Room: Exhibit Hall | Forum 7
Purpose: To investigate the effect of magnetic fields for the response of NACP-02 and Roos parallel-plate ionization chambers in the build-up region, using Monte Carlo (MC) calculations.
Methods: The NACP-02 and Roos chambers were modeled using the cavity code in the EGS code system. The response of parallel-plate chambers in the build-up region was calculated in magnetic fields applied perpendicular to the direction of the photon beam, using the cavity code. The magnetic field strengths were 0, 0.35, 1.0, 1.5, and 3.0 Tesla (T). The 6 MV photons were irradiated with a 10 Ã— 10 cm2 field at SSD of 100 cm in a water phantom. The overall perturbation factor PQ,B for the parallel-plate chambers was also calculated for magnetic fields. In addition, the magnetic correction factor kB for parallel-plate chambers was calculated at a depth of 10 cm. The dose in water and cavity dose in parallel-plate chambers were normalized to measured percentage depth dose (%dd) at a depth of 10 cm without the magnetic field.
Results: The kB for magnetic fields was obtained up to 0.91 and 0.89 for NACP-02 and Roos at 1 T, respectively. The build-up dose for parallel-plane chambers complexly varied depending on magnetic field strengths. The values of PQ,B for NACP-02 and Roos chambers in the build-up region were the largest at 1 T and the values were 0.88-0.94 and 0.79-0.91, respectively. In particular, the variation of PQ,B in the build-up region < 5 mm was large. PQ,B values were almost constant at a depth beyond dmax and the values for NACP-02 and Roos chambers were 0.93 and 0.90 at 1 T, respectively.
Conclusion: The response of NACP-02 and Roos chambers in the build-up region changed strongly depending on magnetic field strengths.