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

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operative target images acquired after surgery. The dedicated treatment
planning system (TPS) CSRAD+ has been developed in order to plan intra-
operative radiotherapy treatments for patients with malignant diseases as
clinically appropriate, using a dedicated mobile accelerator and an imaging
device. The CSRAD+ performs IORT dose distribution calculation relying on
pre-treatment and intra-operative DICOM_RT images. The aim of this work
is to validate the dosimetric output and the performances of CSRAD+ before
its introduction in clinical practice.
Material and methods: The home-made CSRAD+ allows to calculate the
dose distributions of a IORT dedicated mobile linac using Monte Carlo data.
Two dose calculation algorithms have been implemented both with and
without inhomogeneity corrections. The DICOM images of the represen-
tative phantom test cases were acquired using a dedicated CT scan. The
Monte Carlo simulation was performed using DOSXYZnrc MC code.
Gafchromic EBT 3 was used in the test phantoms in order to measure the
planar dose distribution and to determine isodoses. Phantom test cases were
designed to validate the accuracy of the implemented dose calculation al-
gorithms. An independent tool for data analysis has been implementedwith
Matlab®.
Results: The test cases were reproduced experimentally and the refer-
ence measurements were performed with the LIAC mobile IORT accelerator.
The proposed test cases have shown a good agreement between mea-
sured and calculated dose distributions in the relevant experimental setups
containing both horizontal and lateral inhomogeneities.
Conclusion: The developed tool allows independent validation of algo-
rithms implemented within CSRAD+ and MC for absolute dose calculations.
The method can test patient-like geometries and more complicated
setups.
A.42
REFERENCE DOSIMETRY FOR IOERT DEDICATED MOBILE LINACS: HOW
DO WE APPLY INTERNATIONAL DOSIMETRIC PROTOCOLS?
A. Ciccotelli G. Felici A. Soriani M. D’Andrea G. Iaccarino
L. Strigari P. Tabarelli De Fatis M. Liott S. De Stefano F. Marangon
M. Di Francesco A. Leggieri P. Scalchi 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, Roma, Italy; c Fondazione
Salvatore Maugeri, Servizio di Fisica Sanitaria, Pavia, Italy; d Medical Physics
Department, San Bortolo Hospital, Vicenza, Italy
Introduction: In the past years, there has been a growing interest in
IOERT using dedicated electron linacs, as LIAC and NOVAC (SIT, Italy).
These, when compared to traditional external beam radiotherapy accel-
erators, are characterized by smaller field sizes and SSD as well as higher
dose per pulse and degraded energy spectrum, thus making the applica-
tion of international protocols questionable. For instance the standard
two voltage analysis (TVA) for determining the correction factor associ-
ated with ion recombination in ionization chambers leads to significant
errors. This paper investigates systematically the procedures and results
obtained by using and integrating the international protocols (IAEA TRS
398, AAPM TG51 and DIN 6800-2) with the TVA corrected for the high
dose per pulse range.
Material and methods: The measurements of electron beams have been
performed using plane-parallel chambers (PTW Roos and Adv. Markus, IBA
PPC05) in the framework of the three above-mentioned international pro-
tocols, with NOVAC and LIAC. TVA has been executed with the corrections
proposed by the INMRI-ENEA group. The determination of water-absorbed
dose has also been checked by a dose per pulse independent procedure
using reference dosimeters (Fricke, TLD). In addition, dose has been mea-
sured with radio-chromic films. The characterization has been carried out
on more than ten IOERT dedicated mobile linacs.
Results: The absorbed dose differences, evaluated among the three pro-
tocols, were less than 1.2%. The determinations of water-absorbed dose of
ionization chambers and reference dosimeters were in agreement within
3%. The dose measurements with radio-chromic films were within 3% with
respect to ionization chamber evaluations.
Conclusion: High dose per pulse IOERT accelerators can be safely charac-
terized by properly modifying the main international protocols; Fricke and
TLD dosimetry can thus be considered as an auxiliary external audit and
not as a mandatory reference.
A.43
USING STATISTICAL PROCESS CONTROL (SPC) FOR PATIENT-SPECIFIC
VMAT QUALITY ASSURANCE: A SIX-YEAR RETROSPECTIVE ANALYSIS ON
1200 PATIENTS
S. Cilla * P. Viola M. Craus A. Ianiro F. Deodato G. Macchia
C. Digesu’ V. Valentini A. Piermattei A.G. Morganti a Fondazione di
Ricerca e Cura Giovanni Paolo II – Università Cattolica del Sacro Cuore,
Campobasso, Italy; b Policlinico Universitario A. Gemelli – Università Cattolica
del S. Cuore, Roma, Italy; c DIMES Università di Bologna – Ospedale S. Orsola
Malpighi, Bologna, Italy
Purpose: We applied statistical process control (SPC), a tool widely used
in industrial engineering, to our routine VMAT pre-treatment verification
QA.
Materials and methods: Since 2009, more than 1200 patients were treated
with Elekta VMAT at our institution. Plans were re-grouped according to
treatment technique and disease sites: (1) 524 high-modulated complex
SIB treatments (head–neck, brain, gynecological, ano-rectal); (2) 298 pros-
tate treatments and (3) 380 liver, lung and other metastasis treated with
extracranial stereotactic radiotherapy (SBRT). Group 1 and 2–3 plans were
optimized with Oncentra Masterplan (dual-arc) and Ergo++ (single arc) TPS.
A total of 3452 dose measurements were performed with the PTW Seven29
array/Octavius phantom, both on coronal and sagittal planes. Dose com-
parisons were evaluated using 3%/3 mm γ-analysis. Two metrics were
evaluated: (a) points-percentage with g-value less than one (g%) and (b)
mean gamma (gmean). Clinical specifications were: g% > 90% and
gmean < 0.67. Shewhart charts were used to calculate the central (CL), upper
control (UCL) and lower control limits (LCL). The process capability was
evaluated by means of Cp5.15 index.
Results: γ pass-rate values significantly depend on plan complexity. For g%,
CL and LCL were 95.0% and 90.3%, 99.1% and 96.8%, and 98.7% and 96.8%,
for groups 1, 2 and 3 respectively. For gmean, CL and UCL were 0.399 and
0.503, 0.364 and 0.498, and 0.323 and 0.448, for groups 1, 2 and 3 respec-
tively. In all cases, the control limits are well within the clinical specifications.
The Cp5.15 capability indices for g% and gmean resulted equal to 0.62 and
1.70 in group 1; 1.84 and 2.08 in group 2; 2.14 and 2.22 in group 3, re-
spectively. g% process for group 1 was not capable at 3%/3 mm; with 5%-
3 mm specification, CL and LCL resulted 98.4% and 96.1% and Cp was 1.26.
Conclusion: SPC is useful to quantifiably demonstrate the QA process con-
formance to clinical specifications.
A.44
OPTIMAL DOSE PRESCRIPTION IN LINAC-BASED STEREOTACTIC
EXTRACRANIAL RADIOTHERAPY (SBRT) WITH VMAT TECHNIQUE: AN
INVESTIGATION USING “PARETO FRONTS”
S. Cilla A. Ianiro F. Deodato G. Macchia C. Digesu’ P. Viola
M. Craus A. Piermattei V. Valentini A.G. Morgant a Fondazione di
Ricerca e Cura Giovanni Paolo II – Università Cattolica del S. Cuore, Campobasso,
Italy; b Policlinico Universitario A. Gemelli – Università Cattolica del S. Cuore,
Roma, Italy; c DIMES Università di Bologna – Ospedale s.Orsola Malpighi,
Bologna, Italy
Purpose: Pareto fronts are a powerful mathematical strategy to formalize
the trade-off between a given set of mutually contradicting objectives. We
use this strategy to determine the optimal block margin and prescription
isodose for both optimal target coverage and normal tissue sparing for VMAT
treatments in SBRT.
Material and methods: Three spherical-shaped targets (PTVs = 20, 55 and
101 cc) were selected. A single fraction dose of 26 Gy was prescribed (PD).
VMAT plans were generated with Ergo++ TPS using a 10 MV single arc. Pareto
fronts based on (i) different MLC block margins around PTV (ranging from
+4 mm to −2 mm) and (ii) different prescription isodose line (IDS) ranging
from 50% to 100% of PD were produced. For each block margin, the great-
est IDS fulfilling the two criteria: 95% and 99% of PTV reached 100% and
e13 Abstracts/Physica Medica 32 (2016) e1–e70