Abstract ID: 16762 Title: Investigation of the system performance of an online dose verification device
Purpose: To measure the detector response function of an online dose verification device and to
validate our measurement by applying the detector response to Monte Carlo (MC) derived
incident fluence.
Methods: A transmission detector array (IBA Dosimetry, Schwarzenbruck, Germany) was
recently developed as an IMRT quality assurance tool. To understand the detector system
performance for verification of IMRT fluence, the detector response function at 6MV of a
single ion chamber in the detector was measured with a narrow slit collimator. The narrow slit
was formed by two 24 x 72 x 152 mm3 lead blocks, which provided a slit of 0.25 x10 mm2.
The LSF of the detector was obtained by laterally translating it in 0.25 mm steps underneath the
slit. Two head-and-neck IMRT fields were also measured using the device. The same fields
were simulated in our BEAMnrc MC model, and the MC incident fluence was convolved with
the 2D detector response to obtain calculated dose. The measured and calculated dose
distributions were then quantitatively compared using chi comparison (3%/3mm) for in-field
points (defined as those above 10% maximum dose).
Results: The FWHM of the measured detector response for a single chamber is 4.3 mm which
is comparable to the chamber diameter of 3.8 mm. For both examined IMRT fields, the Chi
comparison between measured and calculated data show good agreement with 100% of in-field
points below a chi of 1.0 (chi <0.6 for IMRT field 1 and <0.12 for IMRT field 2).
Conclusions: The detector response function for a new novel online detector has been measured
at 6 MV using a narrow slit technique. The good comparison shown between measured and
calculated dose of the two IMRT fields is a validation of our response function measurement.