Research Article - Biomedical Research (2017) Volume 28, Issue 4

## Monte Carlo study of a flattening filter-free 6 MV photon beam using the BEAMnrc code

**Ankit Kajaria**

^{1*}, Neeraj Sharma^{1}, Shiru Sharma^{1}, Satyajit Pradhan^{2}, Abhijit Mandal^{2}and Lalit Aggarwal M^{2}^{1}School of Biomedical Engineering, Indian Institute of Technology (BHU) Varanasi, UP, India

^{2}Department of Radiotherapy and Radiation Medicine, Institute of Medical Science (BHU), Varanasi, UP, India

- *Corresponding Author:
- Ankit Kajaria

School of Biomedical Engineering

Indian Institute of Technology

Varanasi, India

**Accepted on** August 18, 2016

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### Abstract

Flattening Filter Free (FFF) photon beams have different dosimetric properties from those of flattened beams. The aim of this study is to evaluate the basic dosimetric properties of a flattening filter free 6 MV photon beam. A Monte Carlo simulation model was developed for a 6 MV photon beam based on Varian Clinic 600 unique performance linac operated with/without a flattening filter and dosimetric features including central axis absorbed dose, beam profiles and photon and electron spectra were calculated for flattened and unflattened cases separately. Absolute depth dose calculations showed an increase in dose rate with a factor of more than 2.4 for the unflattened 6 MV photon beam which is dependent on the depth. Percentage Depth Doses (PDDs) values were found to be lower for unflattened beam for all field sizes. The total Scatter Correction Factor (SCP) were found to have less variation with field sizes for unflattened beam indicating that removing of the filter from the beam line can reduce significant amount of head scatter. However surface doses were found to be higher for the unflattened beam due to more contamination electrons and low energy photons in the beam. Our study showed that increase in the dose rate and lower out-of-field dose could be considered as practical advantages for unflattened 6 MV photon beams.

## Keywords

Unflattened photon beam, Flattening filter, Monte Carlo.

## Introduction

Conventional clinical accelerators are equipped with a Flattening Filter (FF) which is primarily designed to produce a flat beam profile at a given depth by compensating for the nonuniformity of photon fluence across the field. But flattening filter decreases the X-ray output considerably and produces quality changes within the primary beam by scattering and absorption of primary photons. The requirement to have a flattened beam profile for treatment delivery is no longer necessary when certain type of treatments such as intensitymodulated radiation therapy or intensity-modulated arc therapy is used. In Intensity Modulated Radiation Therapy (IMRT), the patient dose distribution can instead be shaped by the Multileaf Collimator (MLC) to create the desired clinical effect. In principle, the flattening filter can then be removed, and the leaf sequences can be adjusted accordingly to produce fluence distributions similar to those of a beam with a flattening filter. Removal of flattening filter with its associated attenuation from x-ray beam path increases dose rate [1]. The other possible effect is substantial reduction in head scatter, as the flattening filter is the major source of scattered photons. Flattening Filter Free (FFF) beams in radiotherapy thus have the advantage of shorter treatment delivery time and lower out-of field dose compared with conventional flattened beams. This is especially important where large doses per fraction are prescribed, e.g., stereotactic ablative body radiotherapy [2,3] or where patient motion might affect the efficacy of the delivery or both [4]. Monte Carlo (MC) method has become a powerful tool in radiotherapy dose calculations and many studies have been conducted using this method for analysing linac head components and influencing factors on beam characteristics [5-7] .Thus, the effect of flattening filter on photon energy spectra, absolute absorbed dose per initial electron and beam profiles could be studied by this method [8]. In an Monte Carlo (MC) study on Flattening filter free beams, dose rates increase by a factor of 2.31 (6 MV) and 5.45 (18 MV) and out-of-field dose reductions were reported [9]. In a similar study, a significant improvement in out-of-field dose was reported for small field sizes [10]. Above studies outline the potential benefits of removing the flattening filter. It is therefore important to investigate these properties for a typical modern accelerator such as the Varian Clinic 600 unique performance. This study reports on depth-dose dependencies, dose rates, lateral profiles, out-of-field doses, total scatter factors, and photon and electron fluence in a conventional accelerator and a flattening filter-free system.

## Material and Methods

*The 6 MV Varian linac simulation model*

Calculating spectra with more accuracy requires knowledge of the characteristics of the electron beam incident on the target as well as better tools for modelling the linac. We used the BEAMnrc code system [11,12] to derive best estimates for the mean energy and Full Width at Half Maximum (FWHM) of the electron beam incident on the target. Monte Carlo simulations for monoenergetic beams ranging from 5.5 to 6.2 MeV with Full Width at Half Maximum (FWHM) varied from 0.15 to 0.25 cm were performed to find the best match with Percentage Depth Dose (PDD) and profiles measurements. A monoenergetic source with kinetic energy of the beam 5.7 MeV and Full Width at Half Maximum (FWHM) for the X and Y directions of 0.2 cm was found to give best agreement with measured data. Geometry and materials used to build the Monte Carlo model of the linear accelerator were based on machine specifications as provided by the manufacturer Varian Medical Systems. The linac was structured in the following order: a target slab of tungsten and copper, primary collimator (tungsten), flattening filter, ion chamber, mirror, jaws (tungsten). All materials used in the Monte Carlo simulation were extracted from the 700 ICRU PEGS4 (International Commission for Radiation Units Pre-processor for Electron Gamma Shower) cross section data available in BEAMnrc, and met the specifications for the linac as provided by the manufacturer.

*The structure of the calculation using Monte Carlo
techniques*

In this section we describe the different stages of simulation for
6 MV photon beam produced by Varian Linac using principal
features of the BEAMnrc-DOSXYZnrc code [13,14] which are
shown in **Figure 1**. In the simulation of the full accelerator unit
we have split the calculation into three steps in order to save
time. In the first step, which takes the most computing time,
1.5 × 10^{8} initial histories are initiated and a monoenergetic
electron beam source of kinetic energy of 5.7 MeV with Full
Width at Half Maximum (FWHM) for the X and Y directions
of 0.2 cm was incident on the target. The primary collimator,
flattening filter and ion chamber are included in this step. The
output of this step is a phase space file at plain one as show in **Figure 1**, having information of energy, position, direction,
charge and history variable for every particle exiting
downstream from the end of ion chamber. Since the source and
primary collimator have fixed openings, it is possible to use
this phase space data for the simulation of different field sizes. **Figure 1** list the component module of BEAMnrc code used for
modelling of fixed opening part of treatment head in first step.
This large set of particles produced in first step is used
repeatedly as the input to the next step of simulation. The
second step of the calculation simulates the passage of the
particles through the mirror, adjustable collimator and the air
slab to a plane at Source to Surface Distance (SSD) 100 cm
from target. We simulate different openings of the adjustable collimator to get field sizes from 5 × 5 to 20 × 20 cm^{2} at a
Source to Surface Distance (SSD) equal to 100 cm. **Figure 1** also list the component module of BEAMnrc code used for
modelling of variable opening part of treatment head in second
step of simulation. The output of this step is a phase space file
at plain two as show in **Figure 1**, having information of energy,
position, direction, charge and history variable for every
particle reaching the plain at Source to Surface Distance (SSD)
100 cm from target. The data analysis program BEAMDP [15]
is used to analyse the phase space data files to extract the
various types of spectra of all particles reaching the plane at
Source to Surface Distance (SSD) 100 cm. In the third step of
the simulation, the phase space files for field sizes of 5 × 5 to
20 × 20 cm^{2} at an Source to Surface Distance (SSD) of 100 cm
which are obtain at end of second step are reused by the
DOSXYZnrc code as an input for dose calculations in a water
phantom as shown in **Figure 1**. We transport the particles
through a water phantom of dimension 30 × 30 × 30 cm^{3} with
voxels size of 0.25 × 0.25 × 0.25 cm^{3}. In the simulation of
“unfiltered” 6 MV photon beam all the three step of simulation
are same expect in first step where the flattening filter is being
removed from the beam line. A comprehensive set of
dosimetric data for 6 MV filtered photon beams where
acquired using a three-dimensional (3D) phantom, Blue
phantom 2 IBA Dosimetry GmbH and OmniPro-Accept 7 data
acquisition software. All the measurements were performed
with a Scanditronix/Wellhofer compact ionization chamber
CC13, in the water phantom.

## Results

*Monte Carlo simulation model validation*

Depth-dose curves for filtered 6 MV photon beam for field size
5 × 5 to 20 × 20 cm^{2} were calculated in an on axis cylinder of
radius 1 cm using Monte Carlo simulation and compare with
measured data for the validation of simulation model. The
calculated central axis depth-dose curves were normalized to
unity at the depth, d_{max}, of the maximum dose deposition,
D_{max}. Both results measured and calculated, could then be compared with respect to the relative value of the maximum
dose D_{max} and the corresponding depth d_{max}. **Figure 2** show the
comparison between the calculated depth-dose distributions
and measurements for three different field sizes studied in this
work. The comparison shows that the calculated and measured
data agree within 1% of local relative dose, and 1 mm in depth
at all depths and field size which are summarized in **Table 1**.

A/cm^{2} |
d_{max} (simulated) |
d_{max} (measured) |
Δ D_{max} |
---|---|---|---|

5 × 5 | 1.50 | 1.56 | 0.20 |

10 × 10 | 1.50 | 1.52 | 0.17 |

15 × 15 | 1.48 | 1.50 | 0.13 |

20 × 20 | 1.38 | 1.40 | 0.10 |

**Table 1.** Comparison of calculated and measured central-axis depthdose
profiles at various field sizes. A denotes the field size, d_{max} (cm)
denotes the location of the maximum dose, and Δ D_{max} is the relative
dose difference between the measurement and the calculations at d_{max}.

Lateral beam profiles for the filtered 6 MV photon beam were
also simulated for 5 × 5 to 20 × 20 field sizes at 1.5, 5 and 10
cm depths. The measured and calculated lateral dose profiles
were normalized to unity on the central axis for comparison. **Figure 3** shows the comparison of Monte Carlo calculations to
measured data for a field size of 20 × 20, 10 × 10 and 5 × 5
cm^{2} at depth of 10 cm. The lateral field size at the 50% dose
level (X_{50}) and penumbra widths, P_{90-10} and P_{80-20} (calculated
from the 90% level to the 10% level and from 80% to 20%)
where calculated using Monte Carlo simulation and the results
of the comparisons are summarizes in **Table 2**. The differences
between the measurement and the simulations results in lateral
field size at the 50% dose level, X_{50}, was found to be less than
1 mm.

A/cm^{2} |
Δ X_{50} |
Δ P_{80-20} |
Δ P_{90-10} |
---|---|---|---|

5 × 5 | 0.10 | 1.50 | 0.8 |

10 × 10 | 0.50 | 1.52 | 1.0 |

15 × 15 | 0.40 | 1.20 | 2.0 |

20 × 20 | 0.50 | 1.00 | 2.2 |

**Table 2.** Comparison of measured and calculated lateral dose profiles
at 10 cm depth. ‘A’ Denotes the field size, Δ X_{50} (mm) is the lateral
difference measured at the 50% dose point in the penumbra, and Δ
P_{90-10} (mm) as well as ΔP_{80-20} (mm) describes the difference in width
of the penumbra measured from the 90% point to 10% dose point and
from 80% to 20% dose point respectively.

*Simulations without the flattening filter and
comparison with flattened beam characteristics*

**Absolute dose:** Absolute absorbed dose per initial electron
were calculated for flattened and unflattened beam. For
comparison purposes, we considered the depth of 1.5 and 10
cm as a reference depth for dose rate comparison. The ratios of
absolute depth doses for flattening filter free to standard
flattened beams were calculated and are presented in **Table 3**. It
was observe that absorbed dose per initial electron increased
significantly by removing flattening filter, indicating an
increased in dose rate for unflattened beam per initial electron.
However, the increase in dose rate is decreased with increase in
depth.

A/cm^{2} |
(D_{FFF}/D_{FF}) at d=1.5 |
(D_{FFF}/_{DFF}) at d=10 |
---|---|---|

5 × 5 | 2.472 | 2.420 |

10 × 10 | 2.474 | 2.400 |

15 × 15 | 2.447 | 2.440 |

20 × 20 | 2.444 | 2.380 |

**Table 3.** Ratios of absolute depth doses for flattening filter free to
standard flattened beams at two reference depths for different field
sizes. ‘A’ denotes the field size; ‘d’ denotes the depth inside water
phantom. Absorbed dose calculated without the flattening filter in the
beam line is denoted as D_{FFF} (Flattening Filter Free) and with filter
in beam line is denoted as D_{FF}.

*Percentage depth-dose characteristics*

Percentage Depth Dose characteristics (PDD) curves were
generated using absolute depth dose values. It can be seen from **Figure 4** that percentage depth doses calculated for unflattened
beam is slightly lower than standard beam for all field sizes.
Difference in the Percentage Depth Doses (PDDs) of flattened
and unflattened beams are evident at deeper depths and are
increased with depth for 5 × 5, 10 × 10, 15 × 15 and 20 × 20
cm^{2} field sizes. To verify this difference two parameters are
reported in **Table 4**, namely, the relative dose at a depth of 10
and 20 cm (D_{10}, D_{20}).

A/cm^{2} |
D_{10} |
D_{20} |
||
---|---|---|---|---|

With FF | Without FF | With FF | Without FF | |

5 × 5 | 61.87 | 59.77 | 33.14 | 30.88 |

10 × 10 | 66.67 | 63.4 | 37.32 | 34.5 |

15 × 15 | 68.32 | 66.49 | 39.2 | 36.69 |

20 × 20 | 69.5 | 64.54 | 41.6 | 37.98 |

FF: Flattening Filter |

**Table 4.** Comparison of relative depth doses for flattening filter free to
standard flattened beams at two reference depths for different field
sizes. ‘A’ denotes the field size; D_{10} and D_{20} denotes relative depth
dose at 10 and 20 cm depth.

*Analysis of spectra*

**Photon spectra:** **Figure 5** shows photon spectra as a function
of energy (number of photons per MeV per incident electron
on the target) calculated for central axis. Photon emerging from
target passes through the components of the collimating system
on their way to the scoring plain at an Source to Surface Distance (SSD) 100 cm. Scoring plain is an annular region
around the central axis with radius 0 < r < 2.25 cm. The range
of possible energy of photon is divided into interval (bin) of
0.25 MeV. The number of photon within each energy bin
crossing the scoring plain is being recorded for with/without
flattening filter case separately. The precision of calculated
central-axis photon spectra for all the field size used in the
dose calculations is high and uncertainty in each 0.25 MeV
wide bin is usually between 1 to 5%, except for the high energy
end of the spectra. There is a noticeable increase
observe in the photon fluence when the flattening filter is
removed from the beam line.

*Average energy distribution*

**Figure 6** shows the calculated photon average energies
distribution at 100 cm Source to Surface Distance (SSD) for 20
× 20 cm^{2} field size as a function of off axis distance for with/
without flattening filter case. From above distribution we find
that the mean photon energy for flattened beam to have a value at central axis 1.52 MeV and decrease to 1.3 MeV at off axis
distance of 20 cm which verifies the beam hardening effect
produced by the flattening filter [8] for the filtered beam. For
the unflattened beam, the mean energy of spectra was not
changed significantly with increasing off axis distance and it
was respectively decreased from 1.23 MeV on central axis to
1.19 MeV at 20 cm off axis distance for 20 × 20 cm^{2} field size.

*Contaminant electron fluence spectra*

The electron fluence increase indicate a potential risk of
delivering an elevated skin dose to the patient and also the risk
of placing ion chamber used for the measurement outside the
range of its reliable operation. **Figure 7** shows the calculated
fluence spectra for contaminant electrons calculated for central
axis with radius 0<r<2.25 cm and energy bin of 0.25 MeV at
100 cm Source to Surface Distance (SSD) for with/without
flattening filter case separately.

In our study it is found that the number of electron reaching the
phantom surface increases with removing the flattening filter
from the beam line. The averaged value of electron fluence
spectra calculated for without flattening filter case is found to
be 1.25 times greater than its value for with flattening filter
case for field size 20 × 20 cm^{2}.

*Surface dose*

Surface dose has been calculated for different field sizes for
both with/without flattening filter case and is listed in **Table 5**.

Field size (cm^{2}) |
(D_{max}/D_{min}) with flattening filter |
(D_{max}/D_{min}) without flattening filter |
---|---|---|

5 × 5 | 47.80 | 53.72 |

10 × 10 | 49.40 | 56.20 |

15 × 15 | 53.20 | 59.80 |

20 × 20 | 55.19 | 63.10 |

**Table 5.** Percentage Depth Doses (PDDs) for first scoring voxels as an
indication of the surface dose for different field sizes.

The Percentage Depth Doses (PDDs) of first scoring voxels with 0.25 cm thickness from the top of water phantom surface is taken as a measure of surface dose. There are differences in doses of build-up region between with flattening filter and without flattening filter cases. Surface dose is affected significantly by contamination electrons reaching the phantom surface and due to higher fluence of contamination electron for unflattened beam it is evident that for without flattening filter case, the surface dose is higher than that of with flattening filter case in all field sizes.

*Scatter function*

The total scatter factor, Scatter Correction Factor (SCP) is
defined as ‘the dose rate at a reference depth for a given field
size divided by the dose rate at the same point and depth for
the reference field size (10 × 10 cm^{2}). It was measured at
Source to Surface Distance (SSD) =100 cm and a depth equal
to d_{max} of a 10 × 10 cm^{2} field for different field sizes. The data
for with/without flattening filter case are presented in **Table 6**.
The Scatter Correction Factor (SCP) of the unflattened beams
is found to have less value for lager field sizes than that of the
flattened beams which indicate a reduced head scatter in
unflattened beams compared to the standard flattened beam.

Field size (cm^{2}) |
SCP with flattening filter | SCP without flattening filter |
---|---|---|

5 × 5 | 0.96 | 0.97 |

10 × 10 | 1 | 1 |

15 × 15 | 1.031 | 1.012 |

20 × 20 | 1.048 | 1.027 |

SCP: Scatter Correction Factor |

**Table 6.** Total Scatter Correction Factor (SCP) of 6 MV photon beams
measured for with/without a flattening filter cases. The Scatter
Correction Factor (SCP) was measured at Source to Surface Distance
(SSD) =100 cm, and at the depth of maximum dose d_{max} of a 10 × 10
cm^{2} field size.

*Profile comparison*

Beam profiles for different field sizes were calculated at 1.5, 5
and 10 cm depth for both cases with/without flattening filter in
a water phantom. As a measure of beam flatness the ratio of
maximum to minimum dose within 80% of field width was
calculated and is reported in **Table 7** for profiles measured at a
depth of 10 cm. It is seen that the differences between ratios
for the two cases are increasing with increasing field size. It is
clear that there is nearly no difference between two cases for
small field sizes. This is in the consistence with the results
reported by Jeraj et al. [4] that the profiles of unflattened beam
for field sizes up to 3 × 3 cm^{2} are similar to the flattened beam
profiles. Thus, removing flattening filter may have some
application for radiotherapy techniques, which uses small field
size. For larger fields, in flattened beams, the ratio is 1.10 or less, whereas in unflattened beams it increases with increasing
field size reaching 1.3 for a 20 × 20 cm^{2} field.

Field size (cm^{2}) |
(D_{max}/D_{min}) with flattening filter |
(D_{max}/D_{min}) without flattening filter |
---|---|---|

5 × 5 | 1.07 | 1.04 |

10 × 10 | 1.06 | 1.16 |

15 × 15 | 1.05 | 1.2 |

20 × 20 | 1.1 | 1.3 |

**Table 7.** The ratio of maximum to minimum dose in lateral profiles
within 80% of field size for 6 MV photon beams with/without a
flattening filter. The profiles were measured at a depth of 10 cm, at
Source to Surface Distance (SSD) =100 cm.

For the comparison of lateral profiles of unflattened and
flattened beams, the lateral profiles for 10 × 10 and 20 × 20
cm^{2} field sizes are compared at a depth of 10 cm as shown in **Figure 8**. For this comparison, the flat profile is normalized to
1 on the central axis, and the non-fat profile is normalized by
dose (D_{n}) which is calculated using this formula:

D_{n}= ((D_{u})/D_{f})*D_{CAX}

Where D_{u} is the dose at the inflection point of penumbra
region of the unflattened beam, D_{f} is the dose at the inflection
point of the flattened profile and D_{CAX} is the dose on the
central axis of the flattened beam [16]. It is observe in our
study that the beam profiles for unflattened case to have
relative dose value lower than the flattened beam near the
measured field size edge. The amount of reduction for 10 × 10
cm^{2} field size measured at 4 cm off axis distance is 10% and
for 20 × 20 cm^{2} measured at 9 cm off axis distance is found to
be 20%, respectively.

The dose in out-of-field region for small field size was
investigated in our study for unflattened beam and compared
with that of the flattened beam. **Figure 9** shows the calculated
flattened and unflattened beam profiles for a small field size (5
× 5 cm^{2}) at a depth of 5 cm. The dose at 4 cm off-axis distance
is lower in unflattened beams by 15% and it tends to decrease
faster with increasing off axis distance than in flattened beams.

Our results are in consistence with the results reported by Titt et.al [10]. Faster lateral dose fall-off outside the treatment field will result in lower doses to surround normal tissues.

## Discussion

A large portion of primary photons especially those close to the
central axis of the beam are removed by the flattening filter
thus removing the filter from the beam line should result in
substantial increase in the dose rate and therefore a decrease in
beam-on time should be achieved when radiation treatment is
delivered. In order to verify this effect absolute absorbed dose
per initial electron were calculated for flattened and unflattened
beam at two different depths for different field sizes. The ratios
of absolute depth doses for flattening filter free to standard
flattened beams calculated for field size 10 × 10 cm^{2}, at 10 cm
depth for an Source to Surface Distance (SSD) equal to 100 cm
was found to be 2.4 indicating the possible higher dose rate
deliver by the unflattened beam. Percentage Depth Doses
(PDDs) calculated for unflattened beam is found to be slightly
lower than standard beam for all field sizes. Difference in the
Percentage Depth Doses (PDDs) of flattened and unflattened
beams are evident at deeper depths and are increased with
increase in depth for all the field sizes. The photon spectra as a
function of energy and average energy distribution as a
function of off axis distance for flattened and unflattened
beams are being calculated in our study. It was observed that
the fluence of photon on central axis averaged over the total
surface of the top of water phantom increased 1.84 times with
removing flattening filter but the energy spectrum became
softer and the average energy of photon energy spectrum on
central axis was decreased from 1.52 to 1.23 MeV by removing
flattening filter at the top of water phantom for 20 × 20 cm^{2} field size at 100 cm Source to Surface Distance (SSD). It is due
to the differential attenuation of flattening filter with distance
from central axis of beam. The thick central part of the
flattening filter attenuates more low energy photons, but as the
off axis distance increases more low energy photons are
allowed to penetrate the thin lateral part of the flattening filter
and they contribute to the photon energy spectrum, thus the
mean energy of spectra is decreased. For the flattening filter
free beam, the mean energy of spectra was not changed significantly with increasing off axis distance and it was
respectively decreased from 1.23 MeV on central axis to 1.19
MeV at 20 cm off axis distance for 20 × 20 cm^{2} field size.
Surface dose for the unflattened beam is found to be higher
than that of the flattened beam for all the field sizes. The
average energy difference on the central axis is considered to
be the major reason for the superficial dose difference between
the two kinds of beams. Lower average energy of the
unflattened beam on central axis produce higher superficial
dose. It can be seen that the surface dose for flattened beam
varies more with the field size in compare to the unflattened
beam. The major reason for the field size dependence of the
superficial dose of flattened beam is the scatter component
which originates mostly from the flattening filter. The total
Scatter Correction Factor (SCP), for the unflattened and
flattened beam has been investigated in our study. It can be
seen from the data that the flattening filter free beam Scatter
Correction Factor (SCP) increases more slowly with increasing
field size than that of the flattened beam. This is due to the
forward-peaked profile of unflattened beam, which produces
less Scatter Correction Factor (SCP) because of the reduced
off-axis intensity. The flattening filter free beam has greatly
reduced fluence off axis, hence, less secondary head scatter is
created, which is directed in toward the central axis. Due to
this reason as the measured field size increases, the expected
increase in Scatter Correction Factor (SCP) for flattening filter
free beams is not seen which is found with the flattened ones.
It is observed in our study that flattened and unflattened beams
are similar within a few centimetres from the central axis. Thus
Beam non flatness is unlikely to present a problem for
treatments with small fields and the treatments can also benefit
from an increased dose rate however for lager field sizes there
is a significant difference in beam flatness for the two
cases .The beam profiles for unflattened case are found to have
lower relative dose value than the flattened beam near the field
edge. The main reason for this behaviour is that the flattening
filter elevates relative fluence of primary photons propagating
off-axis and reduced head scatter present in unflattened beams.
The out-of-field dose calculated without the flattening filter is
found to be smaller outside the field edge for small field sizes
when compared to the out-of-field dose calculated with
flattening filter. It can be seen that the out-of-field dose from
the flattening filter free beam is substantially lower, and it falls
off faster with distance. This means that a significant reduction
in the out-of-field dose and enhanced sparing of normal tissues
and organs close to small treatment fields can be achieved.

## Conclusion

A Monte Carlo simulation model was developed for a 6 MV photon beam based on Varian Clinic 600 unique performance linac operated with and without a flattening filter. The basic dosimetric features of both flattened and unflattened beams were calculated and compared. Our study shows that removing flattening filter increases the photon fluence and consequently the dose rate considerably. Absolute depth dose calculations showed an increase in dose rate with a factor of more than 2.4 for the unflattened 6 MV beam which is dependent on the depth. These ratios give an estimate of the amount of primary radiation removed from beam due to the interaction with flattening filter. Percentage Depth Doses (PDDs) values were found to be lower for unflattened beam for all field sizes. Surface doses were higher for the unflattened beam due to more contamination electrons and low energy photons in the beam. The total Scatter Correction Factor (SCP), less variation with field sizes indicate that removing the filter from the beam line can reduce significantly the amount of head scatter photons and therefore doses to normal tissues and organs.

## Acknowledgment

The authors of this article wish to thanks Varian medical systems for providing us with the specifications needed for linac simulations.

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