ESTRO 37 Abstract book

S984

ESTRO 37

fractions to the primary tumour (PTV70.5), high-risk (PTV60.0) and low-risk nodal area (PTV55.5) in a simultaneous integrated boost strategy. The PTV70.5 was also separated into components in tissue (PTVtiss) and air (PTVair) to better understand the impact of air cavities on dose distributions. Differences in dose distribution for PTVs and OARs were assessed using different metrics (mean doses, near-minimum dose (D98%), near-maximal dose (D2%), target coverage (D98%, D95% and V95%), mean doses and doses to 1cc of serial OARs). Results PBC algorithm overestimated dose to PTVs for all considered metrics. For PTV70.5, Dmean and D95% calculated by MC decreased by 3.1% (range:2.1%-4.7%) and 4.7% (range:2.6%-7.6%), respectively. In particular, D95% decreased by 4.5% for PTVtiss and 7.5% for PTVair. D98% decreased by 5.0% (range:2.5%-8.8%), with average values of 4.8% in tissue and 8.8% in air. Percentage differences in V95% was 22.5% (21.8% in PTVtiss and 28.1% in PTVair). The magnitude of dose differences was strongly correlated with the amount of air cavities in PTV70.5. Differences between MC and CCC were found within 1% for all metrics in PTVs and PTVtiss. However, CCC showed a significant underestimation of the Dmean, D95% and D98% doses in the PTVair by 3.0% with respect to MC. With regard to OARs irradiation, doses to 1cc for PRV spine and PRV brainstem were found to be approximately 3.0 Gy lower with MC; mean dose for parotids was lower by 2.7%. Conclusion MC is recommended instead of PBC for avoiding serious overestimation in target doses. A key question remains open: should the prescription dose be adjusted to the actually delivered dose, more accurately predicted by MC algorithm? If radiation oncologists wanted to keep the PBC original dose prescription and the same accepting criteria for target coverage when switching from PBC to MC, up to 8% more radiation doses would be given. EP-1825 Output factors determination for radiosurgery beams using the novel IBA Razor Nano Chamber A. Girardi 1 , T. Gevaert 1 , C. Jaudet 1 , G. Coussement 1 , A. Defauw 1 , M. Boussaer 1 , M. Burghelea 1 , J. Dhont 2 , T. Reynders 1 , M. De Ridder 1 1 Universitair Ziekenhuis Brussel, Department of Radiotherapy-Vrije Universiteit Brussel, Brussels, Belgium 2 Universitair Ziekenhuis Brussel, Department of Radiotherapy-Faculty of Medicine and Pharmacy- Vrije Universiteit Brussel, Brussels, Belgium Purpose or Objective Determine an accurate set of output factors (OFs) for a stereotactic radiosurgery (SRS) delivery system is crucial in order to achieve the highest possible accuracy during treatment, but it is at the same time challenging due to lack of lateral electronic equilibrium and to the features of the used detector, such as dimension of the active volume and non-water-equivalent components. The goal of this study was to determine small field OFs for an Elements (BrainLAB, Feldkirchen, Germany) treatment planning system using the novel Razor Nano Chamber (IBA Dosimetry, Schwarzenbruck, Germany) and to compare its performance with other widespread dedicated methods for small field dosimetry. Material and Methods A Razor Nano Chamber (RNC) was irradiated with 6 MV photon beams using a TrueBeam STx system (Varian, Palo Alto CA) equipped with a HD multileaf collimator (MLC). Small fields ranging from 0.5×0.5 to 10×10 cm 2 were defined by both the X-Y collimator jaws and the MLC. The RNC was used in parallel configuration at a source- surface distance of 100 cm and at 10 cm depth in a 3D SCANNER water phantom (Sun Nuclear, Melbourne, FL). The OFs carried out with the RNC were compared with

Figure 1: Correlation between the proportion of dose from small fields and the maximum and overall uncertainty, respectively. Results are shown for the time- integrated PDIs of the VMAT arcs that use the Varian TrueBeam High Definition MLC, and a beam energy of 6 MV. Conclusion In conclusion, there seems to be a correlation between the model’s maximum uncertainty and the proportion of dose from small fields. The overall uncertainty is lower and more stable in relation to the proportion of dose from small fields. EP-1824 Dosimetric impact of Monte Carlo calculation algorithm for VMAT planning of nasopharyngeal tumors A. Ianiro 1 , F. Deodato 2 , G. Macchia 2 , M. Buwenge 3 , S. Cammelli 3 , F. Romani 4 , V. Valentini 5 , A. Morganti 3 , S. Cilla 1 1 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy 2 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Radiation Oncology Unit, Campobasso, Italy 3 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S. Orsola- Malpighi Hospital, Radiation Oncology Department, Bologna, Italy 4 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S. Orsola- Malpighi Hospital, Medical Physics Department, Bologna, Italy 5 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Radiation Oncology Department, Roma, Italy Purpose or Objective To assess the clinical implications of the Monte Carlo dose calculation algorithm for VMAT treatments of nasopharyngeal tumors (NPC), after a transition from Pencil Beam algorithm (PBC) and Collapsed Cone Convolution (CCC). Material and Methods Ten plans initially produced for patients with NPC using the PBC algorithm were recalculated with the CCC algorithm and the MC algorithm. This last is the XVMC dose engine implemented in the Monaco TPS (Elekta). Doses of 70.5, 60.0 and 55.5 Gy were prescribed in 30