INTRODUCTION
Increasing of death due to prostate cancer shows the growth of this disease,
nowadays prostate cancer is the second wide spread cancer and also is the second
most common cause of cancer mortality among men (Parkin et
al., 2005; CDC, 2001). It is estimated that more
than 300,000 new instances of prostate cancer are discovered in the United States,
that 41,000 of them are certainly fatal (ACS, 2007).
Modern technology and new treatment techniques play important role in effective
external radiotherapy of prostate cancer which leading to lower exposure to
normal tissues and consequently decreasing the side effects of radiation therapy
that improves the quality of life of patients (Vijayakumar
et al., 1993; Dearnaley et al., 1999).
Today, the conformal radiation therapy methods with 3-dimensional treatment
planning are used for carcinoma of prostate in the most modern/developed radiotherapy
centers, while there are too many centers that because of some problems such
as cost use the conventional method with 2-dimensional treatment planning.
Therefore in this study, we measured the received dose by critical uninvolved
organs such as bladder and rectum in two methods using Co-60 unit via anthropomorphic
phantom.
MATERIALS AND METHODS
Phantom: The phantom was consisting of three segments: Head and neck,
trunk and hip (Fig. 1). In this study we used only its hip
region. The phantom is constructed with natural human skeleton and its geometrical
sizes are nearly according to standard human (ICRP., 1975).
(Its total high is 95 cm and the trunk’s thickness is 22 cm). Its internal
organs are made of mixture of wax and salt (sodium chloride), that their average
effective atomic number (Zeff = 6.57) and electron density (3.36x1023
electron/cm3) were close to soft tissue values (Khan,
2003). The amount of percent combination of wax and salt varied (density
range was 0.9-0.97 g cm-3) to making different densities corresponding
to different tissues. To mimicking lung tissue, The porous wood with density
of 0.3 g cm-3 was used. Considering to these qualifications, we can
declare that the phantom is tissue-equivalent and anthropomorphic and could
utilize in radiotherapy applications.
Thermoluminescent dosimetry: Natural LiF TLD-100 (LiF:Mg,Ti) chips with
closely matched sensitivities were used. These TLDs have the dimension of 3.1x3.1x0.9
mm3 (~28 mg), the effective number of 8.14, the main peak in the
glow curve of 195°C and the fading rate of 5% per year at 20°C. The
TLD chips were calibrated with Co-60 gamma rays and the prerequisite corrections
for increasing the precision of dosimetry were done and the values of absorbed
doses were calculated with reading the chips on TLD-reader system (3500 manual
model, Harshaw, USA) (Kron, 1994, 1995;
Wood and Mayles, 1995; McKinlay,
1981).
A cylindrical hole with its central axis in anterioposterior direction crossing
the bladder, center of prostate gland and rectum was established for placing
the chips in hip (Fig. 2).
Treatment planning: The 2-D and 3-D treatment planning was done with
ALFARD and Core-PLAN softwares, respectively, on the tissue-equivalent and anthropomorphic
phantom, in the way of, CT-scan images data of region of hip from phantom downloaded
in treatment planning computer.
In treatment planning, the volume of prostate gland was selected as a Planning
Target Volume (PTV) with 0.8 cm margin, also equally weighted 4-field (Box method)
(Khan, 1998; Perez et al.,
1997), that consist of two pairs of parallel-opposed AP (8x8 cm2)
and right and left (8x6 cm2) fields, to deliver 200 cGy isocenter
point with SAD setup was established.
|
| Fig. 1(a-c): |
Constructed phantom. We used only its hip region (a) Hip,
(b) Trunk and (c) Head region |
|
| Fig. 2(a-c): |
Radiographs of hip region of phantom,
(a) Before and (b) After creation the cylindrical emplacement for placing
the TLD chips within (c) Cylindrical container |
|
| Fig. 3(a-b): |
(a) Cerrobend blocks for shaping each
radiation field were constructed and used in (b) 3-D treatment planning
method |
Cerrobend blocks for shaping each radiation field were constructed and used
in 3-D treatment planning method (Fig. 3). The experiments
were repeated five times and the values of absorbed dose compared with paired
student t-test for 95% confidence interval of the difference.
RESULTS
For measuring absorbed dose in hip of phantom, TLD chips were placed in triple
groups in the different depths from skin (at anterior surface) to 21.5 cm with
2 cm interval distance. The absorbed dose at these depths were obtained in two
methods by averaging of resultant data after 5 times repetitions which each
repetition point is the average of three measurements itself (Fig.
4).
To have the soft tissue’s anatomical
data in phantom, we fused the CT-image of phantom with the CT-image of a patient,
who had the similarity in dimensions with phantom.
The depth of bladder, prostate gland (tumor) and rectum centers in coronal
plane from anterior surface were determined 8.5, 12.5, 16.5 cm, respectively.
The amount of received dose by bladder was obtained by averaging absorbed doses
at depths of 7.5 and 9.5 cm, for prostate (tumor) averaging absorbed doses at
11.5 and 13.5 cm (depth) and for rectum, by averaging at depths of 15.5 cm (anterior
rectal wall) and 17.5 cm (posterior rectal wall) similarity.
|
| Fig. 4: |
Comparison of absorbed dose in terms of depth between two
treatment planning methods with the same exposure conditions |
|
| Fig. 5: |
Comparison of absorbed dose of bladder
and rectum between 2-D conventional and 3-D conformal treatment planning
methods |
Absorbed dose values in critical organs bladder and rectum, in two planning
methods are compared together in Fig. 5.
As shown in Fig. 5 the use of 3-D conformal treatment planning
causes significant reduction in absorbed dose to uninvolved structures such
as bladder and rectum, compare to 2D conventional treatment planning (p<0.01).
To quantify the non-uniformity, we calculate the Uniformity Index (UI) was
calculated in irradiated volume (approximate range: 9.5-15.5 cm in depth) in
two planning techniques with the following equation:
where, DMax and DMin are the maximum and minimum absorbed
dose in irradiated volume, respectively (Khan, 1998).
| Table 1: |
Dosimetric data derived from treatment planning softwares
at critical uninvolved structures (bladder and rectum) in two planning methods |
 |
Based on the Eq. 1, the UI in irradiated volume for 2-D conventional
and 3-D conformal treatment plans were 10.9 and 22.4%, respectively. This means
the 2-D method concedes nearly twice more uniform distribution dose to irradiated
volume than 3-D conformal method.
We also compared the dosimetric data (minimum dose (DMin), maximum
dose (DMax) and mean dose (DMean)) derived from treatment
planning softwares based on dose-volume histograms at critical uninvolved structures
bladder and rectum between two planning methods (Table 1).
It is observed that absorbed dose values in bladder and rectum with 3-D conformal
method are significantly lesser than 2-D conventional method. The reduction
in absorbed dose from 2-D to 3-D planning is more noticeable in rectum (33.5%)
than bladder (23.9%) on the basis of mean dose (DMean).
DISCUSSION
Cerrobend blocks with 3-D conformal treatment planning, significantly reduce
the magnitude but no uniformity of absorbed doses to critical uninvolved structures
in proportion to 2-D conventional radiotherapy of prostate cancer with using
Co-60 unit.
As we know, the use of multiple fields in radiotherapy increases the tumor
dose relative to the dose to surrounding normal tissues and also cause to acceptable
uniform distribution absorbed dose to PTV (Khan, 2003;
Hendee et al., 2005). In this study, the 4-field
or Box method, other than uniformity, was selected because of the lymph nodes
to place in radiation fields (Perez et al., 1997).
As expected from isodose distributions, the absorbed dose in therapeutic volume
has a little non-uniformity in two methods involve 2-D conventional (10.9%)
and the 3-D conformal techniques (22.4%). The uniformity of conformal method
was improper than conventional one because of asymmetry of radiation fields.
The reduction in absorbed dose results from reduction in field size, since
one of effective factors in absorbed dose is the radiation field size (Khan,
2003). For the sufficient large fields, such as those, that are routinely
used in radiotherapy, the absorbed dose at deep layers are result from primary
and secondary photons. However, the relative contribution of scattered radiation
to the absorbed dose increases more rapidly at the depth than at locations or
near the surface because the photons tend to be scattered in the forward direction
(Hendee et al., 2005). Since scattered radiation
depend on irradiative area (Khan, 2003; Hendee
et al., 2005), blocking the fields for shaping beam, results in diminution
in effective or total field size, thus scattered rays decrease and consequently
the absorbed dose is reduced. Another reason, is that, for any x or gamma ray
beam of specific cross-sectional area, the percent depth dose decreases with
decreasing symmetry of the field shape. Although, the volume of irradiated medium
may remain constant, fewer scattered photons reach the central axis of an asymmetric
beam because the average distance is greater the origin of the scattered photons
and the central axis (Hendee et al., 2005).
It seems, increasing the absorbed dose due to left and right lateral fields
with more weightening, will be logical, to compensate the absorbed dose in PTV
up to prescribed dose and even more, but the received dose by anterior rectal
wall must be considered (Vijayakumar et al., 1993;
Lee et al., 1996). In these fields, more volume
of critical organs (bladder and rectum) that were under exposure previously
is removed from radiation fields using blocks and thus the most reduction in
absorbed dose and also in integral dose of these structures are from blocking
the lateral fields.
According to retrospective works, this increas in dose at PTV, without significant
increase at critical organs, cause more control on treatment prostate cancer
and also improves quality of life patients (Dearnaley
et al., 1999; Zelefsky et al., 1998;
Sale et al., 2005).
We conclude that using Cerrobend blocks with 3-D conformal treatment planning
significantly reduces the absorbed dose to bladder and rectum in proportion
to 2-D conventional radiotherapy of prostate cancer.
ACKNOWLEDGMENT
The authors wish to thank Dr. Hadi Hassanzadeh for making the anthropomorphic
phantom in Iran University of medical sciences. The authors also would like
to acknowledge the cooperation of the all radiotherapy center heads in Asia
and Pars Hospitals and their Medical Physicists, Dr. Ahmad Mostaar and Mr. Mohamadreza
Ali-Naghizadeh and Mrs. Montaseri.
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