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

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characterization it is possible to use Gafchromic films, but next experi-
mental measurements are planned with other dosimeters.
A.117
OPTIMIZATION OF BIOLOGICAL DOSE AFTER NOT PLANNED
INTERRUPTION IN PATIENTS WITH HEAD AND NECK SQUAMOUS CELL
CARCINOMAS (HNSCC) TREATED BY EBRT
T. Licciardello L. Barone Tonghi A. Rosso a Università degli Studi di
Catania, Catania, Italy; b ARNAS Garibaldi U.O.S.D. di Fisica Sanitaria, Catania,
Italy; c ARNAS Garibaldi U.O.C. di Radioterapia, Catania, Italy
Introduction: What happens if a radical radiotherapy treatment is inter-
rupted? If it is not completed within the prescribed time but in a longer
time? Radiobiological studies conducted on patients with Head and Neck
Squamous Cell Carcinomas (HNSCC) who underwent unscheduled inter-
ruption of radiation treatment (e. g. due to holidays, technical failures, Linac
maintenance, patients’ personal problems), induced us to highlight how
the interruption itself may compromise the therapeutic efficacy of the ra-
diation treatment.
Materials and Methods: Six-day treatment interruption of a HNSCC was
considered. Standard dose fractioning planning was performed, that is a
total dose of 70 Gy divided in 35 fractions of 2 Gy daily.
In addition to the Physical Dose (or Total Dose) D, administered by pro-
viding a EBRT (being D = N · d, with “N” number of fractions, and “d” dose
per fraction), biological equivalent dose (BED) was also considered. BED
depends on specific tumor radiosensitivity, total time of treatment and ra-
diobiological parameters (doubling time of the tumor cells, kick-off time,
i.e. the time in days in which tumor cell re-population is delayed due to
the irradiation).
Results: In the event of treatment interruptions, to release the entire pre-
scribed Physical Dose it is necessary to postpone the end of the treatment.
Compared with a non-interrupted treatment, our study shows a drop in
BED value on the tumor and in Tumour Control Probability (TCP) index of
9.59% and 3.83% respectively. It is irrelevant the period during which the
six-day interruption occurs.
Conclusions: Therapeutic efficacy collapses when an interruption occurs.
It is thus recommended not to further extend the total treatment time. To
preserve the efficacy of a radical radiation treatment, several compensa-
tion methods must be considered. Different ones can be proposed according
to the period in which interruption occurs, such as weekend irradiation,
twice-a-day irradiation, or calculating new dose per fraction.
A.118
EXPERIMENTAL VALIDATION OF AN IBEAM® EVO ELEKTA COUCH MODEL
IN PHILIPS PINNACLE3 TPS
S. Linsalata *, V. Ravaglia, A. Lazzari. S.C. Fisica Saniatria – Azienda USL n. 2
Lucca, Lucca, Italy
Introduction: Carbon fiber couch modelling in a TPS is of primary impor-
tance in order to deliver accurate radiation therapy treatments, especially
in rotational techniques. This is done in different ways. Here we report the
experimental validation of an hybrid solution adopted in our institution
using a combination of density override for a specific shell ROI represent-
ing the Elekta iBEAM® evo couchtop and the CIVCO UC (Universal Couchtop)
mounted in our CT simulator.
Material and Methods: Measurements of 10 × 10 cm2 fields were done by
ionization chambers in two different experimental setup: (1) PTW 30013
Farmer chamber in a 10 cm thick slab Solid Water phantom; (2) PTW 31010
Semiflex chamber in the PTW Octavius3D phantom. In both cases we rotate
chamber orientation in order to mach gantry orientation.
Calculations were done by the TPS Philips Pinnacle3 v 9.8 on both phan-
toms CTs.
Measurements in slab were used to choose the best values of density over-
ride for the shell ROI couch model at all photon energies (6, 6FFF, 10, and
15 MV), then couch attenuation was measured in Octavius3D phantom for
particular oblique posterior gantry positions.
Results: Couch attenuation dose measurements in slab led us to choose
the values of 0.7 g/cm3 for the outer carbon fiber shell ROI density and of
0.0 g/cm3 for the inner one. Using these values, comparison of measured
and calculated absorbed doses in slab was within 1.0% for all photon energies.
Measurements in Octavius3D phantom for many significant oblique pos-
terior gantry orientations were used to assess couch model reliability. For
these orientations, experimental couch attenuation is between 2% and 7%
and difference between calculated and measured absorbed doses is within
1.5%.
Conclusions: A combination of density override in a shell ROI couch model
and CT scan of our CT simulator couchtop offer a very good model of iBEAM®
evo Elekta couch, showing a general agreement within 1.5% between cal-
culated and measured absorbed doses.
A.119
EVALUATION OF PLANNING TARGET VOLUME MARGINS BY ANALYZING
INTRA-FRACTION PATIENT MOVEMENTS DURING STEREOTACTIC BODY
RADIATION THERAPY (SBRT) OF LUNG LESIONS
M. Liotta A. Fass P. Tabarelli De Fatis I. Meaglia P. Porcu C. Bocci
M. Riboldi G. Baroni G. Ivald a Fondazione S. Maugeri, Servizio di Fisica
Sanitaria, Pavia, Italy; b Politecnico di Milano, dip. Elettronica Informazione e
Bioingegneria, Milano, Italy; c Fondazione S. Maugeri, Servizio Di Radioterapia,
Pavia, Italy
Introduction: To evaluate the adequacy of the adopted planning margins
in SBRT estimating the intra-fraction movements obtained with Cone Beam
CT (CBCT) and a non invasive infrared Optical Tracking System (OTS).
Material and Methods: 14 patients with lung lesions were treated using
6 MV photons volumetric-modulated arcs. OTS SMART-DX (BTS Bioengi-
neering, Italy) was used to record the 3D coordinates of passive markers
(6–8) placed on the patient’s thoraco-abdominal surface. Both ungated CT
for treatment planning and 4DCT for CTV were acquired and a 5 mm iso-
tropic PTV was generated. Before treatment, a CBCT was acquired and
registered to the planning CT to obtain the setup corrections (only trans-
lations). After irradiation a second CBCT was performed and rigidly registered
to the first with a mutual information algorithm focusing on the CTV region.
A rigid transformation was also estimated from surface markers coordi-
nates just before the CBCT scans. Setup corrections were subtracted from
the rototranslation parameters obtained from both CBCT and OTS. Results
for both CBCT and OTS methods were evaluated regardless of rotations co-
ordinates (always less than 1°).
Results: In 39 analyzed fractions the mean absolute values of transla-
tional displacements obtained with the CBCT were 0.6 ± 0.9mm, 0.7± 1.0mm
and 1.0 ± 1.0 mm respectively in latero-lateral (LL), antero-posterior (AP)
and cranio-caudal (CC) direction. The same analysis achieved in 34 frac-
tions with surface markers revealed absolute displacements of 1.0 ± 1.0 mm
in LL, 1.3 ± 0.9 mm in AP and 1.7 ± 1.8 mm in CC. Comparing the shifts ob-
tained in the same sessions, the mean difference was 1.0 ± 1.2 mm in LL,
1.6 ± 1.2 mm in AP and 1.6 ± 1.6 mm in CC.
Conclusions: The intra-fraction displacements obtained through CTV over-
lapping using CBCT and the surface markers registration are consistent with
PTV margin employed in our facility. The non-invasive OTS could be con-
sidered as alternative to a post treatment CBCT to monitor the intra-
fraction movements.
A.120
EVALUATION OF A COMMERCIAL AUTOMATED INVERSE PLANNING
SYSTEM FOR H&N TREATMENTS
G. Loi * M. Fusella F. Puricelli C. Secco L. Masini M. Krengli
a Department of Medical Physics, A.O.U. Maggiore della Carità, Novara, Italy;
b Departmebt of Radiation Oncology, A.O.U. Maggiore della Carità, Novara, Italy
Purpose: To compare the quality of intensity modulated radiation therapy
(IMRT) plans generated by a commercial automated inverse planning with
those planned by experienced operators.
Methods and Materials: Five patients with head and neck cancer were se-
lected. For each patient an IMRT SIB treatment was prescribed to deliver
e35 Abstracts/Physica Medica 32 (2016) e1–e70