Abstract Book

S936

ESTRO 37

Material and Methods In order to measure only the cable effect, the ionization chamber and stem were removed and the cut part was insulated with silicon. And the change in the collected electric charge with respect to the irradiated length of the cable was measured. The ionization chamber was installed at a depth of 10 cm in water. Three kinds (6 MV, 10 MV, 6 MV - SRS) of beams from Varian Novalis-Tx and 6 MV FFF beam from Tomotherapy were used. The applied voltage was varied from 50 V to 400 V and measured for the negative and positive voltages. To evaluate irradiation field size dependence, the polarity effect was measured at irradiation field size 3 cm × 3 cm to 30 cm × The cable structure of the two manufacturers was almost the same, but the change in the charge amount depending on the irradiated length was different. This is due to differences in insulation materials and coatings, and the effect of dose rate dependence. In TM 31016 and 31014, the ionization amount greatly changed depending on the polarity of the applied voltage, and at low voltage, the polarity effect was large. In A1SL, the change in polarity effect correction factor was small and was 1.005 or less. The polarity effect of TM31022 was stable in the range of 150 V to 400 V (1.000 to 1.002). The polarity effect of A 26 was stable in the range of 50 V to 300 V (0.998 to 1.001). It is thought that it depends on the structure, material and working accuracy of the ionization chamber. In TM30013, there was almost no dependence of polarity effect on irradiated field size. In A1SL, the influence of field size was small, 0.2% or less, and its change was small. In TM31022 and A26, the polarity effect correction factor increased as the irradiation field increased, 1.2% in the irradiation field size 30 cm × 30cm. This is because when the ionization volume of the ionization chamber is small, the influence ratio of the cable effect becomes large. Conclusion Changes in the amount of ionization due to the cable effect are small, but contributions of 1 to 2% are considered for micro volume ionization chamber. In the ionization chamber with a large polarity effect, sufficient consideration is also necessary for ion recombination correction, and it is not necessarily good that the high applied voltage is good. Improved micro volume ionization chamber is an excellent ionization chamber with small polarity effect and ion recombination, but it was suggested that attention should be paid to the influence of cable effect in large radiation field. EP-1747 Characterizing proton stopping power ratio uncertainties in a CT/MRI tissue classification model A. Witztum 1 , T. Solberg 1 , A. Sudhyadhom 1 1 University of California- San Francisco, Department of Radiation Oncology, San Francisco, USA Purpose or Objective To analyze the uncertainty in stopping power ratio (SPR) calculations based on potential sources of error in a four- component tissue classification model using CT and MRI imaging. Material and Methods The greatest source of uncertainty in the Bethe-Bloch equation (used to calculate proton SPR) is the mean ionization potential (I m ). One known method to calculate I m is the Bragg additivity rule (BAR) with a summation over all elemental constituents. We propose a reduction to a four-component classification (4CC) system. Using CT and MRI imaging we propose to classify molecules in the human body as proportions of water, fat, and protein (all by MRI) and hydroxyapatite (HA) by CT. In doing so we can calculate I m by a reduction of the BAR to: ln(I m )≈(w water ln(I water ))+(w fat ln(I fat ))+(w protein ln(I protein ))+(w HA ln (I HA )) 30 cm. Results

where w is the mass content percentage and I is the mean ionization potential for each type of molecule. For each molecule, the I-value was obtained from ICRU report 44. For fat and protein, a weighted mean I was calculated using prevalence and I-values for the constituent lipids and amino acids respectively. The ‘true’ mass content percentages were obtained for various soft tissues (tissue) and cortical bone (bone) from ICRU report 44 and others available in the literature. Protein content was considered as the remaining mass. Errors in this value were simulated using a normal distribution of errors between 0-2% (mean=1%) for water and HA, and between 0-10% (mean=5%) for fat, and were randomized in the positive and negative directions. For tissue, HA content was assumed to be 0%. I m calculated with the 4CC system was compared to BAR-calculated values. Proton (E=250MeV) SPR was calculated using I m (BAR) as the reference standard and compared to SPR using I m from: the 4CC system without errors I m,base , mean, and max I m from the distribution with errors. Results Table 1 shows use of the 4CC model results in a <1.7% (tissue) and <1.0% (bone) error in I m compared to BAR. For tissue, I m mean, min, and max errors of up to -1.8%, - 3.5%, and between -0.6-1.2% are seen. Table 2 shows that for tissue and bone, use of the 4CC model results in proton SPR calculations within 0.2% of BAR. The mean SPR from the error distribution still gave results accurate to 0.3%. SPR calculations from the max I m values had an error within 0.4%. Adding the errors for SPR in quadrature to include systematic error from the 4CC system compared to BAR (max 0.2%) and the random error (2σ as a percentage of SPR (4CC) value) from content measurement (max 0.3%) yields a total error of 0.4% for the 4CC model.