ESTRO 38 Abstract book

S906 ESTRO 38

patient size. Aim of this study was to compare between two methods used to evaluate the patient size. Material and Methods The first method is based on determination of the patient’s dimensions for the region of interest (ROI). This includes anterior-posterior dimension and lateral dimension, from which effective diameter (ED) of the patient is calculated as suggested by AAPM TG-204. However, this method is limited as it does not distinguish between low and high attenuated regions. Therefore, an alternative method was suggested by AAPM TG-220 to account for attenuation of the ROI. It requires evaluating the mean CT number of the ROI, and number and area of pixels located in the same region. This allows estimation of the patient size as a water equivalent diameter (WED), from which a conversion factor can be applied to CTDI vol . A MATLAB-based code was developed in house to import the ICRP reference adult phantoms, and to assess ED and WED for each slice of the two reference phantoms that represent an average male and female. Only the trunk was considered, and divided to three regions: thorax, abdomen, and pelvis. Results Figure 1 shows variations between the methods studied for each slice. The variations were found to be comparable for both phantoms. WED values were higher than those for ED by 11% at the upper part of the thorax at most slices due to the higher attenuated area resulting from the shoulder. However, for the slices of the lower part, ED values became higher by up to 12% due to soft tissues existing in this part, e.g. lung and heart. Soft tissues were also spread largely over the abdomen region, which resulted in obtaining ED values that were higher. For the male phantom, ED values were higher by ~5% over most of the slices, but the variations were reduced for the female phantom from 8 to 2% when the slices were moved down. WED values of the male phantom were lower over all the pelvic region by up to 6%. For the female phantom, however, WED values were within ±3% of those for ED at most slices.

Results SSDE factors for the full scan mode were greater than those for the partial mode over all tube potentials studied using the head scan protocol (Fig.1). The full mode factors were higher by up to 7% for small sizes, and the differences were decreased as the phantom size increase reaching ±2% for large sizes. The tube potential, however, was found to play a major role on factors of the body scan protocol (Fig. 1). For 80 kV, factors of the full and partial scan modes were comparable over all sizes, all being within ±1%. However, for higher potentials, factors of the partial scan mode were greater than those for the full mode by (4 – 14%), (8 – 17%), and (9 – 17%) for 100, 120, and 140 kV, respectively. The body variations were also affected by the phantom size as the variations were raised by the size increase.

Figure 1: SSDE conversion factors for the head and body scan protocols using four tube potentials and the full and partial acquisition modes. The factors were derived for patients of sizes equivalent to a water phantom diameter (D w ) in the range of 10 to 40 cm. Conclusion Results of this study show that the SSDE factors derived for the full scan mode using the head scan protocol can be utilized for the partial mode with an over/underestimation reaching up to 7% over all sizes and for the tube potentials studied. For the body, negligible differences found for the lower tube potential, 80 kV, but attention to the differences should be paid if one uses the full mode factors for the partial mode at higher potentials. Moreover, influence of start point on CTDI measurements for partial scan should be taken into consideration. EP-1686 Assessment of patient size for size-specific dose estimates using two methods: a comparative study A. Abuhaimed 1 , C. Martin 2 , O. Demirkaya 3 1 King Abdulaziz City for Science and Technology, The National Centre for Applied Physics, Riyadh, Saudi Arabia ; 2 University of Glasgow, Department of Clinical Physics and Bioengineering, Glasgow, United Kingdom ; 3 King Faisal Specialist Hospital & Research Centre, Department of Biomedical Physics, Riyadh, Saudi Arabia Purpose or Objective Volume CT dose index (CTDI vol ), which is assessed in cylindrical phantoms of reference sizes, is one the main dose indices utilized for CT dosimetry. Since CTDI vol is assessed in standard phantoms, it does not give the user an accurate indication of the dose absorbed by a patient of a specific size, which is different from the reference ones. As a result, the concept of size-specific dose estimates (SSDE) has been developed to overcome this limitation. SSDE is based mainly on determination of the

Figure 1: Variations between the ED and WED methods on the ICRP phantoms. Slice number is increased as the slice moves down of the body. Conclusion The comparison shows that the variations between the methods investigated were within ±12%, ±8%, and ±6% for the thorax, abdomen, and pelvic regions, respectively. Since evaluation of the ED is relatively easier than that for the WED, one may use the ED method to obtain an approximate estimation for the patient dose, taking into consideration the uncertainty associated with this method, and that the WED is the accurate method to assess the dose.