826 / 1096
S810
ESTRO 36
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calculate PDDs, lateral profiles and output factors in a
water phantom for stereotactic cones with 5, 7.5, 10, 12.5
and 15 mm nominal diameter. Results from the
simulations were compared against measurements
performed in water phantom with PTW PinPoint ion
chamber and Scanditronix stereotactic diode. Actual cone
diameter was found by the best match between the
calculated and measured lateral profiles. Sensitivity of
output factor to cone diameter variations was
investigated. For this purpose, nominal cone diameter was
changed by +/- 0.3 mm (which is twice the manufacturer
stated uncertainty of 0.15mm).
Results
Lateral profiles agreed within 2%/0.5mm for all cone sizes.
Actual cone diameters were found to be 5.30, 7.70, 10.15,
12.65 and 15.15 mm. For the actual cone diameter, output
factors agreed within 2% for all cones except for cone 5
mm where the difference was 4%. Cone diameter
uncertainty of 0.3 mm lead to up to 11% variation in the
output factor compared to output factor value calculated
for the nominal diameter.
Conclusion
The MC model of the VersaHD linac was employed for
investigation and characterization of stereotactic cones.
Measured data were verified by the MC calculations.
Differences between nominal and actual cone diameter
were observed. Given the level of manufacturing accuracy
and sensitivity of dosimetric parameters to the cone
diameter variation, accurate commissioning of
stereotactic cones must be performed and comparison
with the data from other centers may be misleading.
EP-1511 Radiation Dose from Megavoltage Cone Beam
Computed Tomography for IGRT
E. Kara
1
, B. Dirican
2
, A. Yazici
1
, A. HICSONMEZ
1
1
Onko Ankara oncology center, Oncology Department,
Ankara, Turkey
2
Gulhane Research and Education Hospital, Radiation
Oncology, Ankara, Turkey
Purpose or Objective
Imaging dose in radiotherapy has generally been ignored
due to its low magnitude in comparison to therapeutic
dose used to treat patients. However, the total number of
fractions can range from 30 to 40 fractions for radical
IMRT. The cone beam computed tomography (CBCT) dose
to patients can be substantial. Daily imaging results in
additional dose delivered to patient that warrants new
attention be given to imaging dose. In this study, we try
to figure out the organ dose of CBCT for head&neck and
pelvis’s critical organs with three different CBCT
protocols. We also compare the image quality of these
protocols and try to find optimum one for dose and image
quality.
Material and Methods
Organ doses were measured for three different
megavoltage CBCT protocols on the Siemens Artiste linear
accelerator treatment machine. Organ doses were
measured by distributing thermoluminescent dosimeters
(TLDs) throughout critical organs of an anthropomorphic
(RANDO) phantom. The selected organs are rectum,
bladder, femoral heads and small intestine for pelvis
imaging and spinal cord, brainstem, tiroid and parotid
glands for head and neck imaging. The CBCT protocols
were 8MU, 15 MU and 8 MU half cycle. Slice size (512x512
pixels), slice thickness (0.54 mm), number of slices (512)
and SID (145 cm) were same for each protocol. The
numbers of projections are 360 for 8MU&15 MU protocol
and 200 for 8 MU half cycle protocol. The placement of
TLDs was done with the guidance of an atlas of the
anatomy. The TLDs placed RANDO phantom was irradiated
by using three different imaging protocol and the doses
were compared. We have also performed image quality
tests for each protocol. The used image quality phantom
was 20 cm diameter with four 2 cm sections: 1 solid water
section for noise and uniformity, 2 sections with inserts
for contrast resolution and 1 section with bar groups for
spatial resolution. We have performed image quality tests
for each CBCT protocols.
Results
We have seen that 15 MU protocol has no difference with
8 MU protocols in the means of image quality and the dose
of critical organs are much higher than the others as
expected. When we compare 8 MU and 8 MU half ring
protocols in the means of organ doses, we have seen that
the doses of organs changes according to the geometrical
placements of organs. Accordingly, while the doses of
organs, such as rectum, spinal cord and brainstem, nearby
the posterior decreases with the use of 8 MU half protocol,
the doses of organs located anterior, such as intestine,
thyroid and bladder, increases. It is observed that both the
contrast resolution and the spatial resolution of the 8 MU
half protocol is better than the 8 MU protocol. It also gives
information about position in a shorter time.
Conclusion
After obtain all this information about MV CBCT protocols,
we figure out that the choice of CBCT protocol should be
done after treatment planning by considering of the doses
and location of the critical organs. than the others as
expected.
EP-1512 Comparison between dose transmission
detector and 3d dosimetry for lung SBRT treatments.
F.R. Giglioli
1
, E. Gallio
1
, C. Fiandra
2
, O. Hammad
3
, R.
Ragona
2
1
A.O.U. Città della Salute e della Scienza- Department of
Medical Physics- Torino- Italy, Medical Physics, Torino,
Italy
2
University of Turin- Radiotherapy Unit - Torino- Italy,
Department of Oncology, Torino, Italy
3
International Center for Theoretical Physics- Trieste-
Italy, ICTP, Trieste, Italy
Purpose or Objective