title: Real time transit dosimetry for the breath-hold radiotherapy technique: an initial experience.
abstract: BACKGROUND
  The breath-hold is one of the techniques to obtain the dose escalation for lung tumors. However, the change of the patient's breath pattern can influence the stability of the inhaled air volume, IAV, used in this work as a surrogate parameter to assure the tumor position reproducibility during dose delivery.
  
  
  METHODS
  In this paper, an Elekta active breathing coordinator has been used for lung tumor irradiation. This device is not an absolute spirometer and the feasibility study here presented developed (i) the possibility to select a specific range epsilon of IAV values comfortable for the patient and (ii) the ability of a transit signal rate S(t), obtained by a small ion-chamber positioned on the portal image device, to supply in real time the in vivo isocenter dose reproducibility. Indeed, while the selection of the IAV range depends on the patient's ability to follow instructions for breath-hold, the S(t) monitoring can supply to the radiation therapist a surrogate of the tumor irradiation reproducibility.
  
  
  RESULTS
  The detection of the S(t) in real time during breath-hold was used to determine the interfraction isocenter dose variations due to the reproducibility of the patient's breathing pattern. The agreement between the reconstructed and planned isocenter dose in breath-hold at the interfraction level was well within 1.5%, while in free breathing a disagreement up to 8% was observed. The standard deviation of the S(t) in breath-hold observed at the intrafraction level is a bit higher than the one obtained without the patient and this can be justified by the presence of a small residual tumor motion as heartbeat.
  
  
  CONCLUSIONS
  The technique is simple and can be implemented for routine use in a busy clinic.
authors.affiliation: Istituto di Fisica, Università Cattolica S. Cuore, Roma, Italy. a.piermattei@rm.unicatt.it
authors.forename: Angelo
authors.initials: A
authors.lastname: Piermattei
authors.forename: Savino
authors.initials: S
authors.lastname: Cilla
authors.forename: Luca
authors.initials: L
authors.lastname: Grimaldi
authors.forename: Pietro
authors.initials: P
authors.lastname: Viola
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Frattarolo
authors.forename: Guido
authors.initials: G
authors.lastname: D'Onofrio
authors.forename: Maurizio
authors.initials: M
authors.lastname: Craus
authors.forename: Andrea
authors.initials: A
authors.lastname: Fidanzio
authors.forename: Luigi
authors.initials: L
authors.lastname: Azario
authors.forename: Francesca
authors.initials: F
authors.lastname: Greco
authors.forename: Cinzia
authors.initials: C
authors.lastname: Digesu
authors.forename: Francesco
authors.initials: F
authors.lastname: Deodato
authors.forename: Gabriella
authors.initials: G
authors.lastname: Macchia
authors.forename: Alessio G
authors.initials: AG
authors.lastname: Morganti
mesh_terms: D005240:Feasibility Studies; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy*; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D015203:Reproducibility of Results; D012119:Respiration; D014057:Tomography, X-Ray Computed