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

49 / 146

tions were compared with the gamma analysis method. The local passing
rates (GP) were evaluated using two acceptance criteria: 2%/2 mm dose
threshold (TH) = 30% and 3%/1 mm TH = 50%. The test was considered passed
if yielded a GP > 90% and the PinPoint dose difference (DD) was < 5.0%.
Results: All of the DQA plans passed the tests for the two criteria. No sig-
nificant differences were found between the static and the dynamic cases.
The mean GP were 98.8 ± 1.3% (2%/2 mm) and 98.1 ± 2.2% (3%/1 mm) for
static cases; 98.4 ± 1.7% (2%/2 mm) and 97.6 ± 2.4% (3%/1 mm) for dynamic
cases. Only in one case the DD was > 5.0%.
Conclusions: Our study confirms the ability of the RTS to follow the tumor
motion with great accuracy. Preliminary results show that our approach
is an efficient method for pre-treatment patient-specific DQA. However, a
detailed study of each case is needed.
A.148
QUALITY ASSURANCE TESTS FOR BRACHYTHERAPY TREATMENTS USING
LEIPZIG APPLICATORS
G. Mazzott P. Cacciafest M.G. Morelli E. Menghi a Azienda USL della
Romagna, Ravenna, Italy; b Istituto Scientifico Romagnolo per lo Studio e la Cura
dei Tumori (IRST), Meldola, FC, Italy
Introduction: Brachytherapy Leipzig applicators are very sensitive to small
errors in source positioning, since the dose is delivered in only one point
at short SSD (16 cm). Thus, a small difference between planned and actual
source position may result in out-of-tolerance difference between planned
and actual delivered dose.
The tolerance on source position is ±1 mm. Errors of up to 1 mm in source
positioning when using other types of applicators where the source stops
in several points to deliver dose, may not cause great calculated/actual dose
discrepancies. However, a ± 1 mm tolerance may be too high for Leipzig
applicators.
We study the maximum tolerance on source positioning for Leipzig ap-
plicators to reach a maximum error on delivered dose within 2% of calculated
dose.
Materials and Methods: 6 Leipzig applicators (3 horizontal, 3 vertical, of
diameters 1 cm, 2 cm, 3 cm for each type).
The source positioning was measured with the dedicated check ruler (ac-
curacy 0.5 mm).
The dose at the Leipzig applicator surface was measured with a Markus
ionization chamber and with EBT3 + GaF Chromic films.
Dose was measured placing the source at different distances from the in-
struments (ion chamber or films). The GaF Chromic films were scanned using
a flatbed EPSON 10000XL scanner, 24h after irradiation.
Results: Preliminary measures with the ion chamber show that the error
in the delivered dose caused by an error in the source position strongly
depends on the applicator type. A 1 mm source misplacement results in a
difference of more than 10% between calculated and delivered dose if using
vertical applicators, whereas the difference remains within tolerance values
if using horizontal applicators.
The GaF Chromic film measures are under analysis and will be available
later.
Conclusions: When using Leipzig applicators, the tolerance on source po-
sitioning should be lower than the tolerance requested when using other
types of applicators, especially when using vertical Leipzig applicators.
A.149
STUDY OF DOSIMETRIC IMPROVEMENT IN LUNG SBRT USING A 6-DEGREE
OF FREEDOM COUCH IN IGRT
S. Menna * A.R. Alitto S. Chiesa S. Teodoli G.C. Mattiucci
N. Di Napoli L. De Filippo M. Balducci V. Valentini A. Piermattei
L. Azario a Istituto di Fisica e Unità Operativa di Fisica Sanitaria, Università
Cattolica S. Cuore, Roma, Italy; b Divisione di Radioterapia Oncologica,
Gemelli-ART, Università Cattolica S. Cuore, Roma, Italy
Purpose: To investigate dosimetric impact of rotational errors on SBRT.
Material and Methods: Patients enrolment included: lung primary or meta-
static tumours preferably not in central position and until 5 cm. A Cone Beam
CT (CBCT) was acquired before dose delivery. After 3D manual match, trans-
lational and rotational shifts were applied using Protura 6-Degree of Freedom
(6-DoF) and Robotic Patient Positioning System (CIVCO Medical Solution).
Using MIM 5.5.2 software, a CT was generated by rigid registration in patient
position at the moment of CBCT, obtaining: (i) a translated CT (tCT) with
only translational errors, (ii) roto-translated CT (rtCT). Initial treatment plan
(TP) was copied to tCT (tTP) and rtCT (rtTP). Finally, dosimetric param-
eters were compared.
Results: From July to September 2015, 13 patients were enrolled; 52 CBCT
studies, 52 tTP and 52 rtTP were performed. Dosimetric evaluation showed
no important variations in PTV: 91% of V95% was >95%. Small differences
(only 3% of variations were >5%) due to rotations were found in Organs at
Risk. Multiple regression and pairwise confront (post-hoc test) showed sig-
nificative linear correlations between: (i) esophagus Dmax and roll
(p = 0.007) and pitch (p = 0.020) rotation, (ii) V12.5 in total lung and roll
(p= 0.015) and yaw (p = 0.012), (iii) V12.5 in total lung and yaw (p = 0.048).
No significant difference in PTV coverage between the tTP and rtTP was ob-
served (Mann–Whitney test p > 0.05).
Conclusion: These preliminary data show an improvement for OARs if ro-
tational shifts are applied. Dosimetric benefits on lung tumours are small
that is PTV margins are optimal for all shifts detected. Dosimetric evalu-
ation in other sites is on-going.
A.150
PROSTATE MOVEMENTS ANALYSIS DURING RADIOTHERAPY USING
VOLUMETRIC INTRAPROSTATIC GOLD COILS INFORMATION
R. Miceli G. Ingrosso E. Ponti D. Di Cristino A. Lancia P.L. Bove
F. De Pasquale R. Santoni a Department of Diagnostic Imaging, Molecular
Imaging, Interventional Radiology and Radiotherapy, Tor Vergata General
Hospital, Roma, Italy; b Department of Surgery, Urology Unit, Tor Vergata
University General Hospital, Roma, Italy; c Institute for Advanced Biomedical
Technologies, Department of Neuroscience, Imaging and Clinical Sciences, G.
D’Annunzio, Chieti, Italy; d Department of Radiology, Santa Lucia Foundation,
Roma, Italy
Introduction: We analyzed the correlation between prostate movements
and rectum and bladder filling during the treatment course.
Materials and Methods: Ten patients affected by prostate cancer under-
went image-guided radiotherapy using CBCTs, after the insertion of 3
intraprostatic fiducial markers (FMs). All patients underwent planning CT
and radiotherapy with empty rectum and full bladder. Planning CTs (CTref),
compared to CBCTs, were used to estimate the reference intermarker dis-
tances. For every patient, after the registration between CTref and CBCTs,
FMs, rectum and bladder were contoured. We recorded the centre of mass
(CM) coordinate of each FM, and computed the differences between the
coordinates (x, y, z) of the CM for each respective FM of the two studies:
CTref and CBCT. In order to check the dependence between the prostate
shift and rectal and bladder volumes we computed a Pearson correlation
coefficient and a linear regression analysis.
Results: 120 CBCTs were analyzed. The mean of prostate displacements ( ±SD)
along the three axes averaged over the 10 patients, and evaluated by the
shifts of the FMs, were: 0.90 ± 0.84 mm in x, 0.00 ± 2.07 mm in y,
−0.80 ± 1.28 mm in z; absolute shifts were: 1.20 ± 0.65 mm in x,
2.10 ± 0.71 mm in y, 1.50 ± 0.80 mm in z. We obtained significant shifts in
the left-right direction (x) in 4 patients (+1 ÷ +2.7 mm), in the anterior-
posterior direction (y) in 5 patients (−2.8 ÷ +2.7 mm), and in the superior-
inferior direction (z) in 4 patients (−3.5 ÷ + 1 mm).Using the Pearson
correlation and the linear regression analysis, we obtained the following
statistically significant anti-correlation: in the y-axis, between prostate dis-
placements and bladder volume variations (p < 0.001); in the z-axis, between
prostate displacements and rectal volume variations (p < 0.05).
Conclusions: In empty rectum conditions, prostate displacements are
minimal, oriented in the y direction, and are mainly due to bladder volume
changes.
e44 Abstracts/Physica Medica 32 (2016) e1–e70