ESTRO 36 Abstract Book

S281 ESTRO 36 2017 _______________________________________________________________________________________________

15 Foligno Hospital- A.U.S.L. Umbria 2, MEDICAL PHYSICS, Foligno, Italy 16 ASUR MARCHE AV3, MEDICAL PHYSICS, Macerata, Italy 17 Arcispedale S. Maria Nuova-I.R.C.C.S., MEDICAL PHYSICS, Reggio Emilia, Italy 18 I.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 20 Ospedale 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 (k s ) 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 k Q,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 Δ ic-fs is -0.5% with an uncertainty of ±0.9%, which shows no systematic deviations between systems. Δ ic-fs dispersion is 3.9% (2σ). 40% of the Δ ic-fs are within ±1%, 70% within ±2% and 90% within ±3%.No significant dependence on electron energy was found, thus confirming k Q,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.

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. Scalchi 1 , G. Felici 2 , A. Ciccotelli 2 , A. Petrucci 3 , V. Piazzi 4 , N. Romeo 5 , A. Pentiricci 6 , F. Cavagnetto 7 , S. Andreoli 8 , F. Cattani 9 , S. Fabbri 10 , P. Tabarelli de Fatis 11 , R. Romagnoli 12 , A. Soriani 13 , B. Augelli 14 , M. Paolucci 15 , P. D'Avenia 16 , M. Bertolini 17 , R. Massafra 18 , E. Moretti 19 , S. De Stefano 2 , L. Grasso 2 , C. Baiocchi 20 , P. Francescon 1 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

Proffered Papers: Novel methods for auditing

OC-0536 Causes of irradiation failures of IROC Houston’s IMRT credentialing phantom S. Kry 1 , J. Kerns 1 , M. Carson 1 , A. Molineu 1 , P. Taylor 1 , D. Followill 1 1 UT MD Anderson Cancer Center Radiation Physics, Radiation Physics, Houston- TX, USA