World Congress of Brachytherapy 10-12 May, 2012 - Estro-events.org

S134 World Congress of Brachytherapy 2012
errors was simulated by rotating 3D dose cloud manually using the 3D
imaging registration system (VelocityAI), as shown in figure. The
rotational errors of ± 1°, ± 3°, ± 5°, ± 7° , ± 10° , ± 30°,± 45° and ±
90 ° along each axis were simulated. Dosimetric effects of the rotated
plans were compared with the original plans. In previous study, the
balloon to skin/chest wall distance, (BtoS Dist.) was the most critical
factor to affect the treatment results. Results for two groups of
patients were analyzed: (1) BtoS Dist. ≤ 7mm (the small BtoS group) ;(
2) BtoS Dist.>7mm (the large BtoS group).
: A comparison of various dosimetric parameters is presented
in the Table. The mean volume of PTV_Eval (1cm rind of breast tissue
outside of the balloon) that received the prescription dose decreased,
for the small BtoS group, from 1.7% ± 2.2% (less than 10°) to 28.1% ±
25.9% (up to 90°); for the large BtoS group, from 0.7% ± 0.5% (less
than 10°) to 3.3% ± 3.4% (up to 90°). In general, the increase in the
volumes of the organs at risk (OARs) receiving the tolerance doses was
small and did not exceed the tolerance, except for cases where the
OARs were in close proximity to the balloon (3% tumor coverage loss and >36%
maximal dose increase to the OARs, for the small and large BtoS
groups, respectively. However, for small BtoS Dist. ≤ 3mm targets
with the elliptical shape of dose cloud, the target coverage and dose
of OARs decreased significantly as rotational errors of 3° or more
were present.
: Rotational errors should be evaluated carefully for
clinical cases involving small balloon to skin/chest wall distance and
for dose cloud with elliptical shape. For the best dosimetric results,
APBI treatments using Contura should have rotational errors ≤ 3° (BtoS
Dist. ≤ 3mm), ≤ 10° (BtoS Dist.≤ 7mm) and ≤ 45° (BtoS Dist.> 7mm)
PO335
INVERSE PLANNING OF CONFORMAL HDRPROSTATE BRACHYTHERAPY
BOOST TECHNIQUE USING TWO CTVS
S. Wolf 1 , G. Bockelmann 1 , F.A. Siebert 1
1 University Hospital SH Campus Kiel, Radiotherapy, Kiel, Germany
: Ultrasoundbased intraoperative HDRprostate
brachytherapy (BT) as boost technique is a well established method in
our clinic. To date the treatment planning is performed manually as
conventional planning technique (CP). Despite the success of this
method, the introduction of inverse planning techniques (IP) is hoped
to reduce planning time and to decrease user dependency of the
planning process itself. In this study the clinical implementation and
evaluation of inverse planning for HDR prostate BT is presented. In
contrast to other HDR prostate techniques the target structure is
segmented into two parts: the peripheral zone of the prostate (CTV2)
and the whole prostate gland (CTV2).
: For the patient cohort considered in mean 12
needles were implanted in the typical ushape form. Using 38
different clinical treatment plans appropriate dose constraints for the
IP algorithm (BrachyVision v8.8, Varian Medical Systems, Palo Alto,
CA) were identified. Prescription doses for CTV1/CTV2 were 15/8.5
Gy. Doses in urethra and rectum should be lower than 10 and 8 Gy,
resp. Because the CTV1 is often badly visible with ultrasound after
insertion of the implant needles, the CTV1 was geometrically defined
by a threedimensional 0.5 cm hull around the implanted needles.
After establishing a general template of dose constraints and their
appropriate weightings, applicable to all patient anatomies, all
treatment plans were calculated according the TG43 formalism using
identical start conditions as well: three seconds dwell times in all
dwell positions, calculation time for the IP algorithm in the treatment
planning system was one minute, a maximum dwell time of 10 seconds
was allowed, and dwell time smooth function was set to maximum
100%. IP results were compared against the existing clinical CP by
using D90CTV1/2, V200CTV1/2, D2ccrectum, D0.1ccurethra, and COIN index.
Significant statistical differences (pvalue < 0.05) were analyzed by
the MannWhitney rank sum test.
: The inverse planning results in this study were available after
one minute. In contrast to this an experienced medical physicist needs
about 10 to 15 minutes per treatment plan. The results of this study
are summarized in table 1. Dosimetric indices are very similar
between CP and IP techniques. Only V200CTV1 is significantly lower for
CP. Moreover the IP technique fairly reduced (not statistical
significant) the D0.1ccurethra.
: This study showed that the usage of IP algorithms for
HDR prostate boost techniques with a complex CTV arrangement can
yield similar dosimetric results as a CP technique. Nevertheless
dosimetric results of an inverse planning must be checked by an
expert physicist before patient treatment.
World Congress of Brachytherapy 2012 S 135
PO336
BRACHYTHERAPY ACTIVITIES IN THE FRAMEWORK OF THE EUROPEAN
METROLOGY RESEARCH PROGRAMME
T. Schneider 1 , U. Ankerhold 1 , J. Solc 2
1
Phys. Techn. Bundesanstalt (PTB), Dosimetry for Radiation Therapy,
Braunschweig, Germany
2
Czech Metrology Institute, Inspectorate for Ionizing Radiation,
Praha, Czech Republic
: The European Metrology Research Programme
(EMRP) enables European metrology institutes to collaborate with
industrial organisations and academia on joint research projects
within specified fields. The EMRP is implemented by EURAMET e.V.,
organised by 22 European National Metrology Institutes (NMIs),
supported by the European Union and has a budget of 400 M€ over an
approximately seven year period. In 2011 a new Joint Research
Project (JRP) in the field dosimetry of ionizing radiation was initiated
and approved after a review conference. A part of this project covers
the recent developments in brachytherapy. In this presentation an
outline of the planned activities for dosimetry in brachytherapy will
be given.
: The work within the new JRP addresses the
establishing of miniature xray tubes for the treatment with low
energy photons as well as the work in the community of medical
physicists to bring up the treatment modality and treatment planning
to the level already achieved in External Beam Radiation Therapy,
manifested in the work of the AAPM TG 186 task group members in
close cooperation with the European society ESTRO.
: The dosimetry system of two miniature xray tubes will be
investigated: one from the vendor Carl Zeiss called 'Intrabeam ® ' and
another one from the vendor Xoft called 'Axxent ® ' tube. The
calibration procedures offered by the vendors are totally different.
The Axxent system uses a well type chamber calibrated in terms of Air
Kerma Strength (AKS) for the radiation field of I125. Calibration
factor in AKS for the Axxent field are under development (NIST,
Univ. Wisconsin). For the Intrabeam device the calibration is based on
measurements in a water phantom in terms of absorbed dose to water
with a soft xray ionization chamber. As no NMI offers calibrations in
terms of absorbed dose to water DW in a certain depth in a water
phantom in the respective radiation field, this calibration factor is
derived from Air Kerma calibrations and a conversion into DW
according to ICRU 17. The goal of the first part of the brachytherapy
work is to characterize the sources, to establish a primary standard in
terms of absorbed dose to water and to give recommendations on a
calibration chain.
: In most publications the results of treatment planning
are compared only to MonteCarlo calculations and not to
experimental data. It is the topic of the second part of the work to
overcome this problem by developing measuring techniques to canvass
the results of treatment planning systems in the presence of clinical
applicators. For NMIs this means a step apart from the standard fields
usually applied to a complex field close to clinical conditions. The
applied measuring devices will be characterized to achieve an
absolute uncertainty below 2% (k=1) in this irregular, nonstandard
field and the uncertainty of the positioning will be below 30 m.
PO337
MAPPING OF RELATIVE DOSE RATE DISTRIBUTION UNCERTAINTY OWING
TO SOURCE CONSTRUCTION
K. Zourari 1 , E. Pantelis 1 , L. Sakelliou 2 , E. Georgiou 1 , P. Papagiannis 1
1
University of Athens, Medical School Medical Physics Laboratory,
Athens, Greece
2
University of Athens, Physics Department Nuclear and Particle
Physics Section, Athens, Greece
: A joint AAPM/GECESTRO report 1 recommends
that brachytherapy source dosimetry investigators using Monte Carlo
simulation (MC) should determine the effect of type B uncertainties
on the quantities proposed for clinical use. This work presents results
of a method to map the total uncertainty owing to source construction
related type B uncertainties based on the ISO Guide to the Expression
of Uncertainty in Measurement 2 and MC.
: Calculations were performed for one
commercially available 125 I LDR seed. 3 Information on the dimensions
of the cylindrical silver marker, the thickness of its radioactive silver
halide coating, and the dimensions of the titanium encapsulation,
were available 3 as: xi±axi. Corresponding standard uncertainties were
calculated assuming uniform probability distributions. Let be a
point in the 2D matrix of dose rate per unit air kerma strength on a
plane including the source long axis.
is a function of xi (i.e. ) and, assuming f is
approximately linear and no correlation exists between xi, its standard
uncertainty can be calculated according to the law of uncertainty
propagation 2 : . MC simulations were
performed to obtain the 2D distribution of around the nominal
source design, as well as source designs where each of the quantities
xi was individually varied, assuming values of xiaxi or xi+axi. This
allowed for the forward and backward finite difference
approximations of the partial derivatives in the above equation, and
hence the calculation of 2D maps of the total standard uncertainty
owing to source construction and the partial contributions to it.
: The figure presents the overall % relative standard
uncertainty of around the source (left) compared to the
corresponding type A MC uncertainty (right). As expected, radioactive
coating thickness, followed by end weld radius, were the major
uncertainty contributors affecting mainly points close to the source
longitudinal axis. Uncertainty results at these points are comparable
to the uncertainty of NIST traceable SK calibration of seeds delivered
to the clinic. 1 The former however is position dependent and
therefore subject to smoothing out when multiple sources are used.
: Results support recommendations 1 that detailed
information on the source should be openly provided by
manufacturers and considered by dosimetry investigators. The
proposed 2D mapping method facilitates the overview of total
uncertainty while providing all the input necessary for the calculation
of uncertainty of TG43 quantities. The brute force implementation of
MC could be replaced by methods of improved computational
efficiency or physically intuitive models.
Acknowledgments
Kiveli Zourari was supported by Irakleitos II.
References
1. L. De Werd et al. Med. Phys. 38, 782801, 2011.
2. Guide to the expression of uncertainty in measurement, ISO, JCGM
100, 2008.
3. P. Karaiskos et al. Med. Phys. 28, 1753–1760, 2001.