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

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verts EPID images to fluence and a pencil beam algorithm to calculate the
dose. It can be used for pretreatment QA verification and for in vivo do-
simetry. In this work we evaluated the suitability of DC software for VMAT
verifications.
Materials and methods: DC (v4.10) was used along with Elekta Synergy®
Linac (6 and 10 MV beams) equipped with a-Si electronic portal imaging
device (EPID) Iview-GT. Twenty VMAT (5 prostate, 5 whole pelvis, 5 lung,
5 head and neck), elaborated by treatment planning system (TPS) Elekta
Monaco® 5.0, were measured. Through-air (EPID T-A) and transit EPID
images were used for three dimensional dose map reconstruction in ho-
mogeneous phantoms. Octavius 4D with 729 2D array was used as a
reference. Gamma analysis at 3% local dose/3 mm DTA was performed. Doses
from though-air measurements were also reconstructed in the planning
CT (T-A in plan TC) and compared with the treatment planning dose maps.
Gamma pass rate of DC dose maps was compared with those of 729 in the
Octavius 4D.
Results: The assessment of VMAT plans shows a mean of 93.9% points with
gamma < 1 for Octavious 4D (3.5% SD), 89.2% and 7.9% SD for EPID T-A, 89%
and 8.5% SD for EPID transit and 89.8% and 6.1% SD for T-A in plan CT. Transit
andthrough-airEPIDacquisitionsproducedsimilargammapassrates.Through-
air EPID images computed by DC in the planning CT showed gamma pass
rate in agreement with those of Octavious 4D in the prostate, whole pelvis
and head and neck, in lung instead, gamma pass rates were lower in 4/5 cases.
Conclusions: DC is a suitable tool for VMAT verifications. The pencil beam
algorithm can be inaccurate in the presence of low-density inhomogeneities.
A.73
CHARACTERIZATION OF HIGH-DOSE-PER-PULSE INTRAOPERATIVE
RADIATION THERAPY ELECTRON BEAMS BY USING A MICRODIAMOND
DOSIMETER
M. Falco M. Marinelli A. Tonnetti G. Verona Rinat M. Pimpinella
A. Ciccotelli S. De Stefano G. Felici F. Marangoni a Department of
Radiation Oncology G. D’Annunzio, Università di Chieti SS. Annunziata Hospital,
Chieti, Italy; b INFN – Dipartimento di Ingegneria Industriale, Università di Roma
Tor Vergata, Via del Politecnico 1, 00133, Roma, Italy; c Istituto Nazionale di
Metrologia delle Radiazioni Ionizzanti, ENEA-INMRI C R Casaccia, Roma, Italy;
d SIT – Sordina IORT Technologies S.p.A., Aprilia, Italy
Purpose: Accurate clinical dosimetry of electron beams produced by linear
accelerators dedicated to intraoperative radiation therapy (IORT) is chal-
lenging due to the very high dose-per-pulse (0.3–10 cGy per pulse) as
compared to conventional accelerators (<0.01 cGy per pulse). The aim of
this work is to characterize electron beams produced by a dedicated mobile
accelerator for IORT by using the microDiamond 60019 PTW Freiburg
dosimeter.
Materials and methods: A dosimetric characterization of electron beams
produced by a NOVAC11 IORT accelerator (S.I.T. – Sordina IORT Technolo-
gies) in the 6–9 MeV energy range, with dose rate from 26 to 105 mGy per
pulse, was performed. The response of the mD was first tested in terms
of dose linearity. Percentage depth dose (PDD) curves, beam profiles and
output factors were then measured and compared with those obtained by
an Advanced Markus ionization chamber (AM-IC), previously character-
ized for measurements in high dose per pulse electron beams, and a PTW
silicon diode E (Si-D).
Results: The microDiamond showed a linear dose response, independent
of dose per pulse, in the dose range from 0.2 Gy to 28 Gy. PDD measure-
ments were found in agreement with the ones obtained by using the AM-
IC and Si-D, with differences in R50 values below 0.3 mm. Profile
measurements evidenced a high spatial resolution of the mD, slightly worse
than the one of the silicon diode, being the differences in penumbra values
no larger than 0.5 mm. OFs measured by the mD were found to be within
2% with respect to those obtained by the AM-IC, in field sizes down to 3 cm
in diameter.
Conclusions: The performed dosimetric tests evidenced that the
microDiamond dosimeter is suitable for accurate relative dosimetry in high
dose per pulse IORT electron beams.
A.74
COMPARISON BETWEEN 2D AND 3D IMRT PRE-TREATMENT
VERIFICATION
Y.A.C. Fiagan F. Simonato A. Roggio M.A. Rossato A. Scaggion
R. Zandonà M. Paiusco a ICTP – Abdus Salam International Centre for
Theoretical Physics, Trieste, Italy; b Istituto Oncologico Veneto IOV – IRCCS,
Padova, Italy
Introduction: Patient-specific pretreatment verification of intensity modu-
lated radiation therapy (IMRT) treatments can be performed using software
that modifies the dose distribution calculated by the treatment planning
system (TPS) according to the dose discrepancies detected during dosim-
etry measurements. The aim of this work is to investigate the sensitivity
of patient-related gamma analysis and DVH-based metrics to the geome-
try of the dosimeter used as verification system.
Materials and methods: 10 prostate and 10 H&N cases were planned with
IMRT technique. Pre-treatment verifications were performed by acquir-
ing planar per-beam dose distributions with MapCHECK and cumulative
dose distributions with ArcCHECK. TPS dose distributions were com-
pared to the dose distributions reconstructed by 3DVH software via
MapCHECK and ArcCHECK measurements. 2D gamma analysis in phantom
geometry, 3D gamma analysis in patient geometry and patient-specific DVH
metrics were used to assess the dosimetric accuracy.
Results:Planar per-beam 2D dosimetry is shown to produce generally larger
gamma passing rate (PR%) than composite pseudo-3D cylindrical dosim-
etry. Similarly, 3D gamma analysis based on planar measurements yielded
larger PR% values than that based on cylindrical measurements. In general
5% ± 3.5% larger PR% is found when using 3D instead of 2D analysis. DVH
analysis revealed generally small differences between TPS dose distribu-
tion and 3DVH dose distribution for both dosimetric systems. DVH variations
based on cylindrical measurement are found to be generally larger than
that coming from planar dosimetry, but no statistically significant devia-
tion from TPS DVH is found.
Conclusions: A pseudo-3D dosimeter is preferable to a planar one if gamma
metric is chosen to evaluate delivery accuracy IMRT pre-treatment veri-
fication. Conversely, the geometry of the dosimeter does not affect DVH-
related metric as obtained from 3DVH software and the two dosimeters
can be considered equivalent.
A.75
PATIENT SPECIFIC CBCT CALIBRATION FOR DOSIMETRIC EVALUATION OF
HYBRID RT-PLAN
A. Fidanzio * S. Menn F. Grec L. Azario A. Porcelli G. Benecchi
A. Piermattei a U.O.C. Fisica Sanitaria UCSC, Roma, Italy; b UO di Fisica
Sanitaria Azienda Ospedaliaro-Universitaria, Parma, Italy
Introduction: Several factors such as patient setup and the anatomical
changes can affect the dose delivery in radiotherapy. The cone beam CT
(CBCT) allows a timely assessment of the treatment and its calibration, in
terms of relative electron densities (RED), allows hybrid plan calculation
useful to adopt an adaptive strategy. However the CBCT calibration suffers
from some problems such as time stability and patient variability. This work
reports the dosimetric assessment of an original patient-specific CBCT cal-
ibration method.
Methods and materials: An automated procedure was adopted to cali-
brate CBCT images in terms of RED adopting the following steps: (1) two
CT and CBCT scans with negligible morphological changes are selected for
a patient, (2) in these images the HU values of different ROIs, relative to
correspondent anatomical regions, are acquired to obtain a correlation func-
tion between CBCT and CT HUs, (3) the correlation function is used to
determine the CBCT calibration curve HUs versus RED from the CT cali-
bration curve; (4) finally the CBCT calibration curve is optimized by an
algorithm that minimizes the differences in the patient’s radiological thick-
nesses measured on the CT and CBCT patient’s slices. The procedure has
been verify for H&N, lung and pelvic body regions in Rando phantom and
for 15 VMAT patients irradiated by a linac Varian TrueBeam STx. Using
Eclipse TPS the dosimetric assessment of the method was based on: the
ratio between isocenter doses; the gamma analysis between the dose ma-
trices of planes passing through the isocenter and DVH comparisons.
e22 Abstracts/Physica Medica 32 (2016) e1–e70