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ESTRO 36 2017
_______________________________________________________________________________________________
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
The new IBA Dolphin (IBA Dosimetry, Germany) is a dose
transmission detector (DTD) mounted onto the gantry for
online treatment verification as well. Aim of this study is
to compare the results of the Dolphin/Compass with the
traditional 3D dosimetry phantom Delta 4 (Scandidos,
Sweden) for lung stereotactic body radiation therapy
treatmentsand to mesure the detector attenuation for
online dose verification.
Material and Methods
At first the two systems were compared in terms of ability
of error detection of leaf position. A box treatment was
measured three times with introduction of a shift of one
leaf bank in steps from 0 to 2 mm and the analysis of
gamma index or DVHs was carry out. Afterward ten
patients with lung cancer, treated by sbrt, were included
in the study. All treatment plans were
simultaneuslyverified with the Dolphin and the Delta
4.The treatment plans were generated by Monaco system
(ver.5.0, Elekta AB, Sweden). Dolphin with the Compass
software (v. 4.0) permits the 3D dose distribution
reconstruction on a patient CT and the Compass itself is a
model-based dose computation, with a collapsed cone
dose engine; the beam model of the Compass was
validated and accepted. For the quantitative analysis