[Investigation of a CT value electron density conversion method in radiotherapy planning CT]

To obtain heterogeneous corrections, it is necessary to convert computed tomography (CT) values into electron density. However, differences in the current conversion tables of the method of acquisition for each institution are not clear. Therefore, we sent a questionnaire survey to 200 radiotherapy institutions in the Kanto area regarding heterogeneous correction data acquisition. When these conversion tables were created, many institutions used phantoms for conversion, and 85% of institutions borrowed them from the factories. On the other hand, some institutions used the factory values as the other conversion method. In the conversion tables of the various institutions, we recognized large differences between institutions, in that relative electronic density (Rel.ED) was greater than 1.0. When we assumed the CT value of bone to be 793 HU, the mean relative electron density at 120 kV was 1.46+/-0.09, with a minimum of 1.36 and maximum of 1.82. We have recognized the need to create guidelines detailing these methods of evaluation.