Application of Gafchromic film in the study of dosimetry methods in CT phantoms

摘要

Gafchromic film has been used for measurement of computed tomography (CT) dose distributions within phantoms. The film was calibrated in the beam from a superficial therapy unit and the accuracy confirmed by comparison with measurements with a 20mm long ionisation chamber. The results have been used to investigate approaches to CT dosimetry. Dose profiles were recorded within standard CT head and body phantoms and scatter tail data fitted to exponential functions and extrapolated to predict dose levels in longer phantoms. The data have been used to simulate both CT dose index (CTDI) measurements with ionisation chambers of differing length and measurements of cumulative doses with a 20mm chamber for scans of varying length. The results show that the length of a pencil ionisation chamber is the most significant factor affecting measurements of weighted CTDI (CTDI_w) and a 100mm chamber would record 50-61% of the dose measured with a 450mm one. The cumulative dose measured at the centre of a 150mm long body phantom records over 70% of the equilibrium dose from a helical scan of a longer phantom. For routine CT dosimetry tests, the determination of correction factors could allow measurements with a 100mm chamber to be used to derive the CTDI that would be recorded with a longer chamber, and cumulative doses measured with a 20mm chamber in shorter phantoms to be used to calculate equilibrium doses for helical scans.