Scatter-to-primary ratio in cone beam computed tomography with extended source to image-receptor distance for image-guided proton beam therapy system

Purpose: The number for clinical systems of proton beam dedicated to the tumor irradiation therapy has been increasing in recent years. In particular, these new systems are likely added with an imaging unit of cone beam CT (CBCT) for image guidance to reduce patient positioning error. In these CBCT systems, the x-ray source to image-receptor distance (SID) of the CBCT unit mounted on the proton beam gantry may be extensively longer than that of CBCT units on linear accelerators. To characterize the scattering effect for these CBCT units with a long SID, we used Monte Carlo computer simulation to calculate the quantity of scatter-to-primary ratio (SPR). Materials and methods: A general-purpose radiation transport code, PENELOPE, was used in our Monte Carlo computer simulation. Analytical uniform and non-uniform phantoms of different materials were used for SPR calculations, including adipose (density: 0.92 g/cm³, relative electron density: 0.49), water (1.00 g/cm³, 1), muscle (1.04 g/cm³, 1.043), and bone (1.85 g/cm³, 1.095). The phantom size was 50×50×20 cm³ in length, width, and height, respectively. The receiving area of flat-panel detector was 40×40 cm². Cone angle was used to represent the maximum angle that the x-ray of cone beam can cover the entire receiving area of x-ray detector. Different SID geometries of 150, 220, and 300 cm with different cone beam x-ray energies were included in this work. Results: In the phantom study with different materials, SPR decreased as either the x-ray energy increased, density of material decreased, or SID increased. The cone angle was 7.59° for 150 cm of SID; 5.19° for 220 cm of SID; 3.81° for 300 cm of SID. A negatively linear relationship was characterized between the cone angle and SID. Our Monte Carlo calculations showed that SPR decreased as the cone angle decreased. A positive correlation was shown between the SPR and cone angle. Conclusions: We used different CBCT systems to determine SPRs and SPR decreased for the CBCT unit with a longer SID..