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

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caudal and latero-side direction. Variations were compared basing on various
anatomical parameters. Instead of carrying out a CTS with subsequent plan-
ning in every BRT fraction, the optimization of the BRT treatment following
the first fraction was not performed and positions and dwell times of the
radiation source in the first BRT fraction were used for the following ones.
The DVHs obtained during these assumed BRT fractions were compared
with the DVHs of each BRT fraction actually performed. D100 and D90, V100
and D2cc of OARs were taken into consideration.
Results: Relevant differences, in terms of geometrical variations and dis-
tance of the IT from anatomical landmarks, were found when comparing
mean values and standard deviations for every parameter including CTV
and OAR volumes or D100 and D90, V100 and D2cc of OARs.
Conclusion: Variables in the different phases of BRT treatment were ob-
served. They not only evidentiate the differences existing between the
positioning of the applicator at different BRT fractions, at contouring of
volumes and at planning DVHs, but they also confirm, in quantity and quality,
that it is fundamental to study target and OARs before every BRT fraction
with CTS and subsequent planning.
A.100
IN VIVO DOSIMETRY IN 3D CONFORMAL RADIOTHERAPY FOR PROSTATE
CANCER WITH GAFCHROMIC® EBT3 FILMS: A FEASIBILITY STUDY
A. Mitillo A. Girardi S. Anglesi E. Trevisiol a AOU San Luigi Gonzaga,
SS. Radioterapia, Orbassano, Italy; b Department of Oncology, Radiation Oncology
Unit, University of Torino, Torino, Italy
Introduction: In vivo dosimetry (IVD) is a method to detect systematic errors
and to assess clinically relevant differences between planned and deliv-
ered dose in external beam radiotherapy. Several works show the reliability
of the films for in vivo measurements of the surface dose in electron beam
radiotherapy, such as intraoperative radiation therapy (IORT) or total skin
electron therapy (TSET). The purpose of this study is instead to evaluate
the feasibility of the use of EBT3 Gafchromic® films for the dose determi-
nation at the depth at which the absorbed dose to water is maximum
(dmax), during 3D conformal radiotherapy (3D-CRT) treatments for
prostate cancer.
Materials and Methods: Build-up caps made in bolus were used to measure
the dose at dmax. A study of dose distribution in the build-up region was
performed in order to define the build-up cups thickness. Film samples were
cut with a dimension of 1 × 1 cm2, irradiated with 10 MV photon beams,
digitized with a flatbed scanner and processed with a software written in-
house (MatLab R2010a). The uncertainties analysis of the dose determination
was also implemented in the same software. A pool of five patients with
prostate cancer, treated with five fields 3D-CRT, was chosen for prelimi-
nary tests in solid water (SW) phantom and IVD. Two sets of films were
irradiated in SW phantom to evaluate the repeatability. The measure-
ments were compared with Oncentra MasterPlan Treatment Planning System
(OM-TPS) calculated doses, both for in phantom and in vivo procedures.
Results: The percentage deviations between measurements and OM-TPS
ranging from −3.9% to 4.7% for SW phantom tests, and from −9.9% to 4.6%
for in vivo measurements.
Conclusions: The agreement between computed and measured dose is well
below ±5% in most situations, suggesting that the use of Gafchromic® EBT3
films is appropriate for in vivo dosimetry.
A.101
OUTPUT FACTORS FOR A LINAC EQUIPPED WITH CIRCULAR CONES:
CORRECTION FACTORS DETERMINATION FOR DIFFERENT ACTIVE
DETECTORS COMPARED TO GAFCHROMIC FILMS
A. Girardi C. Fiandra E. Galli F.R. Gigliol R. Ragon a Department
of Oncology- Radiation Oncology Unit, University of Torino, Torino, Italy;
b Medical Physics Unit, Azienda Ospedaliero – Universitaria Città della Salute
e della Scienza, Torino, Italy
Introduction: The use of small fields is an established practice in stereo-
tactic radiosurgery, although it is hard to measure with accuracy the
parameters for machine commissioning. This is related to the peculiari-
ties of highly collimated beams and to the features of the detector. The first
goal of this study was to determine small fields output factors (OF) with
several active detectors and one passive detector (Gafchromic EBT3 films)
for an Elekta Axesse medical linear accelerator equipped with circular cones.
The second one was to determine the correction factors for active detec-
tors for comparison with film measurements.
Materials and Methods: OF measurements were performed for small fields
beams ranging from 5 mm to 30 mm in diameter with six active detec-
tors (Exradin A26, Exradin A16, PTWmicroLion, PTWmicroDiamond, Exradin
W1 and Razor IBA) and one passive detector (Gafchromic EBT3 films).
Results: OFs measured with Exradin W1 scintillator were in excellent agree-
ment with EBT3 films (better than 2%). A significant underestimation
between the results obtained by radiochromic films and air-filled
microchamber was observed, particularly for the smallest field, up to 12%
for Exradin A16. The results obtained with the PTWmicroLion and the PTW
microDiamond indicate instead an opposite behavior: a dose overestima-
tion for the smaller radiation fields, up to 5% and 8% for the 5 mm-
diameter field for microLion and microDiamond, respectively, was noted.
Razor diode was in good accordance with Gafchromic films for very small
fields (diameter < 10 mm), while an underestimation for larger fields has
been observed.
Conclusions: The present study points out that it is crucial to apply the
appropriate correction factors in order to provide accurate measure-
ments in small beam geometry. The correction factors should be especially
applied for smaller fields and for all detectors used in this work, with the
exception of Exradin W1, which could be employed without corrections.
A.102
A GEOMETRICAL ANALYSIS OF ACCURACY IMPROVEMENT USING A
COUCH 6-DEGREE OF FREEDOM IN SBRT
F. Greco A.R. Alitto S. Chies S. Menna M. Massaccesi M. Ferro
V. Frascino M. Balducc V. Valentini A. Piermattei L. Azari a U. O.
C. di Fisica Sanitaria, Fondazione Policlinico Universitario A. Gemelli, Roma,
Italy; b Divisione di Radioterapia Oncologica Gemelli-ART, Fondazione Policlinico
Universitario A. Gemelli, Roma, Italy
Introduction: The purpose of the study is to determine the relevance of
rotational shifts using 6DoF robotic couch in patients treated with SBRT
to improve setup accuracy.
Material and Methods: Patients affected by lung tumours with a diame-
ter up to 5 cm were enrolled to SBRT. Breast board (CIVCO) was used for
set-up patient. Gross target volume was defined by a radiation oncologist
on 4D TC scan. Treatment planning was carried out in Volumetric arc therapy
with Eclipse™ (Varian Medical System®). Total dose was prescribed on the
basis of tumour position and dimensions. Daily cone beam computed to-
mography (CBCT) was performed before dose delivery. CBCT were compared
with CT scan for radiotherapy planning in order to determine the magni-
tude of set-up error and organ motion: translational (lateral, vertical and
longitudinal) and rotational (Pitch, Yaw and Roll) shifts were identified
(Varian 6D Online Review System). The collected shifts were applied on
the Protura TM Robotic couth 6DOF. Mean translational and rotational shifts
were calculated.
Results: From July to September 2015, 13 patients were enrolled. Fifty-
two CBCT were performed. The maximal translation setup shift was 1.1 cm
vertically, 1.6 cm longitudinally and 1 cm laterally, with 77% of the
shifts < 3 mm. The maximal rotation error was −2° for Pitch, −3.7° for Roll
and −3.4° for Yaw, with 22% of the rotations >1° and 5% of rotations >2°.
No correlation was observed between the magnitude of translational and
rotational shift. A Kruskal–Wallis test showed that there was no statisti-
cally significant difference between the three rotation groups (p > 0.05).
Conclusions: This work confirms that a 6-DoF robotic couch could be useful
to improve accuracy in SBRT. No correlation was found between transla-
tional and rotational errors, but it could reveal important outliers and
corrected. Geometric and dosimetric analyses on other regions are on-going.
e30 Abstracts/Physica Medica 32 (2016) e1–e70