source distance, except where stated otherwise. Scoring regions for depth dose calculations were 10ϫ10 ϫ5 mm 3 . The average spectra were scored in the central 4 ϫ40 cm 2 area, except for the off-axis dependence where the scoring regions were 4ϫ5 cm 2 .…”

Section: Methodsmentioning

confidence: 99%

Radiation characteristics of helical tomotherapy

et al. 2004

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Helical tomotherapy is a dedicated intensity modulated radiation therapy ͑IMRT͒ system with on-board imaging capability ͑MVCT͒ and therefore differs from conventional treatment units. Different design goals resulted in some distinctive radiation field characteristics. The most significant differences in the design are the lack of flattening filter, increased shielding of the collimators, treatment and imaging operation modes and narrow fan beam delivery. Radiation characteristics of the helical tomotherapy system, sensitivity studies of various incident electron beam parameters and radiation safety analyses are presented here. It was determined that the photon beam energy spectrum of helical tomotherapy is similar to that of more conventional radiation treatment units. The two operational modes of the system result in different nominal energies of the incident electron beam with approximately 6 MeV and 3.5 MeV in the treatment and imaging modes, respectively. The off-axis mean energy dependence is much lower than in conventional radiotherapy units with less than 5% variation across the field, which is the consequence of the absent flattening filter. For the same reason the transverse profile exhibits the characteristic conical shape resulting in a 2-fold increase of the beam intensity in the center. The radiation leakage outside the field was found to be negligible at less than 0.05% because of the increased shielding of the collimators. At this level the in-field scattering is a dominant source of the radiation outside the field and thus a narrow field treatment does not result in the increased leakage. The sensitivity studies showed increased sensitivity on the incident electron position because of the narrow fan beam delivery and high sensitivity on the incident electron energy, as common to other treatment systems. All in all, it was determined that helical tomotherapy is a system with some unique radiation characteristics, which have been to a large extent optimized for intensity modulated delivery.

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“…Recently, application of Monte Carlo simulation has experienced tremendous growth, particularly in studying clinical accelerators. [13][14][15][16] This is because Monte Carlo simulation enables us to study the accelerator at a greater depth in identifying individual sources of the backscatter from collimator jaws. The motivation for this work is to model the backscattered radiation as functions of the upper ͑Y͒ and lower ͑X͒ jaw positions.…”

Section: Introductionmentioning

confidence: 99%

Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique

2000

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Dose per monitor unit in photon fields generated by clinical linear accelerators can be affected by the backscattered radiation into the monitor chamber from collimator jaws. Thus, it is necessary to account for the backscattered radiation in computing monitor unit setting for a treatment field. In this work, we investigated effects of the backscatter from collimator jaws based on Monte Carlo simulations of a clinical linear accelerator. The backscattered radiation scored within the monitor chamber was identified as originating either from the upper jaws ͑Y jaws͒, or from the lower jaws ͑X jaws͒. From the results of Monte Carlo simulations, ratios of the monitor-chamber-scored dose caused by the backscatter to the dose caused by the forward radiation, R(x,y), were modeled as functions of the individual X and Y jaw positions. The amount of the backscattered radiation for any field setting was then computed as a compound contribution from both the X and Y jaws. The dose ratios of R(x,y) were then used to calculate the change in photon output caused by the backscatter, S cb (x,y). Results of these calculations were compared with available measured data based on counting the electron pulses or charge from the electron target of an accelerator. Data from this study showed that the backscattered radiation contributes approximately 3% to the monitorchamber-scored dose. A majority of the backscattered radiation comes from the upper jaws, which are located closer to the monitor chamber. The amount of the backscatter decreases approximately in a linear fashion with the jaw opening. This results in about a 2% increase of photon output from a 10 cmϫ10 cm field to a 40 cmϫ40 cm field. The off-axis location of the jaw opening does not have a significant effect on the magnitude of the backscatter. The backscatter effect is significant for monitor chambers using kapton windows, particularly for treatment fields using moving jaws. Applying the backscatter correction improves the accuracy of monitor-unit calculation using a model-based dose calculation algorithm such as the convolution method. © 2000 American Association of Physicists in Medicine. ͓S0094-2405͑00͒00904-4͔

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A Monte Carlo study of accelerator head scatter

Cited by 126 publications

References 0 publications

A Monte Carlo study of a flattening filter-free linear accelerator verified with measurements

A Monte Carlo study of a flattening filter-free linear accelerator verified with measurements

Radiation characteristics of helical tomotherapy

Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique

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