Comparison of Conventional Radiotherapy Techniques with Different Energies in Treating Prostate Cancer, Employing a Designed Pelvis Phantom
The aim of this study is to determine and compare the dosimetric consequences
of prostate and normal structures (rectum, bladder and right femoral heads)
in pelvis region using different conventional radiotherapy techniques
4-field (box), 3-field with one anterior and two oblique 115 and 245 °
fields and anterior-posterior technique) with two different energies of
9 and 18 MV. In this study two high-energy linear accelerators (Neptun
10 and Saturn 20) located in Seyed-Alshohada hospital in Isfahan were
used. An anthropomorphic pelvic phantom was designed and fabricated for
dosimetry applications based on the pelvic CT images of an adult patient
with an average size of prostate cancer patients referring to the medical
center. Measurements of the organ doses was performed in phantom using
TLD (TLD-100) dosimeters, which was suited at different depth especially
in prostate, rectum, bladder and femur head. After drawing the fields
on the phantom, the photon beam at a dose of 200 cGy with various levels
of photon energy (9 and 18 MV) were used. One way ANOVA test was used
to data analysis. The measured percentage depth dose (DD%) in 4-field
technique using photon 9 MV to the prostate, rectum, bladder and right
femoral heads were 94.8, 85.71, 77.51 and 65.81%, respectively and using
18 MV photon beam they were 95.81, 86.73, 77.5 and 63.45%, respectively.
The amount of DD%, in the 3-field technique with 9 MV photon, to the prostate,
rectum, bladder and right femoral heads was found to be 91.7, 78.83, 93.4
and 63.25%, respectively and 92.38, 79.05, 93.31 and 62.05% when 18 MV
photon beam were used. Using the 9 MV photon beam in AP-PA technique,
prostate, rectum, bladder and right femoral heads received 96.23, 96.77,
96.3 and 28.77% of prescribed doses, while with 18 MV photon radiation
they were 95.77, 96.91, 95.82 and 26.69%, respectively. Differences among
the techniques have been found for all of four considered organs with
total prescribed dose of 60 Gy and there was no significant difference
among all considered techniques. Technique 3-filed give the best sparing
of the rectum; the bladder is better spared with technique box and the
best technique for sparing the femoral head is AP-PA. Differences between
energies were low and using 18 MV photons give the more satisfied results.
Carcinoma of the prostate is the most frequent noncutaneous malignant
disease and is the third leading cause of cancer-related death in men
(Jemal et al., 2006). The established risk factors for the disease
include race, age and family history (Bostwick et al., 2004). The
prognosis for patients with prostate cancer is variable and depends on
the tumor-related characteristics at diagnosis.For patients with non-metastatic
prostate cancer, there are many treatment options, including observation,
surgery, external beam radiation therapy, brachytherapy or hormonal manipulation
with or without radiation therapy (Pilepich et al., 1997; Mettlin
et al., 1997; Khoo et al., 2000; Jani and Hellman, 2003;
Thomas and Pisansky, 2006). During the past four decades, External Beam
Radiation Therapy (ERT) has been a mainstay in the management of prostate
cancer and continues to be used in the treatment of almost one third of
all patients receiving definitive therapy (Mettlin et al., 1997;
Bedford et al., 1999; Milecki et al., 2004; Hille et
External-beam radiotherapy has several advantages over others (Kron et
al., 2002; Jani and Hellman, 2003; Hille et al., 2006; Harrison
et al., 2006; Schneider et al., 2007). It is non-invasive
treatment and has no surgical risks. It may be offered to patients for
reasons of age, general health, or specific coexisting conditions might
tolerate prostatectomy poorly. In addition, urinary incontinence is less
common after radiotherapy than after surgery.
The most important disadvantage is the risk of adverse effects caused
by the irradiation of normal organs, particularly the rectum. In addition,
treatment with radiotherapy does not include pathological confirmation
of disease stage; if spread beyond the prostate has occurred, it cannot
be detected directly (Thomas and Pisansky, 2006; Schneider et al.,
The rate of success in ERT is directly related to given dose to the tumor
but the late chronic side-effects limit the dose that can be given in
ERT. Today, with the use of modern radiotherapy techniques such as 3-dimensional
conformal radiation therapy (3D-CRT) and intensity-modulated radiation
therapy (IMRT), increasing the radiation dose to the tumor while minimizing
the normal tissue complication rate is possible. In Iran and many developing
countries the accesses to this new technologies is difficult and the use
of conventional techniques is common. Both radiation dose to the target
and organs at risk is important for the outcome of radiation therapy of
the prostate. The aim of this study is to compare the depth dose of 9
and 18 MV photon beams in the rectum, bladder, right femoral head and
prostate when the pelvis was irradiated in techniques most frequently
used for conventional radiotherapy of the prostate cancer 4-field (box),
3-field (with one anterior and two oblique 115 and 245° fields) and
MATERIALS AND METHODS
This study was conducted in Seyed Al-Shohada hospital of Isfahan, Iran
in 2007. Photon sources used in this study were a Saturn 20 (CGR Ltd.,
France) and Neptun 10 (Zdaj, Poland) Linear accelerators located in Seyed
Al-Shohada hospital of Isfahan, Iran. An anthropomorphic pelvic phantom
was designed and fabricated for dosimetry applications based on the pelvic
CT images of an adult patient with an average size of prostate cancer
patients referring to the medical center. The phantom of pelvic was constructed
using Perspex blocks (Perspex is usually easily accessible, hard enough
to perform the task, can be cut in different necessary thicknesses and
also it is nearly equivalent to the soft tissue). The material used for
bone phantom was Teflon, which has the properties of bone materials (Muren
et al., 2003).
To obtain the necessary sizes for phantom construction, preliminary measurements
of 10 patients referred to the Radiotherapy Department of Seyed Al-Shohada
hospital were performed. Contour sizes of the patients were obtained for
a mean patient size with 95% confidence limits. A photograph of phantom
was shown in Fig. 1.
To measure the mean organ dose and the percentage depth dose (DD%), Thermoluminescent
Dosimeter (TLD) was used. The lithium fluoride chips (LiF:Mg, Tl) is the
most commonly used thermoluminescent material for patient dosimetry. Thermoluminescent
dosimeters were first prepared and calibrated in accordance with manufacturer`s
recommendation and the data was appended manually to the spreadsheet to
provide an alternate assessment of the quantity absorbed dose (including
backscatter). In the phantom suitable holes were made at critical locations
of the pelvis such as prostate, rectum, bladder and right femoral heads.
||Schematics of designed phantom in this study
The treatment field for each technique includes four-field conformal
technique (box treatments), 3-field (with one anterior and two oblique
115 and 245° fields) and anterior-posterior technique (AP-PA) was
drawn on the pelvis phantom by radiotherapist. The irradiation technique
was SAD (source to axis distance) and the applied dose was 200 cGy. After
positioning the TLDs at the predetermined locations of the phantom and
positioning the phantom on the treatment coach of the linacs and adjusting
the radiation field, a dose of 200 cGy was applied to the phantom. The
above procedure was repeated 6 times for both 9 and 18 MV photon beams
of the two linacs. After each irradiation of the phantom, TLDs were removed
and the recorded doses were read using the Solaro 2A TLD reader located
in the Department of Medical Physics of Isfahan University of Medical
Sciences. To measure the dose at different locations, several measurements
was done and an average of the values multiplied to correction factors
(individual and group correction factors) of TLDs was calculated as the
mean dose of each organ.
RESULTS AND DISCUSSION
As can be shown from Table 1, for prostate, box technique
with photon energy of 18 MV is better than the other techniques, while
3-field technique is the worst. For sparing the rectum, 3-field technique
with photon energy of 9 MV has priority to the others, whereas AP-PA technique
is the worst. The exposure of the bladder was significantly lower for
the box technique when compared with both 3-field and AP-PA techniques.
The recommended technique for the femoral head sparing was the AP-PA and
there was no significance differences between 3-field and box techniques.
The measured DD% in 4-field technique to the prostate, rectum, bladder
and right femoral heads were 94.8, 85.71, 77.51 and 65.81%, respectively
when, the photon energy was 9 MV and they were 95.81, 86.73, 77.5 and
63.45% when 18 MV photon beams were used (Fig. 2).
The percentage depth dose (DD%), in the 3-field technique with 9 MV photon,
to the prostate, rectum, bladder and right femoral heads respectively
are 91.7, 78.83, 93.4 and 63.25% and they were 92.38, 79.05, 93.31 and
62.05% when 18 MV photon beam were used. Using the 9 MV photon beams in
AP-PA technique, prostate, rectum, bladder and right femoral heads received
96.23, 96.77, 96.3 and 28.77% of prescribed doses, while with 18 MV photon
radiation they were 95.77, 96.91, 95.82 and 26.69%.
For prostate, box technique with photon energy of 18 MV showed better
results than the other techniques, while 3-field technique was the worst.
For sparing the rectum, 3-field technique with photon energy of 9 MV has
priority to the others, whereas AP-PA technique is the worst. The exposure
of the bladder was significantly lower for the box technique when compared
with both 3-field and AP-PA techniques (Table 1). The
recommended technique for the femoral head sparing was the AP-PA and there
was no significance differences between 3-field and box techniques.
Significant technical advances in recent years have permitted the development
of safe, high dose radiotherapy techniques for localized prostate cancer,
with improved treatment efficacy. Future developments, including improved
imaging techniques for target volume definition, treatment planning algorithms
to optimize radiation dose distributions and methods for verifying precise
geometric and dosimetric accuracy of treatment delivery, hold out the
prospect of further advances in treatment efficacy while reducing treatment-related
side-effects. In conventional radiotherapy field, the studies investigation
various techniques draw differing conclusion concerning the best irradiation
technique. Some studies, comparing four and three-field techniques concluded
the three-field technique to be best in rectal dose sparing (Khoo et
al., 2000; Milecki et al., 2004). Others did not confirm these
results (Bedford et al., 1999; Greco et al., 2003). The
reason for these differing findings is unclear, but seems the different
Clinical Target Volumes (CTV) in theses studies could have been responsible
for different findings.
||The percentage depth dose (DD%) of different organs
(total prescribed dose to the prostate of 60 Gy) in accordance with
3-field (3F), box and AP-PA techniques considering 9 and 18 MV energy
||The mean dose of studied organs for total prescript
dose of 60 Gy
|*Data obtained from six measurements in different locations
In this study, differences among the techniques and energy`s have been
found for all four considered organs with total prescript dose (60 Gy).
Overall, there is no technique that is absolutely better than the others.
Technique 3-filed give the best sparing of the rectum; the bladder is
better spared with technique box and the best technique for sparing the
femoral head is AP-PA. Differences between energies were low and using
18 MV photons give the more satisfied results.
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