ESTRO 36 Abstract Book

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S85 ESTRO 36
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Purpose or Objective
Kilovoltage cone-beam computed tomography (kVCBCT)
has often been regarded as the preferred imaging modality
for the visualisation of soft tissues and verification of
treatment position due to its superior spatial resolution
[1-3]. Transperineal ultrasound (TPUS) is an alternative
imaging tool that can be employed for pre-treatment
verification and in-treatment monitoring as it is non-
invasive and does not involve additional imaging dose [4,
5]. This study aimed to compare the daily inter-modality
derived setup shifts using TPUS versus kVCBCT (gold
standard) for prostate radiotherapy.
Material and Methods
A total of 1927 paired datasets (TPUS versus kVCBCT) from
55 patients were compared in three directions (i.e. x,y,z
shifts representing left/right, anterior/posterior and
superior/inferior directions respectively). The derived
setup shifts were reported to the nearest mm. Data were
analysed using PASW for windows, version 20.0 (SPSS Inc,
Chicago, IL). Observed differences in the derived shifts for
each imaging modality were reported. Statistical tests
were conducted under a two-tailed significance level, at
a minimum 95% confidence interval.
Results
A Shapiro-Wilk test revealed that the data was not
normally distributed (p<0.05). A non-parametric Wilcoxon
Signed Ranks test demonstrated no statistically significant
difference between the derived setup shifts from TPUS
and kVCBCT for all planes; x (p= 0.376), y (p=0.244) and z
(p=0.253). The proportion (%) of datasets where the
difference in the derived shifts between the two imaging
modalities were within 5/4/3mm in the x, y and z
directions are reported in Table 1. Spearman’s rank
correlation coefficients of the derived shifts were
moderate (0.612-0.671) for all three directions (p<0.005),
signifying that the accuracy of TPUS-derived setup shifts
was comparable to kVCBCT.
Table 1: Proportion of datasets where the difference in
the derived shifts between the two imaging modalities
were within 5/4/3mm in the x, y and z directions.
Conclusion
Measured differences were acceptable considering the
planning target volume (PTV) margin expansion was 10mm
in all directions, except posteriorly (6mm). Findings were
in agreement with the recent report by Trivali et al. [6]
who found no significance difference in the x, y and z
coordinates between TPUS and fiducial-based CT
localisation of the prostate gland. With specialised
training and user experience, TPUS is a promising imaging
modality in treatment setup and verification for prostate
radiotherapy without the need for additional exposure to
ionising radiation.
OC-0166 Fast 3D CBCT imaging for Lung SBRT: Is image
quality preserved ?
B. De Rijcke1, R. Van Geeteruyen1, E. De Rijcke1, Y.
Lievens1, E. Bogaert1
1Ghent University Hospital, Radiation Oncology, Gent,
Belgium
Purpose or Objective
Irradiation of Early Stage Non-Small Cell Lung Cancer (ES-
NSCLC), through Stereotactic Body Radiotherapy (SBRT)
requires image guidance. At our institute double pre-
treatment CBCT, with manual registration is performed at
every fraction. Speeding up CBCT gantry rotation and
implementation of automated registration allows for
faster decision taking. It also offers the possibility of
intrafraction CBCT, without severe prolongation of
treatment time. In a first step we investigated the image
quality and performance of a CBCT protocol with lower
dose and faster acquisition time.
Material and Methods
Standard (S) and Fast (F) scan protocols only differed in
gantry speed (180°/min (S) and 360°/min (F)) and were
performed on XVI Elekta ® CBCT. For six patients receiving
lung SBRT (60Gy in 3 or 4 fractions) for upper lobe ES-
NSCLC, dual pre-treatment imaging consisted of a S scan
followed by a F scan. This resulted in 17 useful S and F
image sets. Tumor movement amplitude stayed below 1cm
(1), removing the necessity for 4D-CBCT. All CBCT images
were retrospectively exported to Raystation ® (RaySearch
Laboratories, Sweden) for easy and blended side-by-side
evaluation. The resolution was 1x1x1mm3 for all scans. All
CBCT images were matched to planning CT. WW/WL was
set fixed per patient. Zooming was allowed.
Visual Grading Analysis (VGA) comprised well defined
criteria over the three planes (T, C, S), categorized in
three Image Quality (IQ) Focus groups: bony anatomy
(N=11), tumor characteristics (N=3) and anatomical
landmarks (N=7). Examples are: visualization of corpus
vertebrae (C, S plane), tumor edge (3 planes); carina
bifurcation (C, T plane). Scoring was done independently
by 3 routined RTTs. Possible answers were: equal, better
or worse for ‘upper’ scan (randomly assigned to F or S).
Data were analyzed using SPSS software v24 (IBM Corp.,
New York, NY).
Results
In 73.7 % of all cases, visualization of anatomical
structures was appreciated equally on S and F scans. When
differences emerged, visualization on F scan was
appreciated more in 71.3 % of the cases (71.8 % for bony
anatomy, 75.0 % for tumor characteristics and 67.2 % for
anatomical landmarks). Binary Logistic Regression in these
cases did not reveal significant dependence on patient (for
which BMI or tumor location are most relevant; however
not evaluated separately) (p=0,638), not on IQ focus group
(p=0,540) and not on reader (p=0,883). Thus, in 92.4 % of
all cases, image quality was scored equal or better for fast
imaging protocol compared to the standard protocol
(Figure 1).