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

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The aims of this study are the evaluation of treatment’s feasibility; improv-
ing this new therapeutic approach and validating this schedule of treatment
(WBRT 20 Gy + SIB 40 Gy in 5 fractions); and considering dosimetric cover-
age of target.
Material and methods: Seventeen patients were treated from 2012 to 2015
in our institute. All patients were positioned supine in a custome-made mask
for the TC simulation. The PTVwb was derived by the brain plus 2 mm
margin and the PTVboost by GTVs plus 4 mmmargin. Treatment plans pro-
cessed by ECLIPSE with PTVwb = 20 Gy to a 100% volume and
PTVboost = 40 Gy to a 100% volume in 5 fractions.
Plan evaluation criteria related to PTVs were: Dmean, conformation number
(CN), and homogeneity index (HI). We also considered Dmean to OARs.
To study brain disease response after three months, patients were reval-
ued with the same neuroimaging technique used for diagnosis.
Results: The PTVwb mean volume was 1224.97 cc; PTVboost mean volume
was 28.19 cc. Dmean to PTVwb was 21.37 Gy and to PTVboost was 40.01 Gy.
The calculated CN95% related to PTVwb was 0.79 and HI PTVboost was 0.07.
Our preliminary data about clinical outcomes are the following: the median
survival was 6 months.
Conclusion: VMAT-technique allows to conform doses to PTVs: in effect,
our dosimetric evaluation on PTVwb shows a CN value next to unity, that
confirms the inclusion of brain-volume into the isodose 20 Gy.
PTVboost had an HI value typical for stereotactic result: all of lesions are ra-
diatedwith excellent homogeneity. Our first experience did not showedmore
collateral effects than WB only, despite the poor prognosis of patients.
WBRT plus SIB ensures a better control of intracranial disease with shorter
delivery time, reducing tumor cell repopulation and improving patients’
compliance.
A.172
EVALUATION OF THE COMPASS SYSTEM FOR DOSIMETRIC PRE-
TREATMENT VERIFICATION OF VMAT PLANS: PROPOSAL OF A
CONFORMITY INDICATOR
N. Perna M. Lioc R. Bernard M.G. Leo a Istituto Tumori Giovanni Paolo
II IRCCS – UOSD Fisica Sanitaria, Bari, Italy; b Istituto Tumori Giovanni Paolo
II IRCCS – UOSD Radioterapia, Bari, Italy; c ASL TA – UOSD Fisica Sanitaria,
Taranto, Italy
Introduction: Volumetric modulated arc therapy allows you to achieve
highly-shaped dose distribution. In this study, we describe our experi-
ence of the implementation of the COMPASS System (IBA Dosimetry) in
clinical practice for the verification of VMAT treatment plans, and our
attempt to identify a first indicator of plan compliance.
Material and methods: The COMPASS is a system of 3D quality assurance
for the verification of VMAT treatment plans that reconstructs the dose de-
livered to the anatomy of the patient. It consists of software (COMPASSMain
Application) and an associated measuring device (MatriXX Evolution). The
software can perform both a pure 3D dose calculation and a 3D dose recon-
struction on the basis of the CT data of the patient/phantomand of the fluence
reconstructed by means of MatriXX detector measurements.
In order to validate the COMPASS system, we analyzed 35 RA treatment plans,
planned for different anatomical regions with one or two isocentric arcs. The
calculation dose distributions were compared with the reconstructions ob-
tained with the COMPASS QA system. The validation of this system was
performed with film GafChromic EBT2 for 2D measurements.
Results: From the comparison between Eclipse and GAF, setting a threshold
of 95% for the gamma passing rate, the COMPASS verification system pro-
vides consistent results with our reference verification system EBT2. The
COMPASS system is able to perform various statistical analyses using the 3D
average gamma function as an indicator of conformity of a treatment plan.
Conclusions: The measurements carried out show that the reconstruc-
tion of the fluence measured with the COMPASS system compared with
that obtained with Gafchromic films is reliable. The indicator of confor-
mity of a treatment plan can be the average 3D gamma calculated by
COMPASS on the volume of the patient taking a dose greater than or equal
to 20% of the prescribed dose.
A.173
MULTICENTRE RESULTS OF EPID-BASED IN-VIVO DOSIMETRY
A. Piermattei S. Menna F. Grec S. Cilla R. Caivano V. Fusco
L. Orlandin G. Benecchi R. Nigro D. Falco A. Fidanzi a UOC Fisica
Sanitaria, Università Cattolica S.C. – Fondazione A. Gemelli, Roma, Italy; b UO
Fisica Sanitaria, Fondazione Giovanni Paolo II, Campobasso, Italy; c UOC
Radioterapia, CROB, Rionero, PZ, Italy; d U.O. Fisica Medica, CFO, Firenze, Italy;
e UOC Fisica Sanitaria, Azienda Ospedaliero-Universitaria, Parma, Italy; f UOC
Radioterapia, Ospedale S. Camillo de Lellis, Rieti, Italy; g UOC Radioterapia, OC
S.S. Annunziata, Chieti, Italy
Introduction: The modern radiation therapy requires higher security levels
also during the dose-delivery step, where the presence of the physicists
is poor. In this frame the in-vivo dosimetry (IVD) by EPID images has an
important role to assess dosimetric differences between planned and de-
livered doses, allowing timely QCs. The present work reports the statistical
analysis of 7500 tests obtained in the last 2 years in 7 Italian centres.
Material and methods: SOFTDISO IVD-program, distributed by the Best
Medical Italy, allows 3D-CRT, IMRT and VMAT tests; it is easy to imple-
ment for Varian, Elekta and Siemens linacs, is connected with the Record
and Verify system of the centre and supplies IVD results in a few seconds.
The program perform two checks: (i) the ratio R = Diso/Diso,TPS between
reconstructed and TPS computed isocentre doses, with pass criteria of ±5%,
and (ii) 2D gamma-analysis between EPID images of different sessions adopt-
ing the following pass criteria: Pgamma < 1 higher than 0.90 for 3DCRT and
than 0.95 for IMRT and VMAT; gamma-mean less than 0.5 and 0.3 for 3DCRT
and IMRT-VMAT respectively.
Results: The percentage of off-tolerance tests ranged between 10% and 17%,
depending on the technique used. The causes, in order of frequency, were:
patient set-up, attenuators left in the field, morphological changes, TPS im-
plementation and linac output factor. All the causes of off-tolerances were
justified and once removed, with adequate QC, the patient’s average test
results satisfied the pass criteria. The patient morphological changes, pointed
out by the IVD tests, triggered new TC scans to verify the need of an adap-
tive plane.
Conclusions: The results of this study proved: (i) the great utility of quasi
real time IVD, (ii) the positive role of the physicists during the dose-
delivery, (iii) SOFTDISO allows to understand the causes of dose discrepancies
triggering adequate QC, and (iv) the role of IVD to intercept evident patient
morphological changes to examine for eventual adaptive radiotherapy
analysis.
A.174
A QUANTITATIVE TEST TO EVALUATE THE PERFORMANCE OF THE
MULTILEAF COLLIMATOR
F. Pietrobon P.M. Polloniato C. Arrichiell C. De Toffo a UOA Fisica
Sanitaria, Ospedale S. Martino, ULSS n.1, Belluno, Italy; b UOC Fisica Sanitaria,
Azienda Ospedaliera Universitaria Integrata Verona, Verona, Italy
Purpose: The IMRT treatments require high accuracy on leaf positioning.
The traditional methods are qualitative or require a significant invest-
ment of time. The Autocal (Elekta Oncology Systems, UK) software suite
uses EPID measurements to calibrate the leaf positioning. It includes the
stripe test to perform in a fast way the quantitative evaluation of the ac-
curacy of the leaf position, based on Sastre-Padro work. We analyzed the
Elekta method and verified its application in a MLC quality assurance
program.
Materials and methods: The stripe test consisted of 6 adjacent segments
4 cm wide exposed to the Elekta EPID. These segments produce 5 abut-
ments: the relative position error (RPE) and the absolute position error (APE)
could be measured. We verified the test procedure irradiating Gafchromic
films with the same segments comparing the results. To calculate the RPE
and the APE tolerance required for IMRT treatments (i.e. MLC accuracy less
than 1 mm), we shifted the two leaves of the leaf-pair to a wrong posi-
tion (from −2 mm to 2 mm). Since 2014, every week the stripe test has been
applied to two different MLC Elekta systems so we have defined the short-
and long-term variance of the stripe test.
Results: The stripe test measurements for artificial shift were analyzed and
the results suggested that there are different tolerance levels on the stripe
e51 Abstracts/Physica Medica 32 (2016) e1–e70