ESTRO 36 Abstract Book

S675 ESTRO 36 2017 _______________________________________________________________________________________________

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.

J. Heo 1 , Y.T. Oh 1 , O.K. Noh 1 , M. Chun 1 , J.E. Park 2 , S.R. Cho 3 1 Ajou University School of Medicine, Radiation Oncology, Suwon, Korea Republic of 2 Ajou University School of Medicine, Pediatrics, Suwon, Korea Republic of 3 Ajou University School of Medicine, Laboratory Medicine, Suwon, Korea Republic of Purpose or Objective The objective of this prospective study was to evaluate the relationship between the circulating lymphocyte subpopulation counts during preoperative chemoradiotherapy (CRT) and tumor response in locally In this prospective study, from August 2015 to June 2016, 10 patients treated with preoperative CRT followed by surgery were enrolled. Patients received conventional fractionated radiotherapy (50.4 Gy) with fluorouracil- based chemotherapy. Surgical resection was performed at 4 to 8 weeks after the completion of preoperative CRT. The absolute blood lymphocyte subpopulation was obtained prior to and after 4 weeks of CRT. We analyzed the association between a tumor response and change in the lymphocyte subpopulation during CRT. Results Among 10 patients, 2 (20%) had evidence of pathologic complete response. In 8 patients with clinically node positive, 4 (50%) had nodal tumor response. All lymphocyte subpopulation counts at 4 weeks after CRT were significantly lower than those observed during pretreatment (p < 0.01). A high decrease in NK cell count during CRT (baseline cell count − cell count at 4 weeks) was associated with node down staging (p = 0.034). Conclusion Our results suggest that the change of lymphocyte subset to preoperative CRT may be a predictive factor for tumor response in rectal cancer. EP-1269 Comparison of 2 and 3 arc VMAT versus fixed field IMRT and proton beam therapy in anal cancer C. Kronborg 1 , E.E. Wilken 2 , J. Hansen 1 , L. Nyvang 1 , J.B. Petersen 1 , E. Serup-Hansen 2 , K.L.G. Spindler 1 1 Aarhus University Hospital, Oncology, Aarhus C, Denmark 2 Herlev and Gentofte Hospital, Oncology, Herlev, Denmark Purpose or Objective Chemoradiotherapy is the standard treatment for squamous cell carcinoma of the anus (SCCA) and is the source of both acute and late toxicity. Advanced radiotherapy treatment techniques aim at reducing dose to organs at risk (OAR) while maintaining target coverage and dose homogeneity. Further, VMAT techniques shorten delivery time considerably. We compared dosimetric advantages of fixed field IMRT, 2 and 3 arc VMAT and additional 3- and 4-field pencil beam scanning proton therapy. Material and Methods Twenty patients with SCCA treated at two different centres were included. Standard treatment was 64-51,2 Gy/32 F or 60-49,5/30 Gy/F delivered with 2 or 3 arc VMAT technique and concurrent chemotherapy according to local practice. Alternative treatment plans were generated for all patients using 5- or 6- fixed field IMRT and 3 arc VMAT (All Varian Eclipse planning system). Four patients with doses above normal constraints (ex high V40 Gy to the bowel) were selected for additional proton therapy planning; both 3- and 4- field plans were generated (Eclipse ver. 10 Multi Field Optimization (IMPT)). Bowel was delineated as potential bowel cavity and bladder as total circumference. Results advanced rectal cancer. Material and Methods

Conclusion The system appears to be able to compensate the disomogenities due to the presence of magnetic field through the use of optimizer. EP-1268 Tumor response according to NK cell change during preoperative chemoradiotherapy in rectal cancer