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S189
ESTRO 36 2017
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OC-0357 Treatment planning dosimetry accuracy in
192Ir HDR brachytherapy of lip carcinoma
P. Papagiannis
1
, V. Peppa
1
, T. Major
2
1
National and Kapodistrian University of Athens, Medical
Physics Lab.- Medical School, Athens, Greece
2
National Institute of Oncology, Department of
Radiotherapy, Budapest, Hungary
Purpose or Objective
Advanced dose calculation algorithms h ave become
clinically available for
192
Ir HDR brachytherapy y to
account for the effects disregarded by TG43 based
dosimetry algorithms (heterogeneities, applicators and
patient specific scatter conditions).
The aim of this work is to study the effect of improved
dosimetric accuracy in HDR brachytherapy of squamous
carcinoma of the lip.
Material and Methods
Three anonimized patient cases were studied (treatments
using the
192
Ir microSelectron-HDR v2 source, 27 Gy
planning aim delivered in 3 Gy fractions b.d.).
The plans were imported to OncentraBrachy v4.5 and
dosimetry was repeated using both the TG43 and the
Advanced Collapsed Cone Engine (ACE) TPS algorithms.
The same TRAK was used with both algorithms for the
same patient case. ACE calculations were performed using
the high accuracy option taking into account individual
voxel densities and assuming the elemental composition
of water, average skin and cortical bone for the PTV and
soft tissue, the skin, and the mandible, respectively. The
spatial resolution of TPS dosimetry results was 1 mm,
isotropic.
Corresponding reference data were obtained from patient
specific Monte Carlo (MC) simulations using the MCNP6
code with input files prepared from the parsing of dicom
RT data with the BrachyGuide software tool.
The TPS HU calibration was imported to BrachyGuide to
ensure identical density input to ACE and MC. Dose was
approximated by collision Kerma and kerma to medium in
medium was scored using the F6 tally.
BrachyGuide was also used for the comparison of the three
RT dose files for each patient case (TG43, ACE, and MC).
Results
TG43 clearly overestimates results for all cases as shown
in the left side of Figure 1 for an indicative case. This
cannot be attributed solely to the difference between
patient scatter conditions and TG43 assumptions since
large differences are also observed close to the source
dwell positions. The corresponding comparison between
ACE and MC (right side of figure 1) shows agreement within
MC type A uncertainty up to 5 cm from the implant. While
ACE improves dosimetric accuracy, considerable
differences are still observed close to the source dwell
positions.
Results from the comparison of median DVH parameters in
the Table show large differences between TPS calculations
and MC for high dose PTV volumes (V150 και V200) in
accordance with the above findings. Large differences
between TPS calculations and MC are also observed for
OAR parameters. These differences however correspond
to low dose while the low value of V85 does not raise
particular concern for tissue necrosis.
Conclusion
Considerable differences between TG43 and MC dosimetry
indicate that plan quality of HDR brachytherapy for lip
carcinoma may be compromised.
The ACE algorithm was found to improve dosimetric
accuracy at clinically relevant distances.
TPS dosimetric accuracy close to the source dwell
positions warrants further investigation.
OC-0358 Evaluation of the Advanced Collapsed-cone
Engine dose calculation algorithm for COMS eye
plaques
H. Morrison
1,2
, G. Menon
1,2
, M. Larocque
1,2
, E. Weis
3,4
, R.
Sloboda
1,2
1
University of Alberta, Oncology, Edmonton, Canada
2
Cross Cancer Institute, Medical Physics, Edmonton,
Canada
3
University of Alberta, Ophthalmology, Edmonton,
Canada
4
University of Calgary, Surgery, Calgary, Canada
Purpose or Objective
The current dosimetry protocol for ocular brachytherapy
involves augmenting TG-43 dose calculations with
correction factors or using look-up tables to account for
plaque materials, as the water-based TG-43 calculation
alone overestimates the dose in front of gold eye plaques
by >20%. This work investigates the accuracy with which
the Advanced Collapsed-cone Engine (ACE) algorithm
(Oncentra Brachy (OcB) v4.6.0, Elekta, Sweden) can
account for the ophthalmic applicator materials (gold
backing and Silastic insert) for three different sizes of
COMS eye plaques in a water phantom.
Material and Methods
The 12, 16, and 20 mm COMS eye plaques were introduced
into the applicator library for OcB by creating 3D CAD
models of the plaques and Silastic inserts with virtual
catheter lines along each seed slot. The Nucletron
selectSeed 130.002 I-125 source model for ACE was
created using primary-scatter separated kernel data
(generated by the CLRP (Carleton Laboratory for
Radiotherapy Physics) group) and AAPM consensus TG-43
dosimetry data. Treatment plans were created in OcB for
a single seed in water, a single seed loaded in the central
slot of the 12 and 20 mm COMS plaques (the 16 mm COMS
plaque does not have a central slot), and fully loaded 12,
16, and 20 mm COMS plaques. ACE dose calculations were
performed in high accuracy mode on a high resolution 0.5
mm
3
calculation grid. The resulting dose data was
compared to Monte Carlo (MC) simulated data using
MCNP6, replicating the OcB treatment plans.
Results