ESTRO 36 Abstract Book

914 / 1096

S898 ESTRO 36
_______________________________________________________________________________________________
Material and Methods
Six prostate cancer patients with transperineal placement
of 3mm-long 1mm-diameter polymer-based fiducials
underwent 3mm slice thickness plan CT scan on day 14
after markers implantation, which is consider as a safe
waiting time according to the literature.
All patients were managed with the same IGRT protocol:
before each daily treatment, two planar KV images were
acquired with the OBI 1.4 system (Varian Medical Systems)
at 45º and 315º. A manual marker match between the KV
images and the planning CT DRRs was performed and
automatically transfered to the treatment couch position
to correct the patient position in the three translational
directions (rotations were not taken into account).
Weekly, after patient re-position and just before session
delivery, a CBCT scan is acquired, that is used to assess
rectum and bladder filling (slice thickness between 1mm
and 3mm)
These CBCT images, as being acquired in patient corrected
position, have been used to evaluate the FM locations at
different times during the course of treatment. A total of
37 CBCT images have been analysed to reconstruct the FM
3D coordinates. The displacement of each FM was
calculated relative to its reference position on the
reference planning CT, and also shift of the middlepoint
of each 3 FM set. The distance between markers in each
set at the time of planning CT and during specific
evaluated treatment have also been computed.
Results
The average marker migration observed is 0.68±0.51 mm
(range between 0 – 3.90 mm). This observation seems
independent of the marker position inside the prostate,
but not of the spatial coordinate: the antero-posterior
direction presents the largest FM average displacement.
Although the average migration observed is low, there are
cases among the six patients where the migration
observed an specific day was greater than 2mm. This
observation may be directly related to the degree of
prostate desplacement caused by the influence of the
rectum and bladder, and also with the posible pelvic
rotation in the moment of daily RT (not corrected with the
2D DRR vs KV image comparison).
Changes in distance between pairs of FM in each set have
been, on average, 0.12±0.11 mm (range between 0.02 –
4.38 mm).
Conclusion
The low average FM migration observed is expectable,
according to the waiting time between marker
implantation and the planning CT scan procedure. A futher
investigation should be done in order to reduce this
waiting time.
The fact of having observed cases among all patient with
displacement greater than 2 mm should be taken into
account in the CTV-PTV margins: an adequate expansión
of margins might compensates for this set-up uncertainty.
EP-1654 Clinical set up and first results of EPID in vivo
dosimetry in an overload Chinese Radiotherapy
J. Li1, A. Piermattei2, P. WANG1, S. Kang1, M. Xiao1, B.
Tang 1, X. Liao1, X. Xin1 , L.C. Orlandini1
1 Sichuan Cancer Hospital, Radiation Oncology, Chengdu,
China
2 Fondazione Policlinico Universitario Agostino Gemelli,
UOC Fisica Sanitaria, Rome, Italy
Purpose or Objective
In vivo dosimetry (IVD) is an important tool able to verify
the accuracy of the treatment delivered. In an
environment where several linacs of different types
support daily heavy treatment workload over different
shifts of therapists, physicists and Radiation oncologists,
IVD checks can be strongly recommended to avoid
important dosimetric discrepancies. The work describes
the setup of IVD procedure with electronic portal imaging
devices (EPID) in an overload radiotherapy clinical
workflow, and the preliminary results obtained.
Material and Methods
64 patients that underwent a VMAT or IMRT treatments for
head and neck, brain, breast, lung, thorax, abdomen and
pelvis where scheduled for in vivo dosimetry procedure
with EPID. A commercial software (SOFTDISO, Best
Medical, Italy) was used at this purpose. Two indexes were
analysed: the ratio R between the reconstructed (Diso)
and planned (Dtps) isocenter dose (R=Diso/Dtps) and Pγ%
obtained performing a gamma analysis between the first
EPID image and the next ones acquired. The acceptance
criteria adopted for the ratio R was ±5%, while for the 2D
γ-analysis in term of Pγ index, we adopted Pγ > 90% with
a passing criteria of 3% global difference and 3mm
distance to agreement for head and neck treatment and
5%, 5mm for the others districts. The percentage of
patients P% with Rmean and Pgmean in the tolerance level
P%(Rmean) P%(Pγmean)respectively, and the percentage
of IVD test T% with R and Pγ in the tolerance level T%(R)
and T%(Pγ), were evaluated. For each district P% take into
account the patients with the mean values of the indexes
within the tolerance levels, while the T% is referred to the
number of tests. If one of the indexes resulted out of
tolerance, corrective actions were performed and the test
repeated at the next fraction.
Results
The results of 1211 IVD tests over 64 patients, were
reported in Table 1. All the patients analysed shown both
indexes (Rmean and Pγmean) in tolerance with the
exception of breast and thorax treatments. For VMAT and
IMRT thorax treatments P%(Pγ) decreased to 67%. The
thorax patients were revised considering the high gradient
regions of the isocenter and the positioning set up was
optimized. For IMRT breast treatment, P%(Pγ) decreased
to 50%: two (over four) IMRT breast patients were revised
adjusting the bolus positioning over the mask in order to
realign the reproducibility of the treatment (Pγ index) in
the tolerance level. Adopting the appropriate corrections,
the successive IVD tests guaranteed at the end of the
treatment P% values within the tolerance levels. For
thorax and breast treatments, due to the limitation of IVD
tests acquired, the mean P%(Py) index values after the
correction, were again out of tolerance but the effect of
the correction was always efficient.