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

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3.0 mm (HD) and 0.2 (DSI). T-test confirms these results for all OARs
(p < 0.05) except the brainstem. The mean HD95 in dose registration was
10.9 mm. The DVH dose accumulation comparisons showed differences with
a great variability along patients and OARs.
Conclusion: Preliminary results quantify the improvement of DIR, com-
pared with RIR, in the accuracy with which the accumulated dose has been
determined. However, a valid and efficient QA process of DIR should be done
before its clinical application.
A.25
RAPIDPLAN MODELS BASED ON HELICAL TOMOTHERAPY (HT) PROSTATE
PLANS FOR VMAT OPTIMIZATION
E. Cagni A. Bott R. Micer N. Simoni R. Sghedoni M. Orland
L. Orsingher C. Iott L. Cozzi M. Ior a Fisica Medica, IRCCS-ASMN, Reggio
Emilia, Italy; b Radioterapia, IRCCS-ASMN, Reggio Emilia, Italy; c Radiotherapy
and Radiosurgery, Istituto Clinico Humanitas, Milano, Italy
Introduction: In the radiotherapy planning process automation of
knowledge-based (KB) procedures stands as a possible solution to improve
the quality of the plans, limiting critical factors like the experience of the
operator. RapidPlan (RP) (Varian Medical Systems, USA) uses plan librar-
ies, based on delivery techniques, patient’s anatomy and DVHs, to create
models that predict the DVH of the organs at risk (OARs) and propose op-
timization constraints.
A useful application could be to use KB models based on plans of consoli-
dated technique in order to supply the lack of planning experience with a
new treatment modality.
In this study, HT (Hi-Art, Accuray, USA) plans of prostate cancer patients
were used to create 2 RP models suitable for RapidArc (RA) modality. Fea-
sibility and performance of these models were evaluated.
Material and methods: RP models were created from 2 groups of HT plans
for prostate cancer patients: low risk group, consisting of 35 plans aimed
to deliver 70 Gy/28 fractions to prostate, and intermediate risk group, con-
sisting of 30 SIB plans with prescription of 70 Gy/28 fractions to prostate
and 56 Gy/28 fractions to vesicles.
The HT dose distributions were linked to a virtual RA plan into the Eclipse
TPS.2 full arcs of the 6 MV photon beam and 30°/330° complementary col-
limator angle were set.
Two evaluation groups, consisting of 5 new knowledge-based plans (KBP)
each, were used to validate the 2 models. KBPs were compared with the
corresponding clinical plans (CP) in terms of PTV homogeneity and DVH
endpoints.
Results: The KBP dose-volume constraints were suitable for the RA opti-
mization process. The models’ optimization objectives were consistent with
the criteria chosen by an expert RA planner. Preliminary quantitative results
did not evidence any substantial differences between CPs and KBPs.
Conclusion: RP is able to create models trained by dose distributions of
different modality (HT), suitable to generate clinically acceptable RA plans
for prostate cancer patients.
A.26
SMALL FIELD CHARACTERIZATION OF TRUEBEAM FFF BEAMS WITH A
NEW STEREOTACTIC DIODE: A MULTICENTER STUDY
E. Cagni S. Russo S. Bresciani V. Bruzzaniti M. Esposit D. Fedele
M. Ior S. Naccarato B. Nardiell L. Orsingher G. Raz G. Reggior
A. Rinaldi R. Ruggieri M. Stas A. Stravato L. Strigari M. Zani
P. Mancosu a Fisica Medica, IRCCS-ASMN, Reggio Emilia, Italy; b Fisica Medica,
ASF, Firenze, Italy; c Fisica Medica, Istituto Candiolo-IRCCS, Torino, Italy; d Fisica
Medica, IFO, Roma, Italy; e Radioterapia, Casa di Cura San Rossore, Pisa, Italy;
f Radioterapia, Ospedale Sacro Cuore Don Calabria, Negrar, VR, Italy; g Fisica
Medica, UMPC San Pietro FBF, Roma, Italy; h Fisica Medica, Humanitas Research
Hospital, Rozzano, MI, Italy
Introduction: The Italian Association of Medical Physics (AIFM) insti-
tuted in 2012 a working group dedicated to stereotactic body radiation
therapy (SBRT). The present study was developed in the framework of this
project.
In small fields, the measurement of the output factor is subject to large un-
certainties, impacting the effective delivered dose to the patient. The aim
of this study was to evaluate the dose output ratios (DORs), defined as the
ratio of detector readings without correction factor (Alfonso, Med Phys 2008)
over different TrueBeams (TBs), and to investigate a mathematical descrip-
tion of the DOR curve.
Material and methods: A couple of new unshielded stereotactic diodes
(Razor, IBA) was tested under 7 different TBs using high dose rate (2400
MU/min) 10 MV FFF beams. Small fields, ranging from 6 to 50 mm, were
analyzed in terms of profiles and central axis point measurements. DORs
were normalized to 30 mm field and were calculated as a function of
nominal (NFS) and effective (EFS) field sizes. From DORs acquired using
Razor1 (4 TBs), a theoretical equation was extrapolated by means of a double
exponential fit. The 3 TBs with Razor2 were used to test the mathemati-
cal relationship.
Results: Penumbra and field width (defined as FWHM) analysis over the
7 TBs showed similar results, with mean relative errors of 3.5% and 3.6%
respectively. The EFS were systematically smaller than NFS (p < 0.01) for
all field size ranges, with a mean difference of 0.9 ± 0.5 mm. The DOR fits
using the NFS and EFS had, respectively, R2 = 0.993 and R2 >> 0.999. The test
mean deviations from predicted DORs, using NFS and EFS fits, were 2.9%
and 0.7%, respectively, for field size between 6 and 20 mm. The maximum
deviations (6 mm field size) were 6.1% for NFS and <2% for EFS.
Conclusions: EFS measurements were confirmed to be mandatory when
comparing DORs over different centers. A gold standard curve was tested
and found suitable for DOR calculation using the new Razor diode for TB
10 MV FFF beams.
A.27
QUALITY ASSURANCE OF RAPIDARC RADIOTHERAPY DELIVERY SYSTEM:
RA-COMMISSIONING TEST RESULTS
E. Calabri *, G. Benecchi, R. Rossi, F. Palleri, C. Ghetti. Department of Medical
Physics, University Hospital of Parma, Parma, Italy
Aim: We analyzed the results of 3 RA-commissioning tests which are part
of a VMAT QA procedure in order to evaluate the stability and accuracy of
the system in terms of MLC movement, variable dose-rate and gantry speed.
Materials and methods: We delivered QA files provided by the company
for the accelerator Varian DHX 2100 equipped with Millennium 120 MLC
and Portal Vision imaging system AS 1000. The test images were pro-
cessed with software v.1.3.2 Epiqa Epidos using Artemis module. Test 0.1
(dMLC dosimetry) measures the output at gantry angles of 0°–180°–90°–
270° for a 4 × 10 cm2 DMLC field with a 0.5 cm slit to test the effect of gravity
on leaf position and Clinac dosimetry system. Test 2 (accurate control of
dose rate and gantry speed during RA delivery) uses 7 combinations of dose-
rate, gantry range and gantry speed to give equal dose to seven 1.8 cm strips
in a RapidArc field and an open field irradiation to compensate for beam
profile. Test 3 (accurate control of leaf speed during RA delivery) uses 4
combinations of leaf speed and dose-rate to give equal dose to 4 strips in
a RapidArc field and an open field irradiation to compensate for beam profile.
SW Epiqa displays profiles, values of pre-defined ROIs and deviation from
the reference value within a tolerance value of 2%.
Results: We analyzed the results of monthly tests from February 2010 to
January 2015. For Test 0.1 all 264 data were in the fixed tolerance of 2%
(+1.6% max deviation for 90° and −1.4% for 180° gantry angle). For Test 2
only 5 of 268 data were slightly out of tolerance of 2% (maximum devia-
tion +2.12%), for 2 of these data also Test 3 failed. In Test 3, 12 of 264 data
were more than 2% (max deviations +2.9% and −3.08%).
Conclusions: Clinac performance shows excellent dosimetric stability and
no significant effects of gravity on leaves’ position. Test 3 is the most sig-
nificant, and problems detected in leaf speed were resolved with leaf motors
replacement.
e8 Abstracts/Physica Medica 32 (2016) e1–e70