Poster — Thur Eve — 14: Pretreatment IMRT QA Program for Low‐Dose Control Points Based on Dynamic Noise Correction Using a 2D Matrix of Ionization Chambers

We developed a pre-treatment intensity modulated radiation therapy (IMRT) QA program for verification of low-dose control points registered by a 2D matrix of ionization chambers (ICs). The program eliminates the dose error caused by the induced reading or noise for every chamber electrically. It was found that the low-dose deviation was in the rate of 20–40% outside the field (i.e. in the volume of the normal tissue). Moreover, the voxel-to-voxel dose was found to have non-uniform deviation, which increased linearly with time. The rates of the electrically induced chamber readings were 0.3–1.3 cGy per 0.5 minute, when a beam of 100 monitor units using dose rate equal to 300 MU/min with consideration of the time for the leaf motion, was used in the irradiation with time > 20 s. This means that if low dose point (3–5 cGy) is registered by an IC of the highest induced readings (1.3 cGy), the deviation between the planned and delivered doses would be in the rate of 40−25%. Comparison of the fluence maps may be affected by the non-uniformly induced dose readings. A field function depending on the beam irradiation time to correct the registered dose profile is therefore needed. In this study, a pre-treatment IMRT QA program fully based on absolute dose measurements has been established. The program includes a comparison between the calculated and measured doses. The electrically induced readings of the 2D matrix were subtracted from the beam dose map to correct the measurements.