Effect of Physical Factors on Electromagnetic Radiation Therapy Planning Dose Calculation in Computed Tomography

The tube voltage in computed tomography (CT) changes the Hounsfield Unit (HU) and affects the electromagnetic radiation therapy planning (RTP) dose calculation. In this study, physical factors (tube voltage, tube current) of CT were analyzed for their RTP effects. A CT density phantom (CTDP) was exposed to measure the HU with an RTP system while we controlled the physical factors. The human body phantom was exposed with different CT tube voltages (70 kVp, 80 kVp, 100 kVp, 120 kVp, and 140 kVp) and electromagnetic radiation dose calculations were performed with the RTP system. The HU decreased when tube voltage was increased, in particular, the largest gap was found in bone, which has a high density (1792 +/- 54 at 70 kVp, 1065 +/- 13 at 140 kVp). However, a remarkable HU gap was not observed with the changes in tube current. In RTP the calculated dose increased when the tube voltage was raised, in particular, we observed a 3 % gap in brain tissue and bone, a remarkable HU gap. The chest had a small HU gap because of its relatively low density. The RTP dose calculation with changes in tube voltage had a 3 % error, which is acceptable. However, it can be eliminated of because daily QA/QC was performed and a CT density curve with rational exposure condition can be applied to brain or spine patients who experience large errors. This method enables reduction of the error caused by the physical factors in CT.