METHODS
The patient was simulated in Alderson Rando phantom and irradiated with three different techniques
in Varian DHX (Rapidarc) linear accelerator with 48 Gy in 24 fractions. Doses that occur in the breast
tissue were measured using thermoluminescent dosimeters and compared with each other and the data
obtained from TPS.
RESULTS
The dose values in the right and left breasts were found to be statistically similar to each other (p>0.05),
whereas significant differences were detected between different techniques. The mean calculated breast
doses were 7.16±1.61 cGy in 3D-CRT, 27,75±3,88 cGy in IMRT and 12,20±2,65 cGy in VMAT, respectively.
CONCLUSION
The breast tissue doses are significantly lower in 3D-CRT and VMAT compared with IMRT. This finding
should be considered while choosing a treatment technique, especially in young patients with cervical
cancer.
Keywords: Breast dose; cervical cancer radiotherapy; out-of-field dose; peripheral dose
Radiotherapy is the main component of cervical
cancer treatment in both primary and adjuvant setting.
The dose to the radiation-sensitive breast tissue located outside the treatment field is important, particularly in
younger cervical cancer patients who will receive radiotherapy.
Healthy cells in the surrounding tissue can
be damaged while tumor cells are irradiated during
radiotherapy. The dose-dependent biological effects of
radiation may arise as early or late side effects. To decrease
these side effects, the maximum level of protection
for normal tissues should be provided.[
Despite achieving higher conformality with modern
techniques, unwanted doses may occur in the field
outside the region to be irradiated. These doses are
called out-of-field doses (peripheral doses, PD). Due
to the stochastic effects of ionizing radiation and the
risk of secondary cancers, PD should be determined
by measuring them. They emerge due to the treatment
head of medical linear accelerator, the collimator device,
the scattering from the patient"s body and the
leakage.[
The potential risk of secondary cancers due to
low doses of ionizing radiation is the main problem.
The advanced treatment techniques, such as threedimensional
conformal radiotherapy (3D-CRT) and
intensity-modulated radiation therapy (IMRT), tend
to increase radiation dose at organs out of the radiation
treatment field. In addition, the risk of secondary
cancer has become important since the average life expectancy
of cancer patients has increased with developments
in treatment techniques.
The very low doses received by organs distant to tumor
volume during radiotherapy are known to cause
secondary cancers. These doses cannot be calculated
accurately by TPS.[
In this study, the PD in radiation-sensitive breast
tissue measured during irradiation of human-like
Alderson Rando phantom for virtually created postoperative
stage IB-II cervical cancer with 3D-CRT, IMRT
and VMAT treatment technique and dose values calculated
are compared.
Standard deviation (SD) of the grouped TLD GR- 200A rods was less than ±1.5 was used in our study. Twelve TLD in the right breast and Twelve TLD in left breasts were used. Also, 4 TLDs were stored as a reserve. Two of 12 TLDs were selected for calibration in both breasts.
The response of the TLDs to radiation was determined before the measurements. TLDs were reset by annealing in the TLD oven. Firstly, TLD GR-200A round rods were passed through a baking process in the TLD oven at 220 °C for 15 minutes. Then, 80 TLD GR-200A rods were irradiated with a source-skin distance (SSD) of 80 cm, in a 20x20 cm2 area at a depth of 5 cm to be exposed to 100 cGy using specially designed PTW brand RW3 solid water phantom in Cirus Co-60 machine. To create a backscattering effect, a phantom of 10 cm thickness was placed under the rods. For calibration, TLDs were irradiated for 100 cGy dose at a depth of 10 cm on the Rapidarc linear accelerator using 15 MV energy X-ray, which was used for three techniques during the planning phase. The absorbed dose values of each TLD were found by calculating the ratio between the values read during the measurements and the average of the responses of these calibration TLDs.
The Characteristics of Phantom
The phantom used in this study had no limbs and represented
a 155 cm in length woman with breast tissue
(Fig.
The Characteristics of Radiotherapy Plans
The images of Alderson Rando phantom with 3 mm
sections were taken in computed tomography and
transferred to the Eclipse TPS. The right and left
femoral heads, rectum, bladder, bowel volumes were
delineated as critical organs. PTV was created with 1
cm of safety margin to CTV.
In the study, treatment plans simulating operated stage IB-II cervical cancer were prepared. The 3D-CRT, IMRT, and VMAT treatment plans were created using the AAA algorithm in Eclipse TPS. The center of PTV volume was chosen as the isocenter in all plans. Doses of virtual plans for all techniques were defined as 200 cGy daily with a total dose of 4800 cGy.
3D-CRT plans were created with four different beams with gantry angle 0º, 90º, 180º, 270º using 15- MV energy X-ray (box technique). To obtain a homogeneous dose distribution, 10º virtual wedges were used in beams with 90º and 270º gantry angle.
Seven different beams with gantry angle 0º, 52º, 104º, 156º, 208º, 260º, 312º using 15-MV energy X-ray were used to create IMRT plans. The collimator angle was set at 0o in each area. Sliding Window (SW) was selected as the irradiation technique in all of the IMRT fields. The optimization process was performed by defining dose values for PTV and the tolerance doses for critical organs, and the IMRT plan was created with dose calculation.
A double-arc plan was created using a 15-MV energy X-ray for VMAT plan. Two full arcs by selecting the angle of 179.9° -180.1° and the collimator angle of 30° in the first arc and the gantry angle of 180.1° -179.9° and collimator angle of 330° in the second arc beam were used. The dose rate that was selected to be a maximum of 600 monitor unit (MU)/min. varied dynamically according to the data entered optimization for during irradiation. VMAT plan was created by defining the desired dose values for PTV and the tolerance doses for critical organs.
Conformity index (CI) is defined as a ratio between planning target volume and treated volume (TV) at International Commission on Radiation Units and Measurements (ICRU) Report 62, CI=PTV/TV (equ 1). CI is ideal if this ratio is 1. Homogeneity index (HI) was calculated according to the Formula at ICRU Report 83: HI=(D%2-D%98)/D%50 (equ 2) where D%2=The dose reached in 2% of the PTV volume, D%98=The dose reached in 98% of the PTV volume and D%50=The dose reached in 50% of the PTV volume. The dose distribution in PTV is highly homogeneous if HI approaches 0. The CI and HI values for PTV-cervix and the organ at risk doses in plans with 3D-CRT, IMRT and VMAT techniques are compared.
The PD values measured and obtained from TPS were analyzed and compared with the Wilcoxon sign rank test using SPSS statistical software. Binary comparison of the out of field dose values in different treatment techniques were carried out as 3D-CRT vs. IMRT, 3D-CRT vs. VMAT, IMRT vs. VMAT using the Wilcoxon signed-rank test.
Each of the plans was irradiated three times. The
accumulated dose in the TLD rods of which arrangements
on the breast tissue were indicated in Table
The dose values in TLD rods placed in the upper,
lower, right, left and the center point of the two sections
of the right and left breast were measured and shown in
Table
The average breast doses were found as 7.16±1.61
cGy in 3D-CRT, 27.75±3.88 cGy in IMRT, and
12.20±2.65 cGy in VMAT techniques, respectively
(for 3D-CRT vs. IMRT, 3D-CRT vs. VMAT, IMRT vs.
VMAT, p<0.01). On the other hand, breast dose values
in TPS were zero, which indicated that the doses
measured by TLDs could not be calculated by the TPS
algorithm. In addition, MU values were 676, 1084 and
2228 MU for 3D-CRT, VMAT, and IMRT, respectively.
Taking into account the measured values out of field
breast doses and MU values in three techniques, it can
be clearly seen that breast doses increase in parallel
with MU values (Table
Since the treatment dose was prescribed as 4800 cGy in our planning, breast tissue received approximately 0.15% of the treatment dose in the 3D-CRT technique. The findings showed that the breast tissue doses after irradiation were approximately 0.58% and 0.25% of the treatment dose in IMRT and VMAT techniques, respectively.
When three techniques were compared, the findings indicated that the IMRT method caused nearly four times more doses in breast tissue compared with the 3D-CRT method during lower abdominal irradiation of patient with cervical cancer. In addition, the IMRT technique caused twice more doses in breast tissue compared with VMAT technique, whereas the VMAT technique led to 1.5 times more peripheral doses than the 3D-CRT technique.
To our knowledge, there is no one to one overlapping
study with our study. However, several studies
have been conducted on PD. Jia et al. investigated PD
in irradiation for cervical cancer using X-rays with
6-MV energy (Elekta Synergy Linear Accelerator) with
VMAT and step-and-shoot IMRT techniques. They
measured the doses of the thyroid, breast tissue and
lens using the ionization chamber while irradiating a
human-like phantom formed by combining solid water
phantom for a therapeutic dose of 5000 cGy. The total
breast doses were found as 12.8 and 22.6 cGy in VMAT
and IMRT techniques, respectively.[
Mansour et al. compared PD values between 3DCRT
and IMRT techniques (Varian Ex Linear Accelerator)
in pediatric radiotherapy. They observed lower PD
values in the area near the target volume for the IMRT
technique, which was explained by the potentially decreased
amount of internal scattering due to irradiation
with smaller domains in the sliding-window IMRT technique.
In contrast, IMRT caused higher PD values in the
areas distant to the treatment field compared with the
3D-CRT technique of which cause was suggested to be
the higher MU values as and the leaks from linear accelerator
treatment head. This is consistent with the results
of our study since measured breast doses were found to
be increased in parallel with MU values in three techniques.
For both techniques, dose in nearby areas had a
much smaller value compared with those in remote areas
and the total of PD were close to each other.[
Cemile et al. prepared four treatment plans using
6 MV X-ray energy, anteroposterior-posteroanterior
(AP-PA), noncoplanar 3D-CRT, IMRT and noncoplanar
IMRT treatment techniques for bilateral lung irradiation.
They placed TLDs in RANDO phantom at five
separate points outside the treatment area (brain, thyroid,
left kidney, right kidney and umbilicus) and measured
doses on the Siemens AvantGadre linear accelerator
device. As a result of the measurements, they found
that the dose amount was higher in the thyroid organ
which was closer to the treatment area and the dose
amount was lower in the umbilicus which was farther
away from the treatment area for each technique.[
Cho et al. identified the risk of secondary cancer
in the out-of-field regions for the neck, thorax and
prostate regions 3D-CRT applications (Siemens Onkortm
Linear Accelerator). They found the dose of breast
tissue as 0.7 and 0.9 cGy for a daily dose of 200 cGy
using the two energies, 6 and 10 MV X-ray energy, for
prostate plan measurements, respectively.[
Martín et al. investigated peripheral uterus dose in
breast radiotherapy with plans consisting of two opposing
tangential field with 6-MV X-ray energy with
and without wedges (Saturne 40 Linear Accelerator).
When a treatment dose of 5000 cGy was applied, the
doses of the uterus were measured as 4 and 4.2 cGy
with and without wedges, respectively. In addition, the
insufficiency of TPS in calculating PD was discussed
in their study.[
The use of the protective set to protect the organs
from the radiation scattered from collimator may result
in up to twice reduced risk of secondary cancers
development.[
Kase et al. measured the PD for open fields of different
sizes and defined PD as the exponential function
of distance for all the energies and field sizes (using G.
E. Maxitron for 300 kVp, Varian Clinac 4 for 4 MV X
rays, Siemens Mevatron XII for 8 MV X rays, Picker
C-3000/75 for Co60 Gamma Rays). They found that
PD depended on the collimators, field size and distance
to the treatment field and the contribution of linear accelerator
collimator to the PD dose had a value between
20% and 40%. Although the contribution of radiation
leaks to PD was less than other components, they determined
that leaks became the dominant component
at 60 cm and over distances from the central axis.[
Yousif et al. found the PD in distant organs by applying
current calculation methods in cervical cancer patients
irradiated with the AP-PA field in the Cobalt-60
device. For treatment doses in the range of 8400±600.1
cGy to patients, they found average dose values of
62±9.6 cGy, 56.9±7.5 cGy, 93±12.3 and 74.8±11.6 cGy
in the liver, spleen, right and left kidneys, respectively.
[
The measurement of PD is important in the assessment
of secondary cancer risk potential. Kinhikar
et al. compared the PD in the Varian Trilogy and HIart
II tomotherapy (TT) devices with measurements
using diode and TLDs in patients irradiated for head
and neck cancer (HNC) with IMRT method. They detected
that the TT device constituted 1.2-1.5 times less
PD compared with the Trilogy device.[
As a result, our findings suggest that VMAT or 3DCRT techniques are superior to the IMRT technique concerning lower breast PD, especially in younger patients with cervical cancer.
Peer-review: Externally peer-reviewed.
Conflict of Interest: The authors declare that they have no conflict of interest.
Ethics Committee Approval: This study was conducted in accordance with local ethical rules.
Financial Support: This work has no funding source.
Authorship contributions: Concept - Y.S., U.A., H.B.B.; Design - Y.S., M.T.; Supervision - Y.S., H.B.B.; Funding - None; Materials - Y.S.; Data collection and/or processing - Y.S.; Data analysis and/or interpretation - Y.S., H.B.B., C.K.; Literature search - Y.S., M.T.; Writing - Y.S., M.T.; Critical review - Y.S., H.B.B.
