ESTRO 36 Abstract Book

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S553 ESTRO 36 2017
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1 Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiation Oncology, Amsterdam, The
Netherlands
Purpose or Objective
Our standard IGRT protocol for SBRT of pulmonary lesions
consists of a pre correction CBCT to determine the couch
shift that aligns the tumor, post correction (PCorr) scan
for verification and two intra-arc CBCT scans to monitor
position stability. The intra-arc scans are acquired
simultaneously during VMAT delivery. To limit the number
of scans for patients with centrally located lesions treated
with 8x7.5 Gy, the pCorr CBCT is omitted and the 1st intra-
arc CBCT is used as the verification scan. In this study we
evaluate the positional accuracy of this protocol for the
8x7.5Gy patient cohort and compare this to a 3x18Gy
cohort.
Material and Methods
All 16 patients treated since the implementation of a
setup protocol in April 2016 without a validation scan for
a single centrally located tumor with 8x7.5Gy(Gp1), were
included. Fifty patients were randomly selected from our
3x18Gy(Gp2) database. Patient characteristics were
compared and intra-fraction tumor position variability in
Left/Right(LR), Cranial/Caudal(CC) and Anterior/Superior
AP) was calculated in terms of Group Mean(GM),
systematic(∑) and random(σ) variations for both groups
from either the 1st intra-arc CBCT or a CBCT taken
between the arcs if no intra-arc CBCT was available .
Results
No significant difference in patient characteristics
between the groups was observed; gender: female 44% vs
48%, male 56% vs 52%, median age 78.5 vs 72.0years and
mean GTV 32.2 vs 12.3cc in Gp1 and Gp2 respectively.
Intra-fraction tumor position variability in both groups was
small in all directions. The GM and random error were not
significantly different between the 2 groups whereas the
systematic error was significantly smaller for the 8x7.5Gy
group in the AP direction (table 1). Twelve (75%) patients
completed the new protocol i.e. no validation scan
throughout the whole of their treatment. Four (25%)
patients started with a no validation protocol but reverted
to the standard SBRT IGRTprotocol after a varied number
of fractions (fraction 2-5) when the threshold of ≥0.3cm
was exceeded in the 1st intra-arc scan. For some patients
the intra-arc CBCT was not acquired due to technical
issues or intra-arc CBCT image quality and a CBCT between
the arcs was used for verification. This occurred in 35% of
the fractions. Besides omitting the verification scan some
patients (9% fractions) required an intervention between
arcs.
Conclusion
The setup correction validation scan could be safely
omitted for most patients with central tumors treated
with a 8x7.5Gy dose regimen. Nevertheless, careful
monitoring is recommended for any SBRT dose regimen to
capture patients with larger intra-fraction position
variability.
PO-1012 Traffic Light Protocol as a guide for optimal
registration of LACC complex tumor pathology
E. Bogaert1, L. Van den Berghe1, A.L. Michiels 1, C. De
Wagter1, Y. Lievens1, K. Vandecasteele1
1Ghent University Hospital, Radiation Oncology, Gent,
Belgium
Purpose or Objective
Pre-treatment CBCT imaging for locally advanced cervix
carcinoma (LACC) is challenging. A Dual Registration (DR)
protocol (Mask-based VOI (MVOI) Autoregistration (AR) for
the primary PTV + Clipbox-Based (CVOI) AR for lymph node
(LN) coverage) had been set up with limits of 3 °, 6 mm
for CVOI and 3 mm for MVOI AR. However DR optimizes
target and LN coverage, it does not provide a decision
framework in case anomalies occur. The purpose is to
setup a Traffic Light Protocol (TLP) for RTTs, that includes
DR and where recommendations for Treat, Reposition or
Call Support are clearly outlined and specific actions to
take are visualized in green/orange/red.
Material and Methods
The recently introduced DR protocol was enrolled on 7
patients, counting for 206 CBCT-planning CT (pCT)
registrations. All 3D rotations and translations, together
with categorical data were recorded. (e.g. Did scan lead
to treatment?, Did DR algorithms perform well?, Were
predefined limits exceeded? Final approval (RTT /MD)?
Categories for deviation from pCT: pelvic tilt, bladder
filling, tumor shrinkage,…).
Based on this DR experience, a TLP dedicated for LACC
pre-treatment registration was set up in collaboration
with an RTT, Physician (MD) and Physicist. The MD and RTT
applied the TLP to 30 scans (1 patient) as an initial
validation. A 4-scale score reflecting the degree of
confidence about the RTTs decision was taken up.
Results
The necessity of the DR protocol, in comparison with a
CVOI bony anatomy AR alone, was emphasized by the
absolute mean (and maximum) differences of 0.2 (1.5) cm
(L/R), 0.3 (2.9) cm (S/I) and 0.3 (1.5) cm (A/P) between
the two methods.
In 7.3% of the cases the DR was misled due to high contrast
regions in the intestine (alternated gastrografine and air
bulbs), requiring manual adjustments. For 19.6 % of the
CBCT scans that finally led to treatment and where DR
performed well, anatomical deviations had been reported
and not entirely corrected for (table 1). Some are: pelvic
tilt > 3°, tumor shrinkage, bladder or rectum over- or
underfilling, or exceeding of translational DR limits. In
25.2 % of all CBCT scans, MD support for final decision was
needed.
The color code of the TLP is green= go ahead, orange = go
ahead AND e-mail to attending MD AND copy note into
Record &Verify system; red = do not proceed and call
support of MD. In this way reporting was standardized,
tumor shrinkage is followed more strictly in consideration
of re-planning, and DR failure led to clear
recommendations for manual registration. The TLP’s
results were validated to be ‘correct’. The confidence
level of the RTTs on their final decision, after following
the TLP was high (figure 1).