A practical experience of dose modeling for proton pencil beam scanning in KNCC

Dose modeling of proton pencil beam scanning (PBS) in Eclipse V10 (Varian Medical Systems, Palo Alto) requires absolute integral depth dose (IDD) curves and spot profiles for all energies. IDDs are measured by using a Bragg peak chamber (BPC, PTW; radius: 4.08 cm) and a threedimensional (3D) water phantom (RFA300, IBA, Louvain-la-Neuve, Belgium). The energy range is 99.3 MeV - 226.9 MeV, and the number of measured beam energies is 133, which corresponds to the number of available energy degrader steps. Relative IDDs are converted to absolute IDDs by measuring the outputs at a shallow depth ( i.e ., 2 cm) with the calibrated NDWkq of the BPC. In the present study, in-air spot profiles were collected by using a homemade device at 8 different positions, 0 cm,’ 10 cm,’ 20 cm,’ 30 cm and 40 cm, from the isocenter with and without a range shifter (RS). Additional high monitor unit (MU) measurements were performed to acquire a high signal-to-noise ratio (SNR) in the low-dose region. Without the RS, the smallest in-air spot size at the isocenter was approximately 4.5 mm for 226.9 MeV, and the largest size was 11 mm for 99.3 MeV. The water-equivalent thickness of the RS is 74.4 mm. At National Cancer Center, Korea, we measured 133 IDDs and 2128 x, y spot profiles from Feb. 2014 to July 2014, which sufficed for dose modeling. This is the first proton pencil beam dose modeling data in Korea. Indeed, our experience should prove useful to other institutions, especially where the IBA proton machine is used with Eclipse.