ESTRO 37 Abstract book

S1000

ESTRO 37

Radiation Oncology, Heidelberg, Germany 3 Heidelberg University, Physics Faculty, Heidelberg, Germany 4 Heidelberg University Clinic, Radiation Oncology, Heidelberg, Germany 5 National Centre of Oncological Hadrontherapy CNAO, Medical Physics Unit, Pavia, Italy 6 Heidelberg Ion-beam Therapy Center HIT, Heidelberg, Germany Purpose or Objective To rigorously assess accuracy limits of a Monte Carlo (MC) based treatment planning tool and a novel GPU-based pencil beam algorithm (PBA) under complex heterogeneous conditions using an anthropomorphic phantom head for the four ions ( 1 H, 4 He, 12 C, 16 O) available at the Heidelberg Ion-beam Therapy Center (HIT). Material and Methods Following the experimental setup as detailed in Jäkel et al 2017 [1], half Alderson RANDO head phantom (Radiology support Devices, Long Beach, CA) was attached to the side of a water tank. 6x6x6 cm target structure was delineated within the CT of the phantom for SOBP optimization in the water region. A Monte-Carlo treatment planning platform (MCTP) as described in Tessonnier et al 2017 [2] was used to optimize plans for the four ions through the head. In addition to MC calculation in FLUKA, all plans were recalculated with a dose recalculation platform (FROG: Fast Recalculation and Optimization on GPU) developed in-house and physical measurements were collected through plan deliveries in the experimental beam room. Results MC and physical measurements are found within 5% for all investigated ions. FROG recalculations of the SOBP plans are within 1 - 4% in the high dose volume compared to MC. Comparison with physical measurements and conventional pencil beam algorithms is underway. As an initial test, Figure 1 depicts FLUKA (left) and FROG (right) recalculated dose distributions with the anthropomorphic phantom starting from the plan generated by means of the HIT clinical treatment planning system (TPS). Figure 2 demonstrates agreement of corresponding depth-dose profiles.

Figure 2: Depth-dose profiles for FLUKA (black) and FROG (red) recalculation. Conclusion Insufficient consideration of lateral density heterogeneity in analytical PBA is known to reduce calculation accuracy against physical measurements and the gold standard Monte Carlo predictions. Here, we validated the MCTP against measurements in a more complex clinical-like scenario and demonstrate a new approach to the PBA in an anthropomorphic phantom, representative of commonly encountered cases in the clinic. References: [1] Jäkel et al 2017, A novel phantom setup for commissioning of TPS and beam delivery in light ion beam therapy, Physics in Medicine and Biology, submitted [2] Tessonnier et al 2017, Dosimetric verification in water of a Monte Carlo treatment planning tool for proton, helium, carbon and oxygen ion beams at the Heidelberg Ion Beam Therapy Center. Phys Med Biol. 2017. Electronic Poster: Physics track: Radiation protection, secondary tumour induction and low dose (incl. imaging) EP-1852 Evaluation of two techniques to measure free in air CT dose index (CTDI) of wide beams A. Abuhaimed 1 , O. Demirkaya 2 , C.J. Martin 3 1 King Abdulaziz City for Science and Technology, The National Centre for Applied Physics, Riyadh, Saudi Arabia 2 King Faisal Specialist Hospital & Research Centre, Department of Biomedical Physics, Riyadh, Saudi Arabia 3 University of Glasgow, Department of Clinical Physics, Glasgow, United Kingdom that is measured in standard PMMA phantoms (16 and 32 cm in diameter, and 150 mm long). CT scans based on wide beams (>40 mm) using a digital image plate detector or a wide multiple-row detector are now performed in the clinic. CTDI 100 is not suitable for use with wide beams, so a modification has been proposed to extend the CTDI concept to wider beams, based on correction of the CTDI 100 by a factor determined by free in air (FIA) CTDI measurements. Two methods were suggested for the FIA measurements, and this work aims to compare these methods. Material and Methods The FIA factor is a ratio of CTDI FIA,NT for a wide beam of interest (NT) to CTDI FIA,ref for a reference beam width (<40 mm). In most cases, NT is wider than 100 mm, which is the length of a standard pencil chamber. Thus, the first method suggests stepping the chamber in several steps determined by a minimum coverage (MC) of (NT + 40) mm. One step is required when MC ≤ 100 mm, whereas two would be used for MC ≤ 200 mm, and so on. These steps are taken systematically by covering the beam profile at both sides equally (i.e total coverage / 2 at each side). The second method involves setting the edge Purpose or Objective The main dose index used for CT scans is CTDI 100

Figure 1: FLUKA (left) and FROG (right) plan recalculations for the proton irradiation case, optimized with the facility's clinical TPS.