17 / 146
A.38
IMPLEMENTATION OF A QUALITY ASSURANCE METHOD FOR VMAT
TECHNIQUES ON AN ELEKTA SYNERGY LINAC USING AN IN-HOUSE
DEVELOPED SOFTWARE AND A ROTATIONAL PHANTOM
M. Chieregato
* , a , b ,M. Galelli
b ,S. Ren Kaiser
b ,C. Bassett
i b ,M. Bignard
i b .a
Scuola di Specializzazione Fisica Medica, Università di Milano, Milano, Italy;
b
Fondazione Poliambulanza Istituto Ospedaliero, Brescia, Italy
Introduction:
The relative effectiveness of periodic machine QA or patient
pre-treatment dosimetry for IMRT is debated (Siochi et al., Med. Phys. 40(7),
2013). VMAT machine QA, in particular, are difficult and time consuming.
The goal of our study is to assess the feasibility of linac tests that take into
account the peculiarity of VMAT treatments (gantry rotation with vari-
able speed and dose rate erogation, MLC moving leaves) using an ion
chamber array in a rotational phantom and an in-house developed software.
Materials and methods:
Tests are performed on an Elekta Synergy with a
160 leaf Agility collimator. Doses are measured with a planar 2d array of
27 rows and 27 columns of ion chambers 1 cm apart (PTW 2dArray 729)
in a rotating phantom (PTW Octavius 4D). Custom QA beams are gener-
ated using Elekta iCOMCat software following the recommendations from
the Nederland Commission on Radiation Dosimetry codes of practice for
IMRT (NCS22, 2013) and VMAT (NCS24, 2015). Raw detector data are pro-
cessed with a software written in MATLAB (Mathworks Inc.) in order to
extract measured values (without interpolation) and corresponding gantry
angles from the Octavius inclinometer. Stability of dose/m.u. and dose pro-
files is verified as a function of gantry angle in different conditions (fixed
angles, rotating gantry with constant or variable speed and dose rate
variation).
Results:
We developed a method for VMAT machine testing in a reason-
able time, without tailor-made devices. The same experimental set-up can
be used in the same session for patient QA. The in-house MATLAB-built
software allows analysis customization and makes possible to exploit
MATLAB graphical and statistical functions.
Conclusion:
A linac QA program specific for VMAT can replace most of pre-
treatment patient QA that could be reserved to cases that push the technique
to its limits. Pulling together both kinds of tests should allow an in-
creased efficiency in uncovering clinically relevant dose errors.
http://dx.doi.org/10.1016/j.ejmp.2016.01.042A.39
DOSE ACCUMULATION AND REPLANNING IN H&N PATIENT: A STEP
TOWARD IMPLEMENTATION OF ART IN CLINICAL PRACTICE
A. Ciarmatori
*
, a , b ,G. Guidi
a ,N. Maffei
a , c ,C. Vecchi
c ,A. Bernabei
a ,M.G. Mistretta
a ,P. Ceroni
a ,B. Meduri
d ,P. Giacobazzi
d ,T. Cost
i a .a
Medical
Physics Department, Az. Ospedaliero Universitaria Policlinico, Modena, Italy;
b
Post Graduate School in Medical Physics, Alma Mater Studiorum University,
Bologna, Italy;
c
Physics Department, Alma Mater Studiorum University, Bologna,
Italy;
d
Radiation Oncology Department, Az. Ospedaliero Universitaria Policlinico,
Modena, Italy
Introduction:
Shrinkage of parotid glands (PG) in head and neck (H&N)
patients is a major issue in adaptive radiation therapy (ART). This study
focuses on target and organs at risks (OARs) anatomical variation, their effect
on dosimetric distribution and the possible dosimetric benefit from
re-planning.
Material and methods:
16 patients with H&N cancer treated by
tomotherapy were, retrospectively, enrolled. Dose distribution (DMVCT) has
been re-calculated on pre-treatment MVCT. The planned doses were mapped
(DDVF) using the deformed vector field (DVF) matrix obtained from de-
formable image registration (DIR). DMVCT have been deformed to match
the planning kVCT in a single dataset. DDVF and DMVCT were compared
by performing gamma-analysis (2 mm, 2% of 2.2 Gy). 2 ART strategies were
adopted on the 18th fractions: (1) re-plan on original target and de-
formed OARs (D18,OAR) and (2) re-plan on both deformed target and
deformed OARs (D18).
Results:
DDVF and DMVCT show a discrepancy (gamma 3D
=
85
±
1%,
p
<
0.001). A systematic underestimation between DDVF and DMVCT
(12.2
±
10.3 %) was found in average doses in PG. For what concern human
resources, weekly protocol for 25 patients/day needs besides 1 medical phys-
icist, 30% and 15% of daily time of a dosimetrist and radiation oncologist
respectively. Dose accumulation showed an increase of the average of
2.4 Gy
±
2.2 Gy for PG. Average doses of the PTVs increase by 1.4 Gy
±
1.2 Gy.
Retrospective re-planning analysis showed that 13 out of 16 (81.1%) pa-
tients enrolled could have benefited from ART. By ART the PG average dose
decreased by
−
2.0 Gy
±
1.4 Gy in case of only OARs warping and by
−
3.2 Gy
±
1.7 Gy in case of both target and OARs deformation.
Conclusions:
Anatomical variations are possible cause of PG over-dosage.
Re-planning on the 18th MVCT brought significant benefits in terms of
average dose of PG. Supported by an automated workflow, a weekly re-
calculation seems to be sustainable in clinical practice.
http://dx.doi.org/10.1016/j.ejmp.2016.01.043A.40
MODELLING SEVERE LATE RECTAL BLEEDING: RESULTS ON A LARGE
POOLED POPULATION OF PROSTATE CANCER PATIENTS
A. Cicchetti
* , a ,T. Rancat
i a ,M. Ebert
b , c ,C. Fiorino
d ,A. Kenned
y c ,D.J. Josep
h c ,J.W. Denha
m e ,V. Vavassori
f ,G. Felli
n g ,R. Valdagni
a .a
Fondazione IRCCS
Istituto Nazionale dei Tumori, Milan, Italy;
b
University of Western Australia,
Perth, Western Australia, Australia;
c
Sir Charles Gairdner Hospital, Perth,
Western Australia, Australia;
d
San Raffaele Scientific Institute, Milan, Italy;
e
University of Newcastle, Newcastle, New South Wales, Australia;
f
Cliniche
Humanitas-Gavazzeni, Bergamo, Italy;
g
Ospedale Santa Chiara, Trento, Italy
Purpose:
To develop a model for grade 3 (G3) late rectal bleeding (LRB)
after radical radiotherapy (RT) for prostate cancer, in a pooled population
from 2 large prospective trials: Airopros0102 (Fellin RO2014) and TROG
03.04 RADAR (Ebert IJROBP2015).
Materials and methods:
The trials included patients treated with a con-
ventional fractionated 3DCRT at 66–80 Gy. Planning data were available for
all patients. G3 LRB was scored using the LENT/SOMA questionnaire, with a
minimum follow-up of 36 months. Rectal DVH were reduced to equivalent
uniformdose (EUD) calculatedwith volume parameter n derived by 3 studies:
n
=
0.018 (Defraene IJROBP2011), n
=
0.05 (Rancati RO2011) and n
=
0.06
(Rancati RO2004). EUD was inserted into multivariable logistic regression
(MVL) together with clinical features. Irradiation of seminal vesicles (ISV),
irradiation of pelvic nodes, hormonal therapy, hypertension, previous ab-
dominal surgery (SURG), use of anticoagulants, diabetes, presence of
cardiovascular diseases (PCD) and of acute toxicity were considered as po-
tential dose-modifying factors. Calibration was evaluated with a Hosmer–
Lemeshow (HL) test and AUC was used for evaluating discrimination power.
Results:
1337 patients were available. G3 LRB was scored in 95 patients
(7.1%). EUD calculated with the volume parameter n
=
0.06 was the best do-
simetric predictor for G3 LRB. A 4-variable MVL model was fitted including
EUD (OR
=
1.07 p
=
0.02), ISV (OR
=
4.75 p
=
0.01), SURG (OR
=
2.30 p
=
0.02)
and PCD (OR
=
1.42 p
=
0.18). This model had an AUC
=
0.63, a slope m
=
0.99
(R
2
=
0.89) and a p-HL
=
0.43. Inclusion of acute toxicity (OR
=
2.34 p
<
0.001)
slightly improved AUC (0.65), confirming a possible role of consequential
injury.
Conclusions:
EUD with n
=
0.06 was predictive of G3 LRB in this pooled
population, confirming the importance of sparing the rectum from high
doses. ISV together with the PCD and previous SURG were relevant dose-
modifying factors highly impacting the incidence of G3 LRB.
Acknowledgements:
The study was funded by AIRC IG16087, Fondazione
Monzino, NHMRC (300705, 455521, 1006447).
http://dx.doi.org/10.1016/j.ejmp.2016.01.044A.41
AN INNOVATIVE TOOL FOR IMAGE GUIDED INTRA-OPERATIVE
RADIOTHERAPY (IGIORT): THE VALIDATION PROCESS OF A DEDICATED
TREATMENT PLANNING SYSTEM
A. Ciccotelli
* , a , b ,S. Carpin
o b ,M. D’Andrea
b ,G. Iaccarin
o b ,A. Soriani
b ,G. Felici
a ,M. Benassi
c ,L. Strigari
b .a
R&D Department, S.I.T. Sordina IORT
Technologies S.p.A., Aprilia, LT, Italy;
b
Laboratory of Medical Physics and Expert
Systems, National Cancer Institute Regina Elena, Rome, Italy;
c
Physics
Department, IRCCS Istituto Scientifico Romagnolo per lo Studio e la Cura dei
Tumori IRST, Meldola, FC, Italy
Introduction:
The image guided intra-operative radiotherapy (IGIORT) is
a new methodology based on the planning optimization using intra-
e12
Abstracts/Physica Medica 32 (2016) e1–e70