title: Use of parallel-plate ionization chambers in reference dosimetry of NOVAC and LIAC®mobile electron linear accelerators for intraoperative radiotherapy: a multi-center survey.
abstract: OBJECTIVE
  LIAC®and NOVAC are two mobile linear accelerators dedicated to intraoperative radiation therapy (IORT), generating electron beams in the energy range of 3-12 MeV. Due to high dose-per-pulse (up to 70 mGy per pulse), in 2003 the Italian National Institute of Health (ISS) stated that "for the measure of dose to water in reference conditions, ionization chambers cannot be employed and no published dosimetry protocol can be used". As a consequence, ferrous sulphate (or, alternatively alanine) dosimetry was recommended. Based on a retrospective multi-center survey, a comparison with ferrous sulphate dosimetry is now used to validate the parallel-plate ionization chambers for reference dosimetry of NOVAC and LIAC.
  
  
  METHODS
  The IAEA TRS-398 dosimetry protocol was applied except for the reference irradiation setup and the determination of the ion-recombination correction factor ks. Moreover the depth of maximum dose (R100) instead of zrefas measurement depth was chosen by the majority of centers, thus implying a renormalization of the beam-quality correction factor kQ,Qo, based on water-air stopping power ratios. Regarding the ksdetermination, a previously published method, independent of ferrous sulphate dosimetry, was adopted. All the centers participating in this study had used both ferrous sulphate dosimeters and ionization chambers in water phantoms for dosimetry under reference conditions.
  
  
  RESULTS
  The mean percentage difference between ionization chambers and ferrous sulphate dosimetry was -0.5% with a dispersion of 3.9% (2σ). Moreover, the uncertainty analysis allowed the agreement between ionization chambers and ferrous sulphate dosimetry to be verified. These results did not show any significant dependence on electron energy, thus indirectly confirming kQ,Qorenormalization. The results from the centers using zrefas the measurement depth were similar to the other data, but further focused studies could aim at investigating possible dependences of the dose differences on the chosen reference depth.
  
  
  CONCLUSIONS
  The present study confirms that parallel-plate ionization chambers can properly and accurately substitute ferrous sulphate detectors in reference dosimetry of LIAC and NOVAC mobile linear accelerators. Therefore, we hope that the most commonly used protocols for reference dosimetry in external-beam radiotherapy will be updated in order to provide guidance in the calibration of electron beams from linear accelerators dedicated to IORT, so that users may benefit from specific, authoritative and up-to-date recommendations.
authors.affiliation: Department of Medical Physics, Azienda U.L.S.S. 8, Vicenza, 36100, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: Scalchi
authors.affiliation: R&D Department, SIT, Aprilia, 04011, Italy.
authors.forename: Alessia
authors.initials: A
authors.lastname: Ciccotelli
authors.affiliation: R&D Department, SIT, Aprilia, 04011, Italy.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Felici
authors.affiliation: Department of Medical Physics, S. Filippo Neri Hospital, Rome, 00135, Italy.
authors.forename: Assunta
authors.initials: A
authors.lastname: Petrucci
authors.affiliation: Department of Medical Physics, I.R.C.C.S. Institute of Oncology Giovanni Paolo II, Bari, 70124, Italy.
authors.forename: Raffaella
authors.initials: R
authors.lastname: Massafra
authors.affiliation: Department of Radiotherapy, Multimedica Hospital, Castellanza, 21053, Italy.
authors.forename: Valeria
authors.initials: V
authors.lastname: Piazzi
authors.affiliation: Department of Medical Physics, ASUR MARCHE AV3, Macerata, 62100, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: D'Avenia
authors.affiliation: Department of Medical Physics, IRCCS A.O.U. SAN MARTINO-IST, Genova, 16132, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Cavagnetto
authors.affiliation: Department of Medical Physics, European Institute of Oncology, Milan, 20141, Italy.
authors.forename: Federica
authors.initials: F
authors.lastname: Cattani
authors.affiliation: Department of Medical Physics, Bellaria Hospital, Bologna, 40139, Italy.
authors.forename: Raffaella
authors.initials: R
authors.lastname: Romagnoli
authors.affiliation: Laboratory of Medical Physics and Expert Systems, Regina Elena National Cancer Institute, Rome, 00144, Italy.
authors.forename: Antonella
authors.initials: A
authors.lastname: Soriani
mesh_terms: D004583:Electrons; D005296:Ferrous Compounds; D007432:Intraoperative Period; D010315:Particle Accelerators; D011874:Radiometry / Q000295:instrumentation* / Q000592:standards*; D011878:Radiotherapy / Q000295:instrumentation*; D012015:Reference Standards
chemical_list: D005296:Ferrous Compounds; C020748:ferrous sulfate
keywords: 
  IOERT
  ; electron beams; reference dosimetry