In vivo dosimetry by an aSi-based EPID

Journal Article · Wed Nov 15 00:00:00 EST 2006 · Medical Physics

DOI:https://doi.org/10.1118/1.2360014· OSTI ID:20853727

Piermattei, Angelo; Fidanzio, Andrea; Stimato, Gerardina; Azario, Luigi ^[1]

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1. Instituto di Fisica, Universita Cattolica del S. Cuore, Rome (Italy)

A method for the in vivo determination of the isocenter dose, D{sub iso}, and mid-plane dose, D{sub m}, using the transmitted signal S{sub t} measured by 25 central pixels of an aSi-based EPID is here reported. The method has been applied to check the conformal radiotherapy of pelvic tumors and supplies accurate in vivo dosimetry avoiding many of the disadvantages associated with the use of two diode detectors (at the entrance and exit of the patient) as their periodic recalibration and their positioning. Irradiating water-equivalent phantoms of different thicknesses, a set of correlation functions F(w,l) were obtained by the ratio between S{sub t} and D{sub m} as a function of the phantom thickness, w, for a different field width, l. For the in vivo determination of D{sub iso} and D{sub m} values, the water-equivalent thickness of the patients (along the beam central axis) was evaluated by means of the treatment planning system that uses CT scans calibrated in terms of the electron densities. The D{sub iso} and D{sub m} values experimentally determined were compared with the stated doses D{sub iso,TPS} and D{sub m,TPS}, determined by the treatment planning system for ten pelvic treatments. In particular, for each treatment four fields were checked in six fractions. In these conditions the agreement between the in vivo dosimetry and stated doses at the isocenter point were within 3%. Comparing the 480 dose values obtained in this work with those obtained for 30 patients tested with a similar method, which made use of a small ion-chamber positioned on the EPIDs to obtain the transmitted signal, a similar agreement was observed. The method here proposed is very practical and can be applied in every treatment fraction, supplying useful information about eventual patient dose variations due to the incorrect application of the quality assurance program based on the check of patient setup, machine setting, and calculations.

• OSTI ID: 20853727
• Journal Information: Medical Physics, Vol. 33, Issue 11; Other Information: DOI: 10.1118/1.2360014; (c) 2006 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
• Country of Publication: United States
• Language: English

Related Subjects

62 RADIOLOGY AND NUCLEAR MEDICINE
ADIABATIC SURFACE IONIZATION
CAT SCANNING
CORRELATION FUNCTIONS
DOSIMETRY
ELECTRON DENSITY
IN VIVO
IONIZATION CHAMBERS
NEOPLASMS
ORGANS
PATIENTS
PERIODICITY
PHANTOMS
PLANNING
QUALITY ASSURANCE
RADIATION DOSES
RADIOTHERAPY
SI SEMICONDUCTOR DETECTORS
SIGNALS
THICKNESS