ESTRO 37 Abstract book

S987

ESTRO 37

for a pencil beam of 230 MeV as function of the adaptive aperture (AA) opening. Conclusion This work presents a complete model of the Adaptive Aperture dynamic collimator, where a reduction in the lateral penumbra is achieved without the use of any patient specific hardware. Furthermore, a comparison between MC based simulations of its dynamic function and experimental data is presented EP-1829 Monte Carlo calculation of Photo-Neutron dose produced by circular cones at 18 MV photon beams N. Banaee 1 , H.A. Nedaie 2 , E. Hosseinzade 1 1 Central Tehran Branch- Islamic Azad University, Department of Medical Radiation Engineering-, Tehran, Iran Islamic Republic of 2 Cancer Institute, Cancer Research Center, Tehran, Iran Islamic Republic of Purpose or Objective Small photon fields are one of the most useful methods in radiotherapy. One of the techniques for shaping small photon beams is applying circular cones, made of lead. Using this method in high energy photon due to neutron contamination is a crucial issue. The aim of this study is to calculate neutron contamination at presence of circular cones irradiating by 18MV photons using Monte Carlo code. Material and Methods Initially,Varian linac producing 18MV photons was simulated and after validating the code, various circular cones were also simulated. Then, number of neutrons, neutron equivalent dose and absorbed dose per Gy of photon dose were calculated along the central axis. Results Table 1 shows the maximum values for number of neutrons per Gy of photon dose in specific depth of phantom for different collimators. Table 1. Maximum number of neutrons and depth of maximum values producing by presence of various circular cones. Diameter of field (mm) Max number of neutron per photon dose[n.cm -2 . Gy -1 ] D max (cm) 30 5.08×10 6 2 20 6.02×10 6 2 15 7.61×10 6 2 10 7.76×10 6 2 5 9.20×10 6 2 Table 2 shows Maximum values of neutron equivalent doses at the surface of phantom producing by presence of various circular cones. Table 2. Maximum values of neutron equivalent doses at the surface of phantom producing by presence of various circular cones. Diameter of field (mm) Neutron equivalent dose per photon dose (mSv/Gy) 30 1.08 20 1.12 15 1.33 10 1.31 5 1.48 Table 3 shows the Maximum neutron absorbed dose at the surface of phantom producing by presence of various circular cones. Table3. Maximum neutron absorbed dose at the surface of phantom producing by presence of various circular cones.

Diameter of field (mm)

Neutron absorbed dose per photon dose (µGy/Gy)

30 15 10

78.20 95.77 99.71

5

103.78

Conclusion As the field size gets smaller,number of neutrons, equivalent and absorbed dose per Gy of photon increases. Also, neutron equivalent dose and absorbed dose are maximum at the surface of phantom and then these values will be decreased. EP-1830 Switching from AAA to AXB in head and neck treatments using VMAT: is the spinal cord safe? C. Muñoz-Montplet 1 , J. Marruecos 2 , M. Buxó 3 , M. Bueno 4 , A. Onsès 1 , I. Romera-Martínez 1 , D. Jurado-Bruggeman 1 1 Institut Català d'Oncologia, Medical Physics and Radiation Protection, Girona, Spain 2 Institut Català d'Oncologia, Radiation Oncology, Girona, Spain 3 Institut d'Investigació Biomèdica de Girona Dr. Josep Trueta, Unit of Statistical and Methodological Advice, Girona, Spain 4 Institut de Radioprotection et de Sûreté Nucléaire, Laboratoire de Dosimétrie des Rayonnements Ionisants, Fontenay-aux-Roses, France Purpose or Objective Spinal cord (SC) is a “serial-like” organ. In head and neck (H&N) VMAT treatments it receives maximum doses (Dmax) close to 50 Gy, from which grade 2 myelopathy may appear in more than 1% of the patients. This tolerance was established for existing convolution/superposition algorithms, such as AAA. Switching to more advanced algorithms, such as Acuros XB (AXB), may lead to significant differences in the calculation of the dose distribution. This study aims to determine wether this happens for both water (Dw) and medium (Dm) AXB reporting modes in the SC and therefore to know if there is a potential risk of unexpected toxicities if current constraints are maintained. Material and Methods 114 patients treated with VMAT were enrolled. Simultaneously-integrated boost was used in all cases. 70 Gy were prescribed to the PTV receiving the highest dose. Plans were created for 6 MV photon beam using RapidArc technique in Eclipse v13. Dose calculations were performed with AAA, AXB Dw and Dm for same number of MU and identical beam setup. Mean values, standard deviation (SD) and range of SC Dmax were calculated for each sample. Paired t-tests were used to analyse the differences in mean values between AAA and AXB Dw as well as between AAA and AXB Dm. Mean differences and 95% confidence intervals (CI) were also calculated using SPSS. It was necessary to know if these differences remained constant for all patients. For this purpose, Pitman-Morgan test was computed using R software to determine if the ratio of the variances was equal to 1. Both central tendency and dispersion tests were two-sided and a p-value<0.05 was considered significant. Results Table I summarizes the mean, SD, and range of the SC Dmax over the 114 patients for AAA, AXB Dw and Dm. Table II shows the values of the mean differences, the 95% CI and the significance of the differences between the mean values and between the variances of the samples.