ESTRO 36 Abstract Book

S681 ESTRO 36 _______________________________________________________________________________________________

6 Fondazione Policlinico A. Gemelli, TSRM - Gemelli ART - Radiation Oncology, Rome, Italy Purpose or Objective The MRIdian MRI 60 Co radiotherapy system (ViewRay, Oakwood, Ohio) combines an open split-solenoid MRI scanner equipped for parallel imaging and three 60 Co gamma-ray sources. The quantification of dose distribution perturbations due to the presence of 0.35 T magnetic field represents an issue. The MRIdian TPS is equipped by two Montecarlo based algorithms to calculate the dose distribution: the first one has faster calculation time; and does not account for the presence of magnetic field (B off ). The second one has slower calculation time; and takes into account for the presence of magnetic field (B on ). Hardware and software implementation on MRIdian will be completed at our institution within February 2017. Experimental measurements are planned when the system will become clinically operative. Aim of this study was to compare the two algorithms in order to evaluate which could be more accurate in an in silico treatment planning study designed for locally advanced rectal cancer (LARC). Material and Methods This study includes 7 cases of patients affected by LARC. For each patient of the study two plans were developed. Same priority values for the optimization were applied. The calculation of dose distribution was performed using the two different algorithms object of this study (B off and B on ). Plans were performed in IMRT modality, adopting same beams geometry consisting in one pseudo-arc composed by three beam triplets. The treatment plans were optimized according to usual Quality Assurance protocols adopted in our Institution for Linac IMRT treatments: the PTV1 was represented by tumor and corresponding mesorectum; the PTV2 by mesorectum in toto and pelvic nodes. Isotropic 0.7 cm margins were added to PTVs. The total prescribed dose for PTV1 was 55 Gy and 45 Gy for PTV2 through Simultaneous Integrated Boost. All plans were optimized for PTV coverage and sparing of bowel bag and bladder. For PTVs coverage V95 and V105 were considered. For bowel bag V45 and for bladder the mean dose were considered, respectively. Plans were normalized at target median. Results Table 1 summarizes the median values for PTV coverage and organs at risk sparing obtained in the two cases. No significant differences have been reported between the two algorithms.

energy of the Cobalt source compared to those of clinical X-ray beam lead to increase of the surface dose. A previous study (SIMBAD 01) investigated the feasibility of comparative planning among different radiotherapy technologies (i.e: IMRT vs VMAT vs MRI- 60 Co) optimizing for target coverage and organs at risk sparing. Hardware and software implementation on MRIdian will be completed at our institution within February 2017. Experimental measurements are planned when the system will become clinically operative. Aim of the present analysis is to quantify the dose delivered to subcutaneous tissues in the SIMBAD 01 planning conditions (without specific optimization for subcutaneous tissues). Material and Methods Ten patients affected by locally advanced rectal cancer (LARC) were included in this study. For each patient a VMAT RapidArc, a 5-beams sliding window IMRT and one MRIdian treatment plan were performed. All treatment plans were calculated according to the Quality Assurance protocols adopted in our Institution: the PTV1 was represented by tumor and corresponding mesorectum; the PTV2 by mesorectum in toto and pelvic nodes. Isotropic 0.7 cm margins were added to PTVs. The total prescribed dose for PTV1 was 55 Gy and 45 Gy for PTV2 through Simultaneous Integrated Boost. All plans were optimized for PTV coverage and sparing of bowel bag and bladder.PTV coverage was evaluated by calculating the V95 and V105 values. For bowel bag V45 and for bladder the mean dose was considered. The presence of magnetic field has been taken into account during MRIdian MRI- 60 Co planning procedures.. The evaluation of subcutaneous skin dose was obtained calculating the median dose in a 5 mm wide ring dummy structure contoured 3 to 8 mm far from body surface. The first 3 mm from body surface were not taken into account to avoid inconsistencies related to uncertainties in dose calculation at air-body interface due to different dose calculation algorithms (AAA Collapse Cone for VMAT and IMRT treatments, Montecarlo for MRIdian TPS). Results All plans optimized for this study satisfied the constraints on PTV coverage and organs at risk sparing. Dosimetric values obtained in this planning comparison are listed in table 1. The median skin dose resulted higher using the MRIdian system (1.97 Gy against 1 Gy for IMRT and 0.98 Gy for VMAT) Conclusion A subcutaneous skin dose increase is observed with the employment of MRI- 60 Co RT when compared to LINAC treatment planning, even if still in clinically acceptable constraints. Considering and contouring such a structure seems useful. Specific study will be performed to define how reducing dose to subcutaneous tissues by optimizing for such structures. EP-1267 In silico Evaluation of the impact of Magnetic Field on dose distribution using of MRIdian MRI- 60Co D. Cusumano 1 , L. Boldrini 2 , L. Azario 3 , S. Teodoli 1 , M. Balducci 4 , G.C. Mattiucci 4 , S. Chiesa 2 , G. Chiloiro 2 , N. Dinapoli 2 , M.A. Gambacorta 4 , C. Masciocchi 5 , D. Piccari 6 , M. Rapisarda 6 , M.V. Antonelli 6 , M. Ferro 2 , E. Placidi 1 , A. Piermattei 3 , F. Cellini 2 , V. Valentini 4 1 Fondazione Policlinico A. Gemelli, UOC Fisica Sanitaria - Gemelli ART - Radiation Oncology, Rome, Italy 2 Fondazione Policlinico A. Gemelli, Gemelli ART - Radiation Oncology, Rome, Italy 3 Università Cattolica del Sacro Cuore, Rome, Italy 4 Università Cattolica del Sacro Cuore, Gemelli ART - Radiation Oncology, Rome, Italy 5 Fondazione Policlinico A. Gemelli, KBO Labs - Gemelli ART - Radiation Oncology, Rome, Italy