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

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90% of PD, respectively, was considered as providing the optimal clinical
plan for PTV coverage. Liver Dmean, V7Gy and V12Gy were used against
the PTV coverage (1-V100) to generate the fronts. Conformity index
(CI = V100/PTV), gradient index (GI = V50/V100), homogeneity index
(HI = D2%/PD) and healthy liver irradiation in terms of Dmean, V7Gy and
V12Gy were calculated to compare different plans
Results: About 150 plans per lesion were calculated to generate the fronts.
The fronts for 1 mmMLC margin provided the best plans in terms of minimal
liver irradiation. With 1 mm MLC margin, optimal clinical plans were ob-
tained for IDS equal to 77–82%. GI shows a U-shaped behaviour with respect
to prescribed IDS, CI and HI indexes, with minimum values at 1 mm for
all metrics. The location of these minimal points was found independent
of the tumour dimensions. Minimal GI values were found at HI values ap-
proximately equal to 1.3.
Conclusions: Pareto fronts provide a rigorous strategy to choose clinical
optimal plans in SBRT treatments. We show that a 1 mmMLC block margin
provides the best results with regard to healthy liver tissue irradiation and
steepness of dose fallout.
A.45
EPID-BASED IN-VIVO DOSIMETRY FOR VMAT TREATMENT DOSE
VERIFICATION: INITIAL CLINICAL EXPERIENCE
S. Cilla * A. Ianiro F. Deodato G. Macchi C. Digesu A. Fidanzio
L. Azari M. Craus P. Viola V. Valentini A.G. Morgant A. Piermattei
a Fondazione di Ricerca e Cura Giovanni Paolo II – Università Cattolica del S.
Cuore, Campobasso, Italy; b Policlinico Universitario A. Gemelli – Università
Cattolica del S. Cuore, Roma, Italy; c DIMES Università di Bologna – Ospedale
S. Orsola Malpighi, Bologna, Italy
Purpose: To assess the usefulness of EPID-based in-vivo dosimetry (IVD)
for complex VMAT treatments in clinical routine.
Material and methods: 40 patients (20 with head–neck tumors and 20 with
high-risk prostate cancer) treated with Elekta VMAT using simultaneous
integrated boost were enrolled. IVD tests were evaluated by means of (i)
R ratio between daily in-vivo isocenter dose and planned dose and (ii)
γ-analysis between EPID integral portal images in terms of percentage of
points with γ-value smaller than one (γ%) and mean γ-values (γmean), using
a global 3%-3 mm criteria. Alert criteria of ±5% for R ratio, γ% < 90% and
γmean > 0.67 were chosen.
Results: A total of 620 transit EPID images were acquired during the treat-
ment fractions. The mean R ratios were equal to 1.002 ± 0.018 and
0.993 ± 0.018 for HN and prostate treatments; in both cases, more than 95%
of tests were within 5%. The 2D image γ-analysis showed γmean of 0.42 ± 0.16
and 0.43 ± 0.18 for HN and prostate treatments, respectively; 93.3% and 92.1%
of tests were within alert criteria. Mean γ% were 92.9 ± 5.1% and 92.5 ± 5.8%.
For HN and prostate treatments, 85.9% and 82.1% of tests were within alert
criteria. Relevant discrepancies were observed in 3 HN patients: a set-up
error was detected for one patient and two patients showed major ana-
tomical variations (weight loss/tumor shrinkage) in the second half of
treatment. For prostate treatments, most of tests outside the criteria were
due to random anatomical changes (filling of rectum/bladder) and set-up
uncertainties. All discrepancies would not have been detected by pretreat-
ment verification. IVD results are displayed after 1 minute from the end
of arc delivery.
Conclusions: This procedure was able to detect when the delivery was in-
consistent with the original plans, allowing physics and medical staff to
promptly act in case of major deviations between measured and planned
dose.
A.46
COMMISSIONING OF THE 4-D TREATMENT DELIVERY SYSTEM FOR ORGAN
MOTION MANAGEMENT IN SCANNING PARTICLE BEAMS
M. Ciocca S. Molinell A. Mirandola A. Mairani S. Russ E. Mastella
D. Panizza A. Pella P. Fossati M. Donetti F. Valvo G. Baroni
a Fondazione CNAO, Pavia, Italy; b Politecnico di Milano, Milano, Italy
Introduction: The aim of this work was the clinical commissioning and
validation of the dose delivery procedures for the treatment of moving
targets in hadrontherapy with scanning pencil beams.
Materials and methods: Radiochromic EBT3 films fixed to the moving Anzai
respiratory phantom were exposed to mono-energetic carbon ion scanned
homogeneous fields (E = 332 MeV/u, 6 × 6 cm2 field size, scanning step of
2 mm). To evaluate the interplay effect, field size and flatness for three dif-
ferent scenarios were compared to the static condition: gated irradiation
alone, repainting alone and the combination of both. Respiratory signal was
provided by the ANZAI pressure sensor or the optical tracking system (OTS)
already in use at the CNAO. End-exhale phase and 1 s gating window were
chosen (about 3 mm residual motion). Dose measurements were also per-
formed using a PTW PinPoint ionization chamber inserted into the Brainlab
ET Gating Phantom.
Results:The combination of gating technique and repainting (N = 5) showed
excellent results, being the only approach with a behavior close to the ref-
erence static condition (6% vs 4.3% flatness, identical field size and dose
deviation within 1.5%). Treatment delivery time was comparably accept-
able. The dose homogeneity for gated irradiation alone was poor (9%
flatness). Both the Anzai pressure sensor and OTS appeared suitable for pro-
viding the respiratory signal.
Conclusion: Results of measurements performed on different phantoms
showed that the combination of gating and layered repainting tech-
niques is suitable and safe to treat moving targets using scanning particle
beams. Abdominal compression using thermoplastic masks has also been
assessed as an additional strategy to mitigate the effect of patient respi-
ration in the clinical practice.
A.47
USE OF RADIOCHROMIC FILMS IN HDR IR-192 BRACHYTHERAPY
DOSIMETRY
G. Compagnone F. Roman F. Bisell A.L. Angelin V. Mesisc S. Pin
E. Lombard A. Galuppi G. Frezza A.G. Morgant a UO Fisica Sanitaria,
Azienda Ospedaliero – Universitaria di Bologna, Bologna, Italy; b UO
Radioterapia, Azienda Ospedaliero – Universitaria di Bologna, Bologna, Italy
Introduction:High-dose-rate (HDR) brachytherapy (BRT) procedures present
difficult isodose measurements around the seeds due to steep dose gra-
dient and narrow distances involved. Conventional detectors (ionization
chambers or TLD) are inappropriate at small distances from the sources
because of their relatively large sizes.
In this contribution, the characterization of radiochromic films (RF) as a
2D dosimeter for clinical Ir-192 BRT systems was investigated. Time-of-
film-readout (TFR), best response channel, calibration curve (CC), dose rate
dependence and range-of-use were the focus of our study.
Materials and methods: RF type EBT2 (ISP) was irradiated with two Ir-
192 BRT units (MicroSelectron V2 HDR and PDR, Nucletron). A Farmer-
type ionization chamber 0.6 cc TW30010 (PTW) was used for absolute dose
measurements.
TFR was evaluated for 1 and 10 Gy (2 films, scans after 1, 12, 24, 36, 72 h).
The best response channel was chosen in the range of 1.3–12.4 Gy (5 films,
scans after 24 h, 3 RGB channels). CC was determined in the range of 1–60
Gy (source–film distance 1 cm and 10 cm). Uncertainty analysis of the whole
experimental chain and dose rate dependence (from 4.48 to 50.84 mGy/
h) was performed. All scans were carried out with a commercial flatbed
scanner Epson Expression 1680ProCCD (Seiko).
Results: Stability of the detector response within 2% was reached after 6 h
(1 Gy exposure) and 8 h (10 Gy exposure).
The calibration curve was established considering the green channel readout
and interpolating the experimental data with a double exponential func-
tion. Dose rate independence of EBT2 film was proved. Uncertainty on the
pixel values ranged from 1.1% to 6.0% (dose range: 3–30 Gy) and in-
creased up to >10% for dose > 30 Gy.
Conclusions: The feasibility of RF for Ir-192 BRT sources was assessed. An
accurate calibration makes RF a suitable dosimeter for quality controls and
dosimetry checks of clinical treatments.
e14 Abstracts/Physica Medica 32 (2016) e1–e70