Calculating output factors for photon beam radiotherapy using a convolution/superposition method based on a dual source photon beam model

Liu, H. Helen; Mackie, T; McCullough, Edwin C.

doi:10.1118/1.598111

Cited by 56 publications

(47 citation statements)

References 30 publications

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“…More complex dual source photon beam models reached an agreement within 1% between calculated and measured output factors in water ͑different reference conditions and energies͒. 32,39 No beam model yielded a significant deviation compared to the full MC model because of its relatively large standard deviations. However, the models 1 and 2 led to significant deviations compared to the measurements for all field sizes greater than 15ϫ15 cm 2 , and model 3 only for the field sizes of 15 ϫ15 cm 2 and 40ϫ40 cm 2 .…”

Section: Resultsmentioning

confidence: 84%

Simple beam models for Monte Carlo photon beam dose calculations in radiotherapy

et al. 2000

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Monte Carlo (code GEANT) produced 6 and 15 MV phase space (PS) data were used to define several simple photon beam models. For creating the PS data the energy of starting electrons hitting the target was tuned to get correct depth dose data compared to measurements. The modeling process used the full PS information within the geometrical boundaries of the beam including all scattered radiation of the accelerator head. Scattered radiation outside the boundaries was neglected. Photons and electrons were assumed to be radiated from point sources. Four different models were investigated which involved different ways to determine the energies and locations of beam particles in the output plane. Depth dose curves, profiles, and relative output factors were calculated with these models for six field sizes from 5x5 to 40x40cm2 and compared to measurements. Model 1 uses a photon energy spectrum independent of location in the PS plane and a constant photon fluence in this plane. Model 2 takes into account the spatial particle fluence distribution in the PS plane. A constant fluence is used again in model 3, but the photon energy spectrum depends upon the off axis position. Model 4, finally uses the spatial particle fluence distribution and off axis dependent photon energy spectra in the PS plane. Depth dose curves and profiles for field sizes up to 10x10cm2 were not model sensitive. Good agreement between measured and calculated depth dose curves and profiles for all field sizes was reached for model 4. However, increasing deviations were found for increasing field sizes for models 1-3. Large deviations resulted for the profiles of models 2 and 3. This is due to the fact that these models overestimate and underestimate the energy fluence at large off axis distances. Relative output factors consistent with measurements resulted only for model 4.

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Section: Resultsmentioning

confidence: 84%

Simple beam models for Monte Carlo photon beam dose calculations in radiotherapy

et al. 2000

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“…the S cb (x,y) obtained from the Monte Carlo simulation was also compared to the measured data based on counting the electronic feedback pulses or accumulative charge from the electron target. 1,11 To evaluate the effect of the backscatter on treatment fields using jaw movement, S cp (x,y) was also calculated for treatment fields using dynamic wedges.…”

Section: Methodsmentioning

confidence: 99%

“…In a previous work, 1 we showed that the change of photon output as a function of field geometry is essentially caused by three factors: the head scatter, the phantom scatter, and the backscattered radiation from collimator jaws into monitor chambers. We developed a convolution/superposition algorithm to compute the photon output or dose per monitor unit.…”

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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“…IMRT techniques 4,5 ). High dose gradients are common to all of these stereotactically guided irradiation fields, regardless of their shape, thus precise algorithms are required having to be verified in terms of precise dosimetric measurements [6][7][8][9] .…”

Section: Introductionmentioning

confidence: 99%

Development of a 3-D convolution / superposition algorithm for precise dose calculation in the skull

Mack

Weltz²,

Scheib

et al. 2006

Australas. Phys. Eng. Sci. Med.

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In this paper an algorithm for calculating 3-D dose distributions within the brain is introduced and adapted to the demands of modem radiosurgery. The dose calculation with this model is based on a 3-D distribution of the primary photon intensity which is calculated with a ray casting algorithm. A prelocated matrix takes into account field sizes as well as modifying elements as collimator positions (MLC), blocks, wedges and compensators. Monte Carlo precalculated monoenergetic kernels from 0.1 MeV to 50 MeV were at our disposal. The components of the spectrum were either determined by deconvoluting depth dose curves measured in water or analyzed with a Ge-Li detector system in the case of 60Co. The calculated fluence distribution has to be superposed to the complete kernel containing the spatial energy deposition. Inhomogeneities and tissue interface phenomena (rhoe, Z) have been investigated. The divergence of the rays and the curved surface of the patient are taken into account. Assuming homogenous media, it is possible to shorten the computation time by using the Fast Fourier Transformation (FFT) delivering a first overview within seconds. The algorithm was evaluated and verified under specific conditions of small fields as used in radiosurgery and compared to dose measurements and Monte Carlo calculations. In using both the fast algorithm (FFT) for mainly homogenous conditions on one hand and the very precise superposition for inhomogeneous cases on the other, this algorithm can be a very helpful instrument especially for critical locations in the skull.

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Calculating output factors for photon beam radiotherapy using a convolution/superposition method based on a dual source photon beam model

Cited by 56 publications

References 30 publications

Simple beam models for Monte Carlo photon beam dose calculations in radiotherapy

Simple beam models for Monte Carlo photon beam dose calculations in radiotherapy

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

Development of a 3-D convolution / superposition algorithm for precise dose calculation in the skull

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