ESTRO 2023 - Abstract Book

S1750

Digital Posters

ESTRO 2023

PO-1981 Commissioning the beam model of apertures in a compact proton system using pencil beam scanning

J.M. PEREZ MORENO 1 , J.A. Vera Sánchez 1 , F. Cerrón Campoo 1 , J. Castro Novais 1 , E. Canals de las Casas 1 , I. Lorenzo Villanueva 1 , M. Sallabanda Hajro 1 , M. Montero Feijoo 1 , A. de Pablo González 1 , R. Matute Martín 1 , R. Miralbell 1 , A. Mazal 1

1 Quirónsalud Protontherapy Center, Radiation Oncology, Pozuelo de Alarcón (Madrid), Spain

Purpose or Objective To validate treatment planning system (TPS) beam model for a snout with brass apertures in proton pencil beam scanning (PBS) beams. Materials and Methods The Proteus One proton system has a compact nozzle snout holder compatible the Snout S removable accessory, designed to maintain an aperture and a range shifter to modify PBS beams up to 12 cm diameter. Geometrical, physical and material properties of snout elements can be defined as additional beam line objects for the existing PBS beam model. First, a 10 cm range, 5 cm modulation and 5 cm diameter circular PBS beam was designed. Absorbed dose at 8 cm depth in water was measured for the resulting fixed spot map using circular apertures using IBA Razor ionization chamber (IC). Second, absorbed dose, lateral profiles and depth dose distribution were measured for several single PBS beam plans with different values of range and modulation combined with different apertures of the apertures set. Absorbed dose and depth doses were measured with IC while lateral profiles with IC and PTW Octavius 1600XDR. Finally, dose per beam was measured in an end-to-end test (E2E) defined using Lucy SRS Phantom and 3 beam plan with 2cm aperture. Results Table 1a shows IC-TPS dose difference for the fixed spot map and varying aperture. Last column refers to differences after applying a volume effect correction factor (FCvol) to IC reading. Table 1b shows IC-TPS average dose difference for every aperture of the set of different range, modulation and aperture combination, measured in water and in RW3 phantom respectively, as well for the beams of E2E test measured in Lucy phantom. A set of 4, 3, 2, 1.5 and 1 cm diameter circular and 2 cm square apertures were used to create validation plans.

Measured range R90 and R80 agreement with TPS calculation were better than 0.2 mm in all cases. Maximum observed range difference was 0.35 mm for R20 in one plan with 30 mm aperture. Figure 1a-b shows the IC and TPS agreement in a non-homogeneous SOBP. In Figure 1c-d can be observed X and Y profiles for a heterogeneous dose distribution inside a 20 mm aperture. Maximum deviation in penumbra and field was 0.3 mm for 1600XDR detector, probably due to its limited detector resolution compared with Razor IC.