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radiotherapy delivery, three treatment plans were made
by each institution on each static 3D-CT and on the 4D-CT
using the 15mmd animated by the 20bpm/15A signal. Prior
to phantom measurements, a BOA was performed in water
under reference conditions for the institution chosen
energy. The plans were measured twice using EBT3 films
and a 0.04cc ionization chamber. The films analysis was
done in RIT113. Gamma analyses were performed using
film dose as reference, a normalisation at the centre of
the sphere, a dose threshold at 20%Dmax and 3%dose/3mm
deviation as agreement criteria.
Results
Volume deviations (VD) ( % true volume) were respectively
for the 15mmd and the three motions tested: +10%(+/-
7%), + 1% (+/-17%), +12%(+/-12%) and for the 25mmd:
+6%(+/-7%), +4%(+/-7%).
.
VD were found higher at the end
of inspiration than at the end of expiration 8% (+/-26%)
insp and 1% (+/-3%) exp. The range of motion was
underestimated in all cases: -0,15cm (+/- 0,07cm), the
breathing pattern 10bpm presented the largest error -
0,2cm(+/- 0,2cm) compared to the breathing pattern
20bpm -0.09cm(+/- 0,08cm). Regarding the dosimetric
evaluation, the output dose mean deviation was 0.57% (+/-
1.42%) across institutions, agreement between chamber
and point-planned doses were respectively for the 15mmd
and the 25mmd static 98.9%(+/-1.3%), and 99.9%(+/-
2.8%). Agreement with the planned dose (centre of PTV
taken as ref. point) for the 15mmd in motion was 98.6%
(+/- 0.86%). The film gamma mean pass rates were 70% for
15mmd static, 59% for 15mmd dynamic and 74% for 25mmd
static.
Conclusion
QA of SBRT on moving targets are not yet practice routine,
film dosimetry in 4D-conditions are challenging due to the
absence of a consortium on where to register the films to
the planned dose. Moreover we lack of consistent data to
define acceptability thresholds. These results are a
starting point, with more dataset we hope to correlate
4DCT and dosimetric data to propose relevant evaluation
criteria.
OC-0540 A national cranial stereotactic radiosurgery
end-to-end dosimetry audit
A. Dimitriadis
1,2
, R.A.S. Thomas
2
, A.L. Palmer
3
, D. Eaton
4
,
J. Lee
4
, R. Patel
4
, I. Silvestre Patallo
2
, A. Nisbet
5
, C.H.
Clark
2
1
University of Surrey, Department of Physics, Surrey,
United Kingdom
2
National Physical Laboratory, Radiation Dosimetry,
Teddington, United Kingdom
3
Portsmouth Hospitals NHS Trust, Medical Physics,
Portsmouth, United Kingdom
4
NCRI, Radiotherapy Trials Group, London, United
Kingdom
5
University of Surrey, Physics, Guildford, United
Kingdom
Purpose or Objective
To assess the geometric and dosimetric accuracy of
stereotactic radiosurgery (SRS) in the UK for linac-based
(LB), TomoTherapy (TT), Cyberknife (CK) and Gamma
Knife (GK) radiosurgery.
Material and Methods
26 SRS centres were visited and 28 treatment plans were
assessed (16 LB, 7 GK, 4 CK, 1 TT). The audit methodology
employed an anthropomorphic head phantom with
realistic tissue densities with one irregularly-shaped
target (PTV), modelled on a brain metastasis, located
centrally in the brain and in close proximity to the
brainstem (OAR). The phantom was immobilised, scanned,
planned and treated following the local protocol.
Previously
characterised
near-water
equivalent
dosimeters were placed inside the phantom (EBT-XD film
and alanine pellets) to measure absolute dose, both inside
the PTV and OAR (Figure 1), and compare with TPS
predictions. Film measurements were digitised with
triple-channel-correction and compared to TPS dose
planes on FilmQA Pro using γ-analysis for a range of global
and local criteria.
Figure 1: Schematic representation of detector positions.
Results
Figure 2 shows the alanine measurements inside the PTV.
LB showed the largest range in percentage difference to
the TPS of 5.2% (-1.3% to +3.9%) with a mean of +0.5%. CK
had a range of 2.6% (+1.4% to +4%), with the highest mean
difference in comparison to the other platforms (+2.5%).
GK showed the smallest range at 2.4% (-0.8% to +1.5%)
being comparable to that of CK, with the smallest mean
percentage difference (+0.4%) comparable to that of LB.
Similar trends were observed in the OAR with alanine
measurements showing a range from -1% to +3.6% (mean=
+1.3%), 0% to +1.9% (mean= +0.9%) and -1.1% to +0.9%
(mean= +0.1%), for LB, CK and GK respectively.
The film measurements showed comparable passing rates
between axial and sagittal films, regardless of the
platform used. As expected, higher passing rates were
observed for Global-γ criteria. For 3%-2 mm Local-γ, all
except two films showed passing rates above 75%. For 5%-
1 mm Global-γ, all except 2 films showed passing rates
above 90%.
Figure 2: Results of PTV alanine pellets (dotted lines:1σ,
dashed lines:2σ).
Conclusion
This audit enabled the comparison of all participating
centres in terms of the accuracy achieved during the
delivery. The techniques used differed in many aspects.
The LB group showed the largest variations in agreement
to the TPS, related to more heterogeneous practices
within the group, compared to small variations seen in CK,
and more consistent practices seen in GK. Good overall
agreement with the TPS was observed with only 3 centres
falling above two standard deviations of the mean (2
centres in the target measurements and 1 in the OAR).
Film measurements showed comparable γ-passing rates
for all centres assessed with small differences between
platform groups. The results suggest that good agreement
with the predicted dose distributions is achievable by all
treatment modalities but highlight the need for
standardisation in SRS practices.
OC-0541 Automated treatment planning for
prospective QA in the TRENDY randomized trial on liver-
SBRT for HCC