ESTRO 36 Abstract Book

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S242 ESTRO 36 2017
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Conclusion
The analysis of the potential failures, their causes and
effects allowed us to increase the quality and the safety
in the CK workflow process. The FMECA technique provides
a systematic method to target vulnerabilities before they
generate an error. This framework analysis can naturally
incorporate further quantification and monitoring. The
FMECA method is an effective tool for the management of
risks in patient care.
PV-0459 Prostate CBCT dose optimization : from an
iterative mAs reduction to a sytematic exposure
reduction
E. Jaegle 1, M.E. Alayrach1, A. Badey 1, V. Bodez1, C.
Khamphan1, P. Martinez1, R. Garcia1
1 Institut Sainte Catherine, Physique, Avig non, France
Purpose or Objective
A daily repositioning Cone Beam Computed Tomography
image (CBCT) for prostate radiotherapy is realized using
exposure templates (mAs, kV) which affect image quality
and imaging dose. Settings should be optimized to
minimize patient exposure while maintaining sufficient
image quality to register the initial planning CT with CBCT
using soft tissue matching.
Material and Methods
20 prostate patients (without hip prosthesis) with daily
CBCT (40 fractions) acquired on a TrueBeam™ (Varian
Medical Systems) machine were selected. After the first
fraction using the standard pelvis template (125 kV 1080
mAs CTDIw 14 mGy), the therapists manually applied, day
after day, a low mAs reduction and assessed if the CBCT
image quality was good enough for patient repositioning.
The iterative process stopped when image quality was
assessed too bad and the last proper mAs were selected.
The link between the mAs reduction and corpulence
(patient volume inside CBCT FOV) was studied.
For one example patient, 23 therapists registered CBCT
images with CT for 3 fractions : the first fraction (S0%), a
fraction with 50% mAs reduction (S-50% ) and the fraction
with maximum mAs reduction (S -71% ).
Fisher’s test was applied to every direction, to compare
the variance between S0% / S-50% and S0% / S-71%.
Results
The median mAs reduction for all patients was 64% (13%
to 85%). Patient corpulence was not correlated to the mAs
reduction achieved (Spearman’s correlation rs = 0.465).
Variance analysis, for every direction, shows no significant
difference (p<0.05) between S 0% / S-50% and S0% / S-71%.
Table 1 : For 3 fractions, the variance in matching from
23 therapists and Fisher’s test results
Conclusion
mAs reductions recorded across the 20 patients are highly
variable, due to the subjective assessment of CBCT image
quality, but a median reduction of 64% indicates a great
potential for reducing imaging dose.
For one patient it has been demonstrated that image
quality deterioration has no impact on interobserver
variability.
A 50% mAs reduction for the Pelvis CBCT template is
therefore considered.
PV-0460 Comparison of 3 Image-guided Adaptive
Strategies for Bladder Radiotherapy
V. Kong1, A. Taylor2, T. Craig1, P. Chung 1, T. Rosewall1
1 Princess Margaret Cancer Centre, Radiation Medicine
Program, Toronto, Canada
2Sheffield Hallam University, Faculty of Health & Well-
being, Sheffield, United Kingdom
Purpose or Objective
Due to the significant variation of bladder volume
observed throughout the course of treatment, various
adaptive strategies have been developed to improve the
quality of bladder radiotherapy. The aim of this study was
to use deformable registration and dose accumulation
processes to compare the dosimetric differences of a
population-based PTV approach and three proposed
adaptive strategies: Plan of the Day (POD), Patient-
Specific PTV (PS-PTV) and daily reoptimization (ReOpt).
Material and Methods
Bladder patients (n=10) were included in this
retrospective investigation. Patients were planned and
treated with a full bladder in supine position. Planning CT
and the CBCTs were retrieved and imported into
treatment planning system. After delineating the bladder
and the pelvic lymph node (PLN) on the planning CT, an
expansion of 1.5 cm and 0.5 cm was applied to generate
the population-based Standard PTVWB and PTVPLN ,
respectively. A 7-field IMRT distribution was designed to
deliver a prescription dose of 46Gy in 23 fractions. Each
adaptive strategy was applied according to published
guidelines. After simulating the execution of each
strategy using the daily CBCTs, daily dose was computed
on all CBCTs and then total dose was summed on the
planning CT using the output from the CT-CBCT
deformable image registration. The volume receiving 95%
of prescription dose (V95) was compared against the
Standard for each of the adaptive strategies. p < 0.05 was
considered statistically significant.
Results
Mean V95 (cm3) were 1410 (SD: 227), 1212 (SD: 186), 1236
(SD: 199), and 1101 (SD: 180) for Standard, POD, PS-PTV
and ReOpt, respectively. All adaptive strategies
significantly reduced the irradiated volume, with ReOpt
demonstrating the greatest reduction compared to the
Standard (-25%). This was followed by a mean reduction
of 16% with PS-PTV and 12% with POD. The difference in
the magnitude of reduction between ReOpt and the other
2 strategies reached statistical significance (p = 0.0006).
Conclusion
Previous comparisons between bladder adaptive strategies
have been limited due to the inability to account for the
effect of daily motion of the bladder and surrounding
organs. When deformable registration is used to
reconstruct dose in the presence of organ motion, ReOpt
is the best adaptive strategy at reducing the irradiated
volume due to its frequent adaptation based on the daily
geometry of the bladder. However the resource burden
associated with this strategy needs to be quantified to
further assess the feasibility of clinical implementation.
PV-0461 Integrating diagnostic MRI in radical bladder
cancer radiotherapy: Challenges in image registration.
C.L. Eccles1, H. McNair1, D. McQuaid2, K. Warren-Oseni2,
V.N. Hansen2, A. Sohaib3, M.D. Koh4, R. Huddart4, S.
Hafeez4
1The Royal Marsden NHS Foundation Trust and The
Institute of Cancer Research, Radiotherapy, London,
United Kingdom
2The Royal Marsden NHS Foundation Trust and The
Institute of Cancer Research, Radiotherapy Physics,
London, United Kingdom
3The Royal Marsden NHS Trust, Radiology, London,
United Kingdom
4The Royal Marsden NHS Foundation Trust and The
Institute of Cancer Research, Radiotherapy and Imaging,
London, United Kingdom
Purpose or Objective
Radiographer led soft tissue matching has been
fundamental for implementation of adaptive strategies in