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

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Results: G1 and G2 toxicities occurred in 11 and 2 patients (3DCRT treated),
respectively. A relation between average RR gradient and skin toxicity was
found: (0.13 ± 0.05) redness/Gy for G1 patients; (0.24 ± 0.06) redness/Gy
for G2 patients.
The trend of the fit may be correctly assessed since the first 2 weeks of
treatment. Several hot spots were noticed for the conventional treat-
ments rather than for the volumetric irradiations, that resulted in more soft
homogeneous skin redness.
Conclusions: Digital reflex camera can be used to quantitatively evaluate
skin reactions and seems to be sensitive to the radiotherapic technique. It
should be used to predict skin toxicity since the first 2 weeks of treat-
ment, improving cosmetic results and patients’ quality of life.
A.185
VMAT PATIENT SPECIFIC QA: CHARACTERIZATION AND VALIDATION OF
THE COMPASS SYSTEM
M. Polsoni *, F. Bartolucci, C. Fidanze, F. Rosica, G. Orlandi. Department of
Medical Physics AUSL4 Teramo, Teramo, Italy
Introduction: New hybrid systems for pre-treatment QA are suited for 3D
gamma (GA) and DVH reconstructed analyses. The aim of this work is to
characterize and validate the COMPASS (Iba-Dosimetry) system for VMAT
patient specific QA.
Methods and materials: The work was developed into three phases. (1)
The validation of the commissioned beam model of a Varian Trilogy ac-
celerator. Nine open square fields (from 2 × 2 cm2 to 27 × 27 cm2), AIDA and
CHAIR plans were delivered. The RayStation vs COMPASS reconstructed dose
and RayStation vs COMPASS computed dose were analyzed with a local 2%/
2 mm GA for PDD, XY profiles (Dmax, 5, 10, 20 cm depths) and for the MLC
model. (2) The validation of the inclinometer functionality was done de-
livering a number of static arc plans each composed of n consecutive arcs
(n = 1, 2, 4, 10, 14) to cover a total range of 358°. A local 2%/2 mm GA was
used. (3) The evaluation of the sensitivity of COMPASS to detect any pos-
sible Linac delivery inaccuracies. A TG119 prostate case was optimized using
the VMAT single arc technique. Two types of errors were individually in-
troduced in the RT plan: MU number modification (MUM) and the widening
of both leaf banks (WL). A 3D PTV GA was applied to all plans.
Results: The GA passing rate (PR) analysis in RayStation vs COMPASS showed:
in squared field verification, PR > 98.8% for the computed doses and
PR > 97.2% for the reconstructed doses; for the MLC model validation
PR > 92.9%. The gamma analysis for the inclinometer verification study iden-
tified a PR > 88.9% (system fault was detected for the 4 arc plan). The analysis
on the MUM showed that Compass is sensitive to +2% and−0.5% of MU vari-
ation. The sensitivity for leaf accuracy is WL > 1 mm.
Conclusions: An excellent agreement between RayStation and COMPASS
results as well as good system sensitivity for MUM and WL were found.
Compass results are considered very suitable for VMAT pre-treatment QA.
A.186
SMALL FIELD DOSIMETRY FOR VMAT IMPLEMENTATION: A
MULTIDETECTOR STUDY
M.C. Pressello R. Nigro a San Camillo Forlanini, Roma, Italy; b Ars Medica,
Roma, Italy; c San Camillo, Rieti, Italy
Purpose: Advanced techniques of radiation delivery as IMRT and VMAT are
based on the photon beam decomposition in sub-centimeter fields to achieve
complex dose distribution in the patient. Relative output factors (ROF) of
such small fields have to be determined with high reliability for clinical
commissioning of treatment planning systems (TPS) able to plan such com-
plicated dose delivery. However, dose determination with ionization chamber
(IC) in such small fields includes some serious challenges. This work pres-
ents a multi-detector determination of small field ROF for the
implementation of VMAT with Elekta Agility Linac and Pinnacle3 TPS.
Material and methods: Exradin A26 micro-IC, PTW 31016 pinpoint IC, IBA
Razor Diode and IBA CC01 IC were irradiated in 6 MV photon beams with
field size ranging from 4 × 4 mm2 to 50 × 50 mm2. The detectors were po-
sitioned at 10 cm depth in water. Effective field size and positioning accuracy
were checked with cross profile acquisition. Environment conditions were
taken into account and beam stability was checked with an independent
dosimetric system. ROF were determined by normalizing data to 50 × 50mm2
field and results were compared with Pinnacle3 calculated values (cone con-
volution superposition algorithm, 2mm × 2mm grid). TPS was commissioned
providing it ROF measured with IBA stereotactic diode.
Results: Measured ROF show differences of less than 1% down to
10 × 10 mm2 field. A poorer agreement was found for smaller fields. As ex-
pected, below 20 × 20 mm2 ICs begin to underestimate dose with respect
to diode. Measured and calculated ROF show a difference of less than 1%
down to 20 × 20 mm2 field and swiftly worse beyond, up to 15%.
Conclusion: The good practice to restrict the lower beamlet area to
20 × 20mm2 in clinical VMAT and IMRT planning is validated from the results
presented. Small field ROF have to be determined with high consistency
and possibly with several detectors to enhance confidence in data collect-
ed for TPS clinical commissioning.
A.187
CHARACTERIZATION OF EXRADIN A26 MICRO IONIZATION CHAMBER IN
UNFLATTENED PHOTON BEAMS
M.C. Pressello A. Petrucc A. Sorian a San Camillo Forlanini, Roma, Italy;
b ASL RME, Roma, Italy; c Istituto Regina Elena, Roma, Italy
Purpose: Current techniques of radiation delivery widely use the compo-
sition of sub-centimeter and unflattened (FFF) photon beams to achieve
complex dose distributions.
Accurate absolute dose determination is a challenge for the presently avail-
able ion chamber (IC) in such non standard condition because of issues as
small field dimensions (non-CPE condition) or flatness and dose-rate for
FFF beams.
Exradin A26 is a micro-IC recently proposed as reference class IC, over-
coming the not-ideal performance of A16.
In this work A26 behavior is investigated in conventional and FFF photon
beams.
Materials and methods: According to IAEA398, the raw ionization reading
needs correction to account for polarity effects (Kion) and ion recombina-
tion (Ks). These correction factors were determined by irradiating A26 at
10 cm depth in water in 6 MV Primus Siemens, 6 MV tomotherapy (FFF)
and in 6 MV and 10 MV TrueBeam (FFF) photon beams. Dose-rates at IC
were, respectively, 1.4, 12.3, 15.6 and 26.8 Gy/min.
Readings of an additional dosimetric systemwere used to check beam output
stability. Environment conditions were also taken into account.
IC response was studied over a range of polarizing voltages from −400 to
+400V plotting 1/Q versus 1/V. Ks was also determined with two voltage
method choosing different voltage ratios. Kion was determined at two
extreme voltages. IC response at increasing doses was also evaluated.
Results: Linearity in dose was found within 0.2%.s
Saturation curves (Q/Qmax versus V) clearly show a different polarization
dependence in the four beams types. In the tested conditions, the use of
the two-voltage method is valid.
Ksat values ranges within 1.001 and 1.007 and Kpol varies in 0.993–1.005
interval.
Conclusions: A study of A26 performance in terms of dose linearity, ion
recombination and polarity effects was carried out and the results show
that several operating conditions are identifiable for clinical commission-
ing it in FFF beams.
A.188
PRELIMINARY INVESTIGATIONS ON THE USE OF PTW T60020 DOSIMETRY
DIODE PR IN CLINICAL PROTON BEAMS AT CATANA PROTONTHERAPY
FACILITY
L. Raffaele * F. Romano G. Candiano G. Cirrone A. Gueli
S. Spampinato a Laboratori Nazionali Del Sud, INFN, Catania, Italy; b Scuola
Di Specializzazione In Fisica Medica, Universita’ Di Catania, Catania, Italy
Introduction: Plane-parallel (PP) ion chambers are recommended for ref-
erence and absolute dosimetry of low energy proton beams, such as those
e55 Abstracts/Physica Medica 32 (2016) e1–e70