Abstract Book

S1000

ESTRO 37

profile at both sides equally (i.e total coverage / 2 at each side). The second method involves setting the edge of the chamber at one side at (NT/2 + 20) mm and moving the chamber over the beam profile in several steps to cover the whole primary beam and a large part of scatter tail, i.e. the coverage in one side would be larger than the other. Measurements of both methods are performed free in air using the pencil chamber. Monte Carlo simulations were used to simulate a Varian kV on- board-imager (OBI) system with BEAMnrc/EGSnrc, and a 100 mm chamber (Radcal 10X6-3CT) with Cavity/EGSnrc. Beams of width (80 – 300) mm and a tube voltage of 120 kV were used to simulate head and body protocols. In order to compare the methods, CTDI FIA,NT values of each method were normalized with respect to CTDI FIA,500 , which was assessed for each beam width over a 500 mm long chamber that is assumed to represent an infinite length. Results Figure 1 shows a comparison between the methods for the head and body protocols. CTDI FIA,NT values of both methods were comparable, where the variations between the values were all within ±1%. Moreover, both methods reported CTDI FIA,500 values with low underestimations, where the differences were up to 2.5% and decreased with the beam increase (Figure 1). Conclusion Both methods were comparable (within ±1%), and they can be used to replace a need for a long chamber with an underestimation of up to 2.5%. EP-1853 Investigation of a practical approach to assess dose length product (DLP) of Cone Beam CT scans A. Abuhaimed 1 , C.J. Martin 2 , O. Demirkaya 3 1 King Abdulaziz City for Science and Technology, The National Center for Applied Physics, Riyadh, Saudi Arabia 2 University of Glasgow, Department of Clinical Physics, Glasgow, United Kingdom 3 King Faisal Specialist Hospital & Research Center, Department of Biomedical Physics, Riyadh, Saudi Arabia Purpose or Objective The overall energy delivered by a CT scan is represented by the dose length product (DLP). However, this dose index is not available for CT scans obtained with wide beams, i.e. cone beam CT (CBCT) scans. The aim of this study is to propose an equivalent dose index called dose width product (DWP) to estimate the overall energy deposition from CBCT scans. Material and Methods DWP has the same form as that of DLP, but is based on multiplying CTDI w for CBCT scans by the axial beam width (W) to estimate a value equivalent to DLP. CTDI w of CBCT is assessed as proposed by the International Electrotechnical Commission (IEC), which recommends applying a correction factor to the CTDI 100 measured in standard PMMA head and body phantoms with a fan beam (<40 mm). The correction factor is a ratio of two CTDI values measured free in air, that for the wide beam of interest and that for a measurement in the fan beam used in the phantom. Monte Carlo user codes BEAMnrc and Cavity were used to simulate a Varian x-ray system, on-board-imager (OBI), integrated into a TrueBeam linac to calculate the CTDI values for beams of width 80 – 300 mm and tube voltages of 80 - 120 kV. In order to evaluate the efficiency of the DWP proposed, values were

normalized with respect to the total absorbed energy estimated from the dose profile integral (DPI w ) using 600 mm long CTDI phantoms. Results The efficiency of the DWP values for the head and body protocols is shown in Figure 1. DWP values were a factor of 0.81 of the total energy absorbed for the head and 0.73 for the body. The tube voltage was found to play a minimal role in head scans, for which variations were within ±1%, but the variations were larger for body scans reaching ±3%. The low variation in DWP values with beam widths and tube voltage give the proposed index possibility to be applied in the clinic instead of DLP that has limitations at wide beams.

Conclusion Efficiency of the dose index (DWP) shows to be comparable to that for DLP that plays a major role in CT dosimetry. DWP is a potential dose index that can be utilized to overcome the limitations at wide beams. EP-1854 Variations of coefficients for estimating effective doses of cone beam CT scans A. Abuhaimed 1 , C.J. Martin 2 , O. Demirkaya 3 1 King Abdulaziz City for Science and Technology, The National Center for Applied Physics, Riyadh, Saudi Arabia 2 University of Glasgow, Department of Clinical Physics, Glasgow, United Kingdom 3 King Faisal Specialist Hospital & Research Center, Department of Biomedical Physics, Riyadh, Saudi Arabia ) (also called k- factors) are commonly used to estimate effective doses from CT scans. These coefficients were derived for CT scans acquired with small beams (<40 mm). The aim this study is to investigate the suitability of applying these coefficients to cone beam CT (CBCT) scans acquired with wide beams. Material and Methods Values of conversion coefficients to estimate effective doses (E) for CT scans are derived by normalizing E with respect to dose length product (DLP). For CBCT scans, however, DLP shows some limitations due to unsuitability of CTDI 100 for wide beams. In this study E values have been derived by Monte Carlo simulation and normalized with respect to a dose width product (DWP) that is equivalent to the DLP. It is based on multiplying a corrected CTDI 100 , also called CTDI IEC , for the beam of interest by its width (W). A Varian kV on-board-imager (OBI) system, was simulated with BEAMnrc code, and Cavity code was used to calculate DWP in standard CTDI head and body phantoms. E values for the ICRP adult male and female reference computational phantoms were assessed using DOSXYZnrc code. Four scan protocols (head, thorax, abdomen, pelvis) were investigated using beams of width 80 – 320 mm. A tube voltage of 100 kV was applied for the head scan, whereas 120 kV was used for other protocols. This allowed to derive conversion coefficients (CC E ) for CBCT scans. Results Figure 1 shows influence of the beam width and the patient gender on CC E values. Ranges of the CC E values over the beam widths were (0.0033 – 0.0038), (0.0232 – 0.0284), (0.0214 – 0.0237), and (0.0132 – 0.0174) (mSv/mGy.cm) for both phantoms scanned by head, Purpose or Objective Generic conversion coefficients (CC E