ESTRO 36 Abstract Book

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S281 ESTRO 36 2017
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angle, MLC position, and MU were 0.16° ± 0.01° (range,
0.12°–0.17°), 0.08° ± 0.00° (range, 0.0.7°–0.08°), 0.08 ±
0.02 mm (range, 0.04–0.11 mm), and 0.37 ± 0.05 MU
(range, 0.30–0.44 MU), respectively. In the delivered dose
reconstruction, the means ± SDs of the dose difference of
the all dose-volumetric indices were 0.5% ± 0.8% (range,
0.0%–4.2%) and 0.2% ± 0.2% (range, 0.0%-0.7%) for the
brain and prostate tumors, respectively.
Conclusion
We have established patient-specific QA procedure for the
DWA using ArcCHECK and log files. Our results have shown
that DWA with Vero4DRT delivered the accurate dose
distribution.
OC-0535 Multicenter validation of ion chambers in
reference dosimetry of two IORT-dedicated electron
linacs
P. Scalchi1, G. Felici2, A. Ciccotelli2, A. Petrucci3, V.
Piazzi4, N. Romeo5, A. Pentiricci6, F. Cavagnetto7, S.
Andreoli8 , F. Cattani9, S. Fabbri10, P. Tabarelli de Fatis11,
R. Romagnoli12, A. Soriani13, B. Augelli14 , M. Paolucci15, P.
D'Avenia16, M. Bertolini17, R. Massafra18, E. Moretti 19, S.
De Stefano2, L. Grasso2, C. Baiocchi20, P. Francescon1
1 Ospedale San Bortolo, MEDICAL PHYSICS, Vicenza, Italy
2 SIT, R&D, Aprilia, Italy
3 S. Filippo Neri Hospital, MEDICAL PHYSICS, Rome, Italy
4 Multimedica Hospital, RADIOTHERAPY, Castellanza,
Italy
5 S. Vincenzo Hospital- ASL Messina, RADIOTHERAPY,
Taormina, Italy
6 ASL Umbria 1, RADIOTHERAPY, Città di Castello, Italy
7 IRCCS A.O.U. SAN MARTINO-IST, MEDICAL PHYSICS,
Genova, Italy
8 ASST Papa Giovanni XXIII, MEDICAL PHYSICS, Bergamo,
Italy
9 European Institute of Oncology, MEDICAL PHYSICS,
Milan, Italy
10 S.Anna University-Hospital, MEDICAL PHYSICS, Ferrara,
Italy
11 Fondazione Salvatore Maugeri, MEDICAL PHYSICS,
Pavia, Italy
12 Bellaria Hospital, MEDICAL PHYSICS, Bologna, Italy
13 National Cancer Istitute Regina Elena, Laboratory of
Medical Physics and Expert System, Rome, Italy
14 Spoleto Hospital- A.U.S.L. Umbria 2, RADIOTHERAPY,
Spoleto, Italy
15Foligno Hospital- A.U.S.L. Umbria 2, MEDICAL PHYSICS,
Foligno, Italy
16ASUR MARCHE AV3, MEDICAL PHYSICS, Macerata, Italy
17Arcispedale S. Maria Nuova-I.R.C.C.S., MEDICAL
PHYSICS, Reggio Emilia, Italy
18I.R.C.C.S. Institute of Oncology Giovanni Paolo II,
MEDICAL PHYSICS, Bari, Italy
19“Santa Maria della Misericordia" University-Hospital,
MEDICAL PHYSICS, Udine, Italy
20Ospedale San Bortolo, RADIOTHERAPY, Vicenza, Italy
Purpose or Objective
LIAC and NOVAC (SIT, Italy) are two mobile linacs
dedicated to IORT generating electron beams in the
energy range of 3-12 MeV. Due to the large amount of
scattered electrons from the collimator walls inside the
IORT field, their energy spectra are very different from
the traditional linacs on which are based the international
dosimetry protocols. In addition, the methods
recommended by these protocols to determine the ion-
recombination correction factor (ks) fail under these high
dose-per-pulse beams. Hence in 2003 the Italian Health
Institute stated that ion chambers cannot be used for
reference dosimetry of these linacs. Based on a
retrospective multi-center survey, a comparison with
ferrous sulphate dosimetry is now used to validate
parallel-plate ion chambers for this purpose.
Material and Methods
17 centers participating in this study had modified the
IAEA TRS-398 dosimetry protocol regarding the reference
irradiation setup and the determination of k s, for which a
previously published method, independent of ferrous
sulphate dosimetry, was adopted. Ferrous sulphate
dosimeters and ion chambers had been irradiated in water
phantoms. When both were positioned at the depth of
maximum dose the beam-quality correction factor kQ,Qo
was renormalized based on water-air stopping power
ratios. The equivalence between the dosimetry systems
was checked by verifying the deviations Δ ic-fs between ion
chambers and ferrous sulphate dosimetry together with
the associated uncertainties.
Results
The mean Δic-fs is -0.5% with an uncertainty of ±0.9%, which
shows no systematic deviations between systems. Δic-fs
dispersion is 3.9% (2σ). 40% of the Δic-fs are within ±1%, 70%
within ±2% and 90% within ±3%.No significant dependence
on electron energy was found, thus confirming kQ,Qo
renormalization. The influence of both chamber type and
polarizing voltage on k s was also analyzed. Ion chambers
with lower electrode spacing or polarizing voltages higher
than the normal operating ones allows a reduction of k s,
providing the chamber performance at these voltages has
previously been checked for linearity of 1/Q versus 1/V.
Conclusion
Parallel-plate ionization chambers can properly and
accurately substitute ferrous sulphate detectors in
reference dosimetry of LIAC and NOVAC mobile linacs.
Therefore, we hope that the main dosimetry protocols for
reference dosimetry in external-beam radiotherapy will
provide guidance in the calibration of electron beams from
linacs dedicated to IORT.
Proffered Papers: Novel methods for auditing
OC-0536 Causes of irradiation failures of IROC
Houston’s IMRT credentialing phantom
S. Kry1, J. Kerns1, M. Carson1, A. Molineu1, P. Taylor1, D.
Followill1
1UT MD Anderson Cancer Center Radiation Physics,
Radiation Physics, Houston- TX, USA