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calculated. Such comparsion was done for 12 times on
different patients.
Results
The average position difference between two radiographs
in the breath-hold reconstruction was 1.3 ± 0.5 mm among
different patients. Such difference was greatly increased
to 6.5 ± 2.5 mm in free-breathing reconstruction. Assume
the position difference in the reconstruction due to
breathing motion was independent from other factors such
as isocenter precision and reconstruction calculation
accuracy, the derived average position error of catheter
in the reconstructions due to breathing motion was 6.4 ±
2.5 mm.
Conclusion
Our study showed that in Intraluminal Brachytherapy for
lung treatment, the breathing motion can significantly
affect the catheter position by 6.4 ± 2.5 mm on average.
Position margin of such value should be added in the
treatment length during Intraluminal Brachytherapy
planning to compensate such effect.
PV-0185 Retina dose as risk factor for worse visual
outcome in 106Ru plaque brachytherapy of uveal
melanoma
G. Heilemann
1
, L. Fetty
1
, M. Blaickner
2
, N. Nesvacil
3
, D.
Georg
3
, R. Dunavoelgyi
4
1
Medical University of Vienna/AKH Vienna, Department
of Radiotherapy, Vienna, Austria
2
Austrian Institute of Technology GmbH, Health and
Environment Department Biomedical Systems, Vienna,
Austria
3
Medical University of Vienna/AKH Vienna, Department
of Radiotherapy/Christian Doppler Laboratory for
Medical Radiation Research for Radiation Oncology,
Vienna, Austria
4
Medical University of Vienna/ AKH Vienna, Department
for Ophthalmology and Optometry, Vienna, Austria
Purpose or Objective
Visual acuity is a common side effect in
106
Ru plaque
brachytherapy. The purpose of this study was to evaluate
the retina dose as a risk factor associated with visual
outcome.
Material and Methods
45 Patients treated with
106
Ru plaque brachytherapy were
included in this retrospective study. A minimum of 100 Gy
was prescribed to the tumor apex using one of two
available plaque (types CCB, CCA) manufactured by BEBIG
(Eckert & Ziegler, Germany). Treatment planning and dose
calculation was performed using an in-house developed 3D
treatment planning system with Monte Carlo based dose
calculation. Dose volume histograms (DVH) were
generated for both physical absorbed dose and biological
equivalent dose (BED), according to the definition
introduced by Dale and Jones [1]. Visual acuity was
reported using Snellen charts. To analyze potential
predictors in anterior tumor locations, a subgroup of 20
patients was selected presenting with a minimum distance
of 5 mm between tumor and macula. Statistical
calculations were performed in SPSS (version 21, IBM). Risk
factors associated with loss of visual acuity were
evaluated using the Cox proportional hazards models. The
loss of visual acuity was correlated to risk factors using
Pearson correlation coefficients. Statistical significance
was assumed to be p ≤ 0.05.
Results
Median follow-up time was 29.5 months (IQR, 15.0-29.8).
A median apex dose of 131 Gy (IQR, 113.0-150.4) was
delivered to tumors with median apex heights of 4.6 mm
(IQR, 3.5-6.0)), largest basal diameters of 10.8 mm (IQR,
8.3-12.6) and smallest diameter of 9.3 mm (IQR, 7.9-
11.4). The baseline visual acuity (Snellen 0.82 ± 0.23 SD)
was significantly higher (p < 0.001) than the mean visual
acuity at last individual follow-up (0.59 ± 028 SD). The
Pearson Correlation analysis showed a significant
correlation of visual acuity loss with the mean (r = 0.49,
p = 0.001) and maximum (r = 0.47, p = 0.001) retina dose
and tumor basal diameter (r = 0.50, p < 0.001). The dose
to the macula showed no correlation with visual outcome
(r = 0.24, p = 0.12). In the subgroup of patients with
anterior tumor locations the maximum retina dose
remained the only predictive factor (r = 0.46, p = 0.043).
Evaluating the Cox proportional hazards model yielded a
significantly higher risk for visual acuity loss (of more than
0.3 Snellen) for patients receiving a maximum dose of
500 Gy or higher (p = 0.009). A Cox multivariate analysis
including the macula dose (p = 0.11) and basal diameter
(p = 0.78) showed that a high maximum retinal dose is the
highest risk factor (p = 0.017). The evaluation of the BED
metrics showed no better correlation with the
investigated endpoints and in some cases BED was even
inferior.
Conclusion
The study showed that retina dose (D
2
and D
mean
) is a
suitable predictor for visual acuity loss, especially in case
of anterior tumors where other risk factors (i.e. basal
diameter) fail.
References
[1] R.G. Dale and B. Jones. The clinical radiobiology of
brachytherapy. Br. J. Radiol.
71
, 465-483 (1998)
PV-0186 MaxiCalc: a tool to calculate dose distributions
from measured source positions in HDR brachytherapy
M. Hanlon
1
, R.L. Smith
2
, R.D. Franich
1
1
RMIT University, School of Science, Melbourne, Australia
2
The Alfred Hospital, Alfred Health Radiation Oncology,
Melbourne, Australia
Purpose or Objective
Dosimetric treatment verification via source tracking in
HDR brachytherapy requires evaluation of the delivered
dose as source dwell positions are detected. Current TPSs
are not configured to perform this function, hence a fast
dose calculation engine (DCE) that can accept the input of
arbitrary dwell positions from the source tracking system
is required. Here we present a TG-43 based DCE that
computes 3D dose grids for measured dwell positions and
performs a comparison with the treatment plan.
Material and Methods
The DCE, dubbed MaxiCalc, takes the input of measured
dwell positions and times and calculates a dose grid of
nominated dimensions and grid spacing for direct
comparison to the treatment plan. MaxiCalc was validated
against Oncentra Brachy (OCB v4.3) at 27 single dose
points, as per OCB commissioning, as well as a 3D dose grid
of 13 dwells.
Dwell positions and times delivered in a phantom were
measured by our source tracking system, as previously
published.
1
The measured dwell positions were then used
as input to MaxiCalc and the resultant dose grid compared
to that from OCB. Observed dose differences due to
source position measurement uncertainties were
investigated.
Results
For the 27 dose points, MaxiCalc differed from OCB by a
mean of 0.08% (σ=0.07%, max 0.41%) demonstrating
differences that are similar to those between published
values
2
and OCB. In a multi-source plan for doses between
50-200% of the prescription dose, MaxiCalc yields a
maximum difference of <1%, which arises due to minor
calculation differences in the steep dose gradients near
the source. There was a gamma pass rate of >99% at
1mm/1%.
A dose grid was calculated for a plan of 25 dwell positions
acquired using our source tracking system, there was
maximum difference of 12.2% (mean = 0.7%). The
maximum difference arises from a small shift in the
apparent dwell positions causing large differences due to
the high dose gradients near the source, which is only
significant within 10 mm of the source. For this volume of