SU-E-J-25: Dosimetric Characterization of an OBI System Using Gafchromic XRQA2 Film

Accurate knowledge about the radiation dose due to the imaging component of an OBI supported linear accelerator is very important. Though films provide excellent spatial resolution, there are problems regarding sensitivity and energy dependence at kV energies. XRQA2 film (0.1-20 cGy) is primarily for measurements in kV range. We demonstrate their use for absolute dosimetry for an OBI system.

H&D curves were generated using a kV therapy x-ray unit in the range of 0-15 cGy. A 25-cm diameter water-equivalent cylindrical phantom with 2.5-cm thick slabs was used on Varian Trilogy linac. XRQA2 films were cut into circular diameter and sandwiched between 2 slabs of the cylindrical phantom to measure the percentage depth dose of the CBCT scans. Additional film pieces were wrapped around the cylinder for surface dose. To get sufficient signal, 5 CBCT scans were taken per imaging acquisition template e.g. head standard dose. After 24 hours post-irradiation, films were scanned using Epson V700 scanner in reflection mode. Only red channel was used for dosimetric analysis using RIT software.

High-Quality head acquisition template resulted in a dose at 0.2, 1.3, 2.7, and 2.7 cGy on AP, left-lateral, posterior and right-lateral positions of the phantom, respectively. Low-quality thorax acquisition resulted in 1.4, 1.6, 1.5, and 1.4 cGy. The PDD at 12.5-cm (isocenter) was of the order of 40%. While the PDD was fairly uniform from left to right, the PDD along the AP/PA direction were showing almost continuous fall off with PDD at isocenter of ∼40%. Analysis of low-quality head PDDs show little fall off dose with depth though the overall magnitude was less with PDD at the isocenter only marginally lower than the maximum along the medial and anterior posterior axis.

Gafchromic XRQA2 film provides a useful tool to evaluate and characterize an OBI system.