Field size analysis of patient-specific quality assurance in scanned carbon ion radiotherapy

Purpose To evaluate the dose difference between measurement and double Gaussian beam model prediction according to the field size and correct the measurements in patient-specific quality assurance (QA). Methods The field size dependence of the dose was evaluated with volumes of 20 x 20 x 80 mm(3), 40 x 40 x 80 mm(3), 60 x 60 x 80 mm(3), and 80 x 80 x 80 mm(3) of 1 Gy uniform dose at three depths. Additional two 80 x 80 x 80 mm(3) volumes of nonuniform fields were created: one high-dose field was given 1 Gy at the central 40 x 40 mm(2) and 0.5 Gy in its surrounding, and the other low-dose field was given 0.5 Gy in the middle and 1 Gy at the periphery. The dose in the center of the spread-out Bragg peak (SOBP) was measured in a water phantom and compared with the treatment planning system (TPS) predication. A field factor based on the two-dimensional (2D) dose distribution was proposed to estimate the field size. The field factor was first evaluated against the dose difference in the square fields, and then used to analyze and correct the patient-specific QA results. Results TPS overestimated dose for fields smaller than 80 x 80 mm(2). A practically positive correlation was observed between the measured dose and the field factor. In the patient-specific QA, measured doses were lower than the TPS predication as they were calculated a relatively small field factor. The corrected dose differences were no longer field factor dependent. Conclusions Using the proposed field factor, we have shown that all the measurements with a large dose deviation were due to the small-sized field. It is clinically relevant to take into consideration the field size in the QA analysis as long as the double Gaussian beam model being used for the dose calculation. Correction to the measurement can be made based on the field factor.