title: Breast in vivo dosimetry by a portal ionization chamber.
abstract: This work reports a practical method for the determination of the in vivo breast middle dose value, D(m) on the beam central axis, using a signal S(t), obtained by a small thimble ion chamber positioned at the center of the electronic portal imaging device, and irradiated by the x-ray beam transmitted through the patient. The use of a stable ion chamber reduces many of the disadvantages associated with the use of diodes as their periodic recalibration and positioning is time consuming. The method makes use of a set of correlation functions obtained by the ratios S(t)/D(m), determined by irradiating cylindrical water phantoms with different diameters. The method proposed here is based on the determination of the water-equivalent thickness of the patient, along the beam central axis, by the treatment planning system that makes use of the electron densities obtained by a computed tomography scanner. The method has been applied for the breast in vivo dosimetry of ten patients treated with a manual intensity modulation with four asymmetric beams. In particular, two tangential rectangular fields were first delivered, thereafter a fraction of the dose (typically less than 10%) was delivered with two multi leaf-shaped beams which included only the mammarian tissue. Only the two rectangular fields were tested and for every checked field five measurements were carried out. Applying a continuous quality assurance program based on the tests of patient setup, machine settings and dose planning, the proposed method is able to verify agreements between the computed dose D(m,TPS) and the in vivo dose value D(m), within 4%.
authors: U.O. di Fisica Sanitaria - Centro di Ricerca ad Alta Tecnologia nelle Scienze Biomediche dell'Universitá Cattolica S. Cuore, Campobasso.
authors: Luca
authors: L
authors: Grimaldi
authors: Guido
authors: G
authors: D'Onofrio
authors: Savino
authors: S
authors: Cilla
authors: Andrea
authors: A
authors: Fidanzio
authors: Gerardina
authors: G
authors: Stimato
authors: Luigi
authors: L
authors: Azario
authors: Francesco
authors: F
authors: Deodato
authors: Gabriella
authors: G
authors: Macchia
authors: Alessio
authors: A
authors: Morganti
authors: Angelo
authors: A
authors: Piermattei
mesh_terms: D001940:Breast / Q000473:pathology*; D001943:Breast Neoplasms / Q000473:pathology* / Q000532:radiotherapy*; D002138:Calibration; D004867:Equipment Design; D005260:Female; D006801:Humans; D007477:Ions; D019047:Phantoms, Imaging; D011786:Quality Control; D011874:Radiometry / Q000379:methods*; D011878:Radiotherapy / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014867:Water / Q000737:chemistry
chemical_list: D007477:Ions; D014867:Water