ESTRO 36 Abstract Book

S432 ESTRO 36 2017 _______________________________________________________________________________________________

Conclusion In this study, we measured spectra for external neutrons and characterized neutron dose equivalents for a single gantry proton system, whose use in the United States and worldwide is increasing. Poster: Physics track: Treatment plan optimisation: algorithms PO-0816 LRPM for fast automated high quality treatment planning – towards a novel workflow for clinicians R. Van Haveren 1 , B.J.M. Heijmen 1 , W. Ogryczak 2 , S. Breedveld 1 1 Erasmus Medical Center Rotterdam Daniel den Hoed Cancer Center, Radiation Oncology, Rotterdam, The Netherlands 2 Warsaw University of Technology, Control and Computation Engineering, Warsaw, Poland Purpose or Objective The aim is to create a novel efficient workflow for clinicians, where high quality treatment plans are ready to be inspected minutes after the delineation is finished. In the current clinical workflow, plans are automatically generated using the in-house developed Erasmus-iCycle optimiser, but planning times can be in the order of hours. Therefore, we propose an extension of Erasmus-iCycle to substantially reduce computation times, but maintain plan quality.

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.

Material and Methods We developed the Lexicographic Reference Point Method (LRPM), a fast algorithm to automatically generate multi- criterial treatment plans in a single optimisation run. In contrast, the currently implemented sequential method in Erasmus-iCycle requires multiple optimisations to generate a plan. We validate the LRPM by comparing automatically generated VMAT plans (mimicked by 23 static beams) with the LRPM and the sequential method for 30 prostate cancer patients and 15 head-and-neck cancer patients. For these treatment sites (and others), Erasmus-iCycle is in clinical use. Results For the 30 prostate cancer patients, plan differences between the LRPM and the sequential method were found neither clinically nor statistically significant. The LRPM reduced the average planning time from 12.4 to 1.2 minutes, a speed-up factor of 10. For head-and-neck, the LRPM reduced the planning times from 99.7 to 4.6

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.