Introduction
For patients with mastectomy, postoperative radiotherapy
is the preferred treatment modality to improve
local control and survival.[,] Postmastectomy radiotherapy
(PMRT) is usually performed with 3D conformal radiotherapy (3D-CRT) or intensity-modulated
radiotherapy (IMRT) techniques using high-energy
photons of 6 MV.[,] In the PMRT, the chest wall and,
if necessary, the lymph nodes form the target volume.
Generally, the boost treatment is not applied to the
PMRT.[] A total dose of 45?50 Gy is given to the chest wall as a dose of 1.8-2 Gy per day-fraction.[] When
considering the possibility of cancer cells remaining in
the skin, the skin tissue should be also included to the
target volume in the breast-conserving surgery radiotherapy
or PMRT applications.[]
High-energy photons have skin-sparing-effect properties
because they transmit their maximum energy to
more depth than low-energy photons do. In this case, to
treat the superficial lesions, a bolus is required to increase
the maximum dose toward the surface.[,] Bolus is a
tissue equivalent material that helps to smooth out the
surface of the skin by filling in various tissue deficiencies
in patients.[,] Although the bolus is used to increase
the surface dose, the treatment-planning computers
cannot correctly calculate the surface doses. Chung H.
et al. observed a reduction in treatment planning surface
dose from 18.8% to 7.4% compared to that in the film
surface dose.[] And, the amount of CTV shrinkage into
the tissue is controversial when tumor volume definition
is performed in the treatment planning systems.[,]
In this study, we evaluated the skin dose variation
on a breast phantom using the bolus IMRT technique
when the amount of CTV shrinkage into the tissue is 3
mm and 5 mm.
Methods
Target Volume Definitions and Treatment Planning
For treatment planning of the phantom, Alderson Rando
phantom (Supertech, USA) was placed on the computed
tomography (CT) table at head-gantry and supine
position. In phantom treatment plan applications,
the projections of the lasers were marked on phantom
surface, and the lead markers were placed in order on
the phantom surface to make it easier to adjust the isocenter.
Alderson Rando phantom CT scan was scanned
for 1-cm real bolus and no-bolus (virtual bolus) with
4-mm phantom slice thickness.
In the first part of the measurements, to determine
the effect of 100% real bolus and 100% virtual bolus
on the surface dose, bolus in all treatments plans were
created as virtual and as real in treatment planning
systems. Doses of 50 Gy were defined to CTV in 25
fractions. Treatment planning system was Eclipse (8.9,
Varian Medical Systems, Palo Alto, CA, USA).
In the second part of the measurements, to determine
the shrinkage effects on the surface dose for virtual
bolus, CTV was separately created two times for
3-mm and 5-mm shrinkage. Three different plans were
performed with no-bolus, 50% bolus, and 100% bolus.
Doses of 50 Gy were defined to CTV in 25 fractions.
The energy photons of 6 MV have a dmax of 1.5
cm. In this study, the bolus thickness of 1 cm has been
preferred because 1.5 cm thick bolus was not flexible
enough in the clinical applications. The IMRT treatment
fields were placed at 30° intervals. Total seven
fields were used, and the outermost and innermost
fields were tangential.
Dose Measurements:
For the dose measurements, FIMEL (French) brand
GR200 (3.2 mm diameter, 0.3 mm thick) TLDs placed
on the Alderson Rando phantom surface were used.
According to the manufacturer, the dose range of the
dosimeters was between 0.5 ?Gy and 12 Gy. The TLDs
were read with a Fimel LTM manual TLD reader. They
were calibrated using a 6-MV beam from a Varian Trilogy
linear accelerator (Varian Medical Systems, Palo
Alto, CA, USA) and RW3 solid water phantom (PTW,
Freiburg, Germany). The TLDs are grouped so that the
difference in reading values will be a maximum of 1%.
During the irradiation, phantom was placed on the
LINAC treatment table while paying attention to the
control lines marked on the CT. With the help of table
scrolling data from TPS, phantom was positioned to
the treatment condition. The CT axial slices were used
to determine the TLD positions on the phantom surface.
The TLD chips were placed on these dotted places
(Fig. 1). During the irradiations, a real bolus material,
which was created in the TPS planning as virtual (1-cm
thickness), was placed on the phantom. Each plan was
irradiated three times, and the average readings were
used for comparison. The mean TLD readings were
compared with dose values at the same point on the
treatment planning computer (Fig. 2).
Fig 1: TLD placement on Rando phantom.
Fig 2: Positions of TLDs in TPS.
Results
There are two different ways of making a bolus during
the treatment planning phase: real bolus and virtual
bolus. For treatment planning with the real bolus, a real
bolus is placed on the chest wall during the patient's CT
scan. On the other hand, for treatment planning with
a virtual bolus, a virtual bolus is drawn on the skin in TPS. In this study, the effect of both conditions on TPS
skin dose was investigated with a margin of 3-mm and
5-mm CTV shrinkage margins.
In the first part of the study, only 100% virtual and
100% real bolus treatment situations were examined.
Table 1 shows the effect of 100% real bolus and 100%
virtual bolus on the surface dose between the TPS values
and TLD validation measurements. As shown in
Table 1, there is no significant difference between the
real and virtual situation. It was determined that there
is 1% difference between the TPS values and TLD validation
measurements for 3-mm shrinkage, and there is
0.5% difference between the TPS values and TLD validation
measurements for 5-mm shrinkage.
Table 1: shows the effect of 100
In the second part of the study, the no-bolus (0%
virtual bolus and 0% real bolus) situation, 50% virtual
bolus, and 100% virtual bolus cases were examined.
The values obtained from the measurements are given
in Table 2. Table 2 shows the mean values and differences
between the TLD validation measurements and
TPS calculations on CTV with 3-mm shrinkage. The
differences between the TPS and TLD validation readings
are 20.3%, 18.0%, and 12.6% for no-bolus, 50%
bolus, and 100% bolus, respectively.
Table 2: TPS surface dose comparisons for no-bolus, 50% virtual bolus, and 100% virtual bolus plans using the TLD validation
As similar, for CTV with 5-mm shrinkage, Table
2 shows the mean values and differences between the
same points in the TLD and TPS. The differences between
the TPS and TLD validation measurements are
5.4%, 2.6%, and 2.9% for no-bolus, 50% bolus, and
100% bolus, respectively. The minimum difference between
the TPS and TLD average readings for 5-mm
shrinkage was 2.6% in 50% bolus plan.
Discussion
Yokoyama S. et al. performed several phantom measurements
with 6 MV-energized photons using con-formal open field and IMRT fields.[] They observed
that the surface dose in the IMRT fields was 10% lower
than in the conformal open fields. On the other hand,
in dosimetric measurements for surface doses of 6-10
MV photons, Laurence E. Court et al. found a reduction
of up to 20% in the Eclipse treatment planning
doses compared to the values measured for the IMRT
plans.[] These studies show that the dose received by
the skin in the IMRT plans is lower than the dose prescribed
for the target. Therefore, a convenient option
is to use a bolus material to achieve an enough dose in
the skin.[]
100% bolus treatment is not preferred because the
skin recieves too much radiation. Because the skin receives
too much radiation, 100% bolus treatment is not
preferred. Therefore, a part of the treatment is irradiated
using a bolus. In an international study conducted
by T.T.T. Vu et al. in 2007, preferences of world clinics
regarding the use of bolus material in the PMRT
treatments have been shown.[] A total of 87.5% of
the PMRT treatments are performed with high-energy
photons, 9% with electrons, 1% with Co-60 source,
and the remaining 2.5% with photon-electron combination.
A total of 68% of the clinics stated that bolus
material was used in all PMRT treatments, 6% of clinics
stated that bolus material was not used, and 26%
of clinics stated that they were behaving according to
the situation. A total of 33% of the clinics who prefer a
bolus material use a bolus in each fraction (100% bolus),
and 48% of clinics prefer one bolus in two fractions
(50% bolus).[] In our clinic, we also perform a
PMRT radiotherapy with 50% bolus.
An-Cheng S. et al.[] reported that the accuracy
of the surface dose depends on the TPS calculation in
the head and neck IMRT plans. They demonstrated
that the difference between the measurement and the
calculation is more than 10% in case of 5-mm CTV
shrinkage margins. As the depth increases, the accuracy
of the calculation improves. For example, the value is between 2.5% and 5.5% for 7-mm CTV shrinkage
margin. They also emphasized that when the tumor
invaded to the superficial region as breast cancer, the
bolus was the best way to deliver a sufficient dose.
Conclusion
We recommend that 5-mm shrinkage with 50% bolus
(1-cm thickness) should be used for the better TPS
surface dose calculation because the accuracy of TPS
calculations increases with the decrease in differences
between the TPS and TLD validation readings. Because
there is no significant difference between the TPS
calculation and the TLD validation measurements for
real and virtual bolus, virtual bolus can be used instead
of real bolus.
Peer-review: Externally peer-reviewed.
Conflict of Interest: The authors declare that there is no
conflict of interest.
Financial Support: None.
Authorship contributions: Concept - M.O.; Design -
M.O., N.D.; Supervision - M.O.; Materials - M.O., N.D.;
Data collection &/or processing - N.D., M.O.; Analysis and/
or interpretation - M.O., N.D.; Literature search - N.D.;
Writing - M.O.; Critical review ? M.O.
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