To quantify the correlations between Elekta XVI cone beam CT dose and various scanning protocols, providing mathematical models to assess the protocol-dependency of imaging dose during image guided radiotherapy.

Based on standard protocols and various combinations of kVp and mA on an XVI mounted on an Elekta Versa HD accelerator, the air KERMA was measured at various positions in a standard PTW CTDI body phantom using calibrated PTW 30009 kV chamber and UNIDOS webline electrometer. Weighted CT dose index (CTDI_w) was computed thereafter. SigmaPlot 10.0 was used to fit the measurements against mA and/or kVp yielding empirical functions.

Under standard protocols, the CTDI_w of Varian OBI was only 11.23% (chest) and 9.15% (pelvis) of Elekta XVI. Using the default and other 4 investigated kVp values, the central and peripheral KERMA were both proportional to mA, and yet the slope value a varied dramatically from 0.479 to 6.679. Major affecting factors included kVp settings, measurement locations, and dosimetric metrics, etc. None linear regressions were used to fit kVp against KERMA at various locations and CTDI_w (R²>0.997). The differences between all coefficients were statistically significant (P<0.05). The impact of changing both mA and kVp on the dose to phantom center can be described as mGy=(5.917 -0.197 ×kVp+ 0.002 ×kVp²-5.063 ×10^-6×kVp³)×mA.

Imaging dose of Elekta XVI is strongly dependent on scanning parameters. The proposed mathematical models can be used as efficient and robust indicators of such dependency.

Cone beam CT; Elekta XVI; Radiation dose; Radiological protection; Image guided radiotherapy