Physica Medica: European Journal of Medical Physics - Volume 32 - Supplement 1 - February 2016

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Results: DIBH technique provided a significant dose reduction in heart mean
dose (1.3 Gy FB vs 0.4 Gy BH) and LAD mean dose (10.7 Gy FB vs 2.0 Gy BH).
Better PTV coverage (V95% equal to 88.9% FB vs 92.6% BH) in DIBH plans
and no difference in lung parameters (V10, V20 and Dmean) were achieved.
Inter-fraction variability before treatment was extremely reduced (<1 mm
in translations and <0.5° in rotations). Intra-fraction variability was <2.1 mm
in translations and <1° in rotations.
Conclusions: In our experience DIBH is a reproducible and stable tech-
nique for left breast irradiation showing significant reduction of mean dose
to the hearth and LAD and a limited inter-fraction and intra-fraction DIBH
variability.
A.202
DOSE PROFILES OF LOW ENERGY BEAMS FOR DOSIMETER CALIBRATION
A. Santaniello F. Quarant L. Strigari L. Sportell a Physics Department,
University of Calabria, P. Bucci, I-87036 Rende, Italy;b Laboratory of Medical
Physics and Expert Systems, Regina Elena National Cancer Institute, via E
Chianesi 53, I-00144 Roma, Italy
Introduction: A low energy X-ray irradiator was recently used to expose
innovative solid-state sensors to photon beams. The dose reading was cali-
brated by electron paramagnetic resonance (EPR) dosimetry (E. Pikhay et al.,
submitted). The sensors, with a linear size of about one centimeter and a
thickness of a fraction of a millimeter, were irradiated at different ener-
gies (several ten keV), in an interval of dose values (1–50 Gy) including doses
of clinical interest. Lithium formate (LiFo) pellets (about 1 cm2 × 2 mm), pre-
viously calibrated by ionimetry of clinical beams (dose uncertainties within
about 5%), were used for the dose determination. Because of the pair-
wise mounting of the sensors within their housing, the radial distances from
the axis of the beam are not the same for the sensors, nor for the LiFo pellet.
Given the (expected) strong in-depth and lateral spatial dependence of the
intensity of low energy photon beams, the dose reading as determined by
EPR might differ from the actual dose absorbed by each of the sensors. The
larger thickness of the pellets, as compared to that of the sensors, inte-
grates the dose depth profile of the beam, affecting the measurement
especially at the lowest energies (40 keV). The intensity of the lateral beam
profiles is step-wise integrated over about 1 cm, the diameter size of the
pellet.
Material and Method: To characterize the above spatial heterogeneity, we
determined the dose profiles of the low energy photon beam by gafchromic
film dosimetry. The films were irradiated at different energies and dis-
tances from the source.
Results: The intensity profiles as a function of the radial distance from the
beam axis were measured, for different energies, at fixed positions along
the beam axis. The profiles were dose calibrated by comparison with films
exposed to known doses from clinical beams. In addition, by also expos-
ing for each irradiation a LiFo pellet at a fixed position, we compared to
the dose delivered by the irradiator as measured by EPR dosimetry.
A.203
MODELING OF COUCH TRANSMISSION IN THE RAYSTATION TREATMENT
PLANNING SYSTEM
A. Savini *, F. Bartolucci, C. Fidanza, F. Rosica, G. Orlandi. AUSL 4 Teramo –
Medical Physics Department, Teramo, Italy
Introduction: We present our methods and results regarding the model-
ing of a carbon fiber couch top (Varian Exact IGRT) in RayStation v4.5.
Materials and Methods: Couch attenuations were measured irradiating a
6MV, 10 × 10 cm2 field at several gantry angles on a cylindrical PMMA
phantom with a 0.65 cc Farmer ion chamber at its center, positioned at the
linac isocenter. Three contour models (CMs) were implemented in the TPS
to represent the thick, transition and thin region of the couch. Materials
and mass densities of each CM component were tuned to maximize the
agreement between measured and calculated attenuations. In addition, a
couch CT scan was acquired and dosimetrically compared with the CMs.
For clinical validation, we optimized VMAT plans for the TG-119 cases along
with 5 prostate and 5 H&N clinical cases. Plan-specific QA was performed
with a 2D-array. Measured dose distributions were compared with com-
puted doses which were generated including or not the CMs in the
calculation.
Results: Attenuations up to 4.3% were found. Optimized material and density
for the shell contour was: Carbon Fiber, 0.6 g/cc (thick, transition), 0.5 g/
cc (thin) and for the internal contour: PMMA, 0.052 g/cc. Despite to the
couch CT, optimized CMs gave a better agreement with measurements
and were less dependent on the dose grid resolution. Average deviation
between measured and calculated attenuations was 0.16% (max 0.98%).
When including the CMs in plan-specific QA, global 2%/2 mm γ-pass rates
showed an average improvement of 4.8% (p-value < 0.001, max + 18.6%).
3%/3 mm γ-criterion showed a lower sensitivity in catching the error of
not considering the couch. The couch reduced the mean dose to targets
up to 2.36% of the prescribed dose for prostate cases and up to 1.40% for
H&N cases.
Conclusions: RayStation accurately considers the implemented couch CMs
which justifies their introduction in clinical routine. Suggested materials
and densities are given for the Exact IGRT couch.
A.204
RAPIDARC PROSTATE TREATMENTS: RELATION BETWEEN DOSIMETRIC
ACCEPTABILITY AND PLAN QUALITY
A. Scaggion *, A. Roggio, S. Bacco, N. Pivato, M. Paiusco. Istituto Oncologico
Veneto IOV-IRCCS, Padova, Italy
Introduction: This work aims to assess the relation between clinical plan
quality, plan complexity and results of pretreatment QA procedure for
RapidArc prostate cancer treatments.
Materials and Methods: Ten high risk prostate patients were treated with
RapidArc technique: 5 with a single full arc and 5 with two full arcs. For
each patient, 6 treatment plans were proposed by 6 independent plan-
ners. TPS and Linac were the same for every plan and planner. Each plan
underwent clinical approval and pre-treatment verification using a 3D cy-
lindrical array of diodes (ArcCHECK, Sun Nuclear Corporation). Delivered
dose distributions on the patient were reconstructed via 3DVH software
(Sun Nuclear Corporation). Plans were compared by means of different plan
quality metrics, different plan complexity indices and results of pre-
treatment QA were evaluated in terms of both gamma index metric and
patient-specific metric.
Results: It is proved that plans with various degrees of clinical quality and
complexity can be observed even when a single set of dose–volume con-
straints is used. A better plan quality, in term of DVH metrics, is found to
be related to a higher delivery complexity and a worse agreement between
planned and delivered dose, measured with both gamma passing rate (PR%)
and DVH differences. For each single patient the spread of clinical plan quality
correlates with the PR% standard deviation.
Conclusions: The request of a highly conformed dose distribution,
which also spares healthy tissues, can conflict with the need to have a
trustworthy deliverable plan. The established correlations allow the def-
inition of a priori QA acceptance criteria that guarantees the treatment
delivery accuracy even if it can limit the maximum achievable clinical
plan quality.
A.205
ADVANCES IN KILOVOLTAGE X-RAY BEAM DOSIMETRY BY MONTE CARLO
CALCULATIONS AND MULTIPLE DETECTOR INTERCOMPARISON
P. Scalchi C. Avoss R. Hill P. Francescon A. Piermattei a Azienda
ULSS 6 – Fisica Sanitaria, Vicenza, Italy; b Univ. Cattolica Sacro Cuore, Roma,
Italy; c Chris O Brien Lifehouse, Camperdown, Australia
Introduction: KV x-ray beams are used in radiotherapy for skin cancer,
keloids and IORT. High dose gradients, detector volume and energy re-
sponse, and also the use of small fields, makes choosing appropriate
detectors critical.
We studied percentage depth-dose (PDD) and output factor (OF) perfor-
mance of different detectors by comparison with Monte Carlo (MC)
calculations. Two extrapolation methods to determine surface dose, and
e60 Abstracts/Physica Medica 32 (2016) e1–e70