ESTRO 2021 Abstract Book

S1507

ESTRO 2021

Results Surprisingly highest dose rate was on top of couch, not near LINAC head, in fact maximum dose rate (5.4μSv/h) was detected in correspondence of far end of the couch (patient feet side). In that position the signal decreases with half time of about 3 minutes. Accurate signal half time (T1/2) was evaluated by means of a mono-exponential fit of count rate (T1/2=140s, r2=0.90). Energy spectrum shows an isolated peak at 1.770MeV. There are other peaks in the range 0.4-1.2MeV, from activations in LINAC head were detected. Spectra acquired in other positions show a similar behaviour. Exposure around LINAC head, immediately after beam off, ranges between 2.3 and 1.8μSv/h, near entrance door is 0.3μSv/h, at 1m from coach is 2.3μSv/h. Conclusion Performed measurements are strongly consistent with the activation of aluminium, via the reaction 27 Al(n,γ) 28 Al, which decays beta to 28 Si* which subsequently de-excites via gamma emission to 28 Si. Aluminium components are located in correspondence of the far end of the couch. The workers dose can be evaluated by considering a conservative scenario: a distance of 70cm from the aluminium case, residence time 2min per patient; typically, 15 patients are treated per workshift per day and 20% with high energy beams. The workers dose results below 120μSv/y. This value is about a factor eight lower than the annual dose limit for not exposed workers and people of the public. Moreover, the use of new equipment inside LINAC treatment room (including furniture) should be accurately considered and verified in order to keep workers exposure from neutron activation ALARA. PO-1781 Assessment of water equivalent diameter of CT patients based on effective diameter A. Abuhaimed 1 , C. Martin 2 1 King Abdulaziz City for Science and Technology (KACST), The National Center for Applied Physics , Riyadh, Saudi Arabia; 2 University of Glasgow, Department of Clinical Physics and Bioengineering, Glasgow, United Kingdom Purpose or Objective Size-specific dose estimate (SSDE) is a recommended dose index to be considered to assess the dose delivered to an individual patient from a given CT scan. SSDE overcomes the shortcoming of CTDI vol that is measured in phantoms of reference sizes by using conversion factors that are determined by the patient size. There are several methods used for determination of the patient size, some of which are effective diameter (ED) and water-equivalent diameter (D w ). The purpose of this study is to develop correlations between the two methods under different conditions. Materials and Methods The ED method is based on the patient dimension for the area of interest, whereas the D w method depends on attenuation of the area, which means that it is affected by the photon energy. Therefore, D w values were calculated using equivalent monoenergtic beams, which represent tube potentials of 80 – 140 kVp. A total of 363 paediatric and adult phantoms of a wide range of sizes starting from a new born baby to adult patients were involved. The phantoms set is known as the NCI phantom library, and its weights and heights were in the range of 4 – 125 kg and 51 – 190 cm, respectively. A MATLBA program was developed to calculate ED and D w of each phantom over six scan areas: chest, abdomen, pelvis, chest-abdomen (CA), abdomen-pelvis (AP) and the whole trunk (CAP). In order to obtain accurate correlations, phantoms were separated into two groups, paediatric patients up to 15 years old, and adult patients. Results Influence of tube potential on D w values was minimal, where all differences were within ±2%. This allowed good correlations between ED and energy-independent D w values under the different conditions. For both patient groups, R 2 of the correlations were above 0.94 over all the scan areas studied. The best-fit of correlations are shown in figure 1.