ESTRO 36 Abstract Book

S436 ESTRO 36 _______________________________________________________________________________________________

released recently by Varian with new scan protocols. This study aimed to investigate the influence of parameters of the new protocols on the effective dose (E) compared to the previous version (V1.6). Material and Methods Effective dose of three scan protocols (head, t horax, and pelvis) were estimated using Monte Carlo s imulations. BEAMnrc and DOSXYZnrc user codes were used to simulate the OBI system integrated into a TrueBeam linac, and to calculate organ doses resulting from the protocols employed. Organ doses were evaluated for the ICRP adult male and female reference computational phantoms. The main differences between the software versions (V1.6) and (V2.5) are: (1) the beam width was extended to 214 mm instead of 198 mm, (2) the mAs values were increased to (150, 270, 1080) compared to (147, 267, 1056) for head, thorax, and pelvis, respectively, and (3) the projections number was increased to 500 for head scan compared to 367, and to 900 for thorax and pelvic scans instead of 660. Results The use of the scan protocols implemented in V2.5 resulted in increasing E of head scan by 13% and 12%, where E of V1.6 was 0.27 mSv and 0.44 mSv for male and female phantoms compared to 0.31 mSv and 0.49 mSv for V2.5, respectively. Parameters of the new protocols, also, led to rise E of thorax and pelvic scans by 16% and 17% for male, respectively, and by 16% for female. E of thorax and pelvic scans increased from 3.32 mSv and 5.95 mSv to 3.86 mSv and 6.88 mSv for male, respectively, and from 3.97 mSv and 11.38 mSv to 4.65 mSv and 13.16 mSv for female, respectively. Conclusion CBCT scans play a major role in radiotherapy treatment. The scan protocols with the new parameters were implemented into the new software to improve the image quality acquired with the scans, and to extend the field of view. This helps to improve the patient positioning on the treatment couch and deliver the specified dose to the patient with a high accuracy, and hence optimising the treatment output. The new head, thorax, and pelvic scans only increased E values by 12 – 13%, 16 – 17%, and 16%, respectively, for male and female. These increases are acceptable when compared to improvement of the treatment output. PO-0815 External neutron spectra measurements for a single room compact proton system R. Howell 1 , E. Klein 2 , S. Price Hedrick 3 , M. Reilly 4 , L. Rankine 5 , E. Burgett 6 1 UT MD Anderson Cancer Center Radiation Physics, Radiation Physics, Houston- TX, USA 2 Northwell Health System, Medical Physics, Lake Success, USA 3 Provision Center for Proton Therapy, Radiation Oncology, Knoxville, USA 4 Washington University, Radiation Oncology, St. Louis, USA 5 The University of North Carolina, Radiation Oncology, Chapel Hill, USA 6 Idaho State University, Nuclear Engineering, Pocatello, USA Purpose or Objective Secondary external neutrons are produced within the physical components of the proton beam line e.g., the double scatterer, modulation wheel, compensator, and field aperture. In passive scattered proton therapy, external neutrons account for a majority of neutron dose equivalent for small fields and up to 50 % for large fields. Spectra measurements are needed to fully and accurately understand neutron dose equivalent from external neutrons. Such data should be reported for proton beamlines from each manufacturer. Here, we focused on the single room compact proton system manufactured by

Mevion (Mevion Medical systems, Littleton, MA) whose use is rapidly increasing in the United States and worldwide. Material and Methods Measurements were performed using a 250-MeV passively scattered proton beam with a range of 20 cm, modulation of 10 cm with the small aperture in place. Measurements were done with a solid brass plates fully filling the aperture opening to achieve a 'closed jaw configuration”. This configuration was selected because it is the most amount of high-Z material that can be in the beamline, thus representing the maximum external neutrons produced for the small field designation. We performed measurements at isocenter and off axis at 40 and 100 cm from the isocenter with the gantry rotated to 90 o or 0 o and couch rotated 0 o or 270 o , Figure 1. All measurements were performed using an extended range Bonner Sphere Spectrometer (ERBS). The ERBS had 18 spheres including the 6 standard Bonner spheres and 12 extended spheres with various combinations of copper, tungsten, or lead. Each set of measurements was performed with all 18 sphere combinations in air with the 6 LiI(Eu) scintillator. Data were unfolded using the MAXED MXD_FC33 algorithm and normalized per unit proton Gy to isocenter.

Figure 1: Schematic diagram of measurement locations. Results The measured neutron spectral fluence at each of the six measurement positions are shown in Figure 1. The average energies, total fluence, and ambient dose equivalents per proton Gy are listed in the table imbedded within figure 1. The average energy, total fluence, and ambient dose equivalent were all highest at isocenter and decreased as a function of distance from isocenter. While the energy distributions for each of the fluence spectra (Figure 1) were similar, with a high-energy direct neutron peak, an evaporation peak, a thermal peak, and an intermediate continuum between the evaporation and thermal peaks, there were a higher fraction of direct neutrons at isocenter compared to 40 and 100 cm from isocenter.

Figure 2: Measured neutron fluence spectra at each of measurement position. For each fluence spectrum, the average energy, total fluence, and ambient dose equivalent [H*(10)] are listed in the imbedded table.