ESTRO 36 Abstract Book

S810 ESTRO 36 _______________________________________________________________________________________________

calculate PDDs, lateral profiles and output factors in a water phantom for stereotactic cones with 5, 7.5, 10, 12.5 and 15 mm nominal diameter. Results from the simulations were compared against measurements performed in water phantom with PTW PinPoint ion chamber and Scanditronix stereotactic diode. Actual cone diameter was found by the best match between the calculated and measured lateral profiles. Sensitivity of output factor to cone diameter variations was investigated. For this purpose, nominal cone diameter was changed by +/- 0.3 mm (which is twice the manufacturer stated uncertainty of 0.15mm). Results Lateral profiles agreed within 2%/0.5mm for all cone sizes. Actual cone diameters were found to be 5.30, 7.70, 10.15, 12.65 and 15.15 mm. For the actual cone diameter, output factors agreed within 2% for all cones except for cone 5 mm where the difference was 4%. Cone diameter uncertainty of 0.3 mm lead to up to 11% variation in the output factor compared to output factor value calculated for the nominal diameter. Conclusion The MC model of the VersaHD linac was employed for investigation and characterization of stereotactic cones. Measured data were verified by the MC calculations. Differences between nominal and actual cone diameter were observed. Given the level of manufacturing accuracy and sensitivity of dosimetric parameters to the cone diameter variation, accurate commissioning of stereotactic cones must be performed and comparison with the data from other centers may be misleading. EP-1511 Radiation Dose from Megavoltage Cone Beam Computed Tomography for IGRT E. Kara 1 , B. Dirican 2 , A. Yazici 1 , A. HICSONMEZ 1 1 Onko Ankara oncology center, Oncology Department, Ankara, Turkey 2 Gulhane Research and Education Hospital, Radiation Oncology, Ankara, Turkey Purpose or Objective Imaging dose in radiotherapy has generally been ignored due to its low magnitude in comparison to therapeutic dose used to treat patients. However, the total number of fractions can range from 30 to 40 fractions for radical IMRT. The cone beam computed tomography (CBCT) dose to patients can be substantial. Daily imaging results in additional dose delivered to patient that warrants new attention be given to imaging dose. In this study, we try to figure out the organ dose of CBCT for head&neck and pelvis’s critical organs with three different CBCT protocols. We also compare the image quality of these protocols and try to find optimum one for dose and image quality. Material and Methods Organ doses were measured for three different megavoltage CBCT protocols on the Siemens Artiste linear accelerator treatment machine. Organ doses were measured by distributing thermoluminescent dosimeters (TLDs) throughout critical organs of an anthropomorphic (RANDO) phantom. The selected organs are rectum, bladder, femoral heads and small intestine for pelvis imaging and spinal cord, brainstem, tiroid and parotid glands for head and neck imaging. The CBCT protocols were 8MU, 15 MU and 8 MU half cycle. Slice size (512x512 pixels), slice thickness (0.54 mm), number of slices (512) and SID (145 cm) were same for each protocol. The numbers of projections are 360 for 8MU&15 MU protocol and 200 for 8 MU half cycle protocol. The placement of TLDs was done with the guidance of an atlas of the anatomy. The TLDs placed RANDO phantom was irradiated by using three different imaging protocol and the doses were compared. We have also performed image quality

tests for each protocol. The used image quality phantom was 20 cm diameter with four 2 cm sections: 1 solid water section for noise and uniformity, 2 sections with inserts for contrast resolution and 1 section with bar groups for spatial resolution. We have performed image quality tests for each CBCT protocols. Results We have seen that 15 MU protocol has no difference with 8 MU protocols in the means of image quality and the dose of critical organs are much higher than the others as expected. When we compare 8 MU and 8 MU half ring protocols in the means of organ doses, we have seen that the doses of organs changes according to the geometrical placements of organs. Accordingly, while the doses of organs, such as rectum, spinal cord and brainstem, nearby the posterior decreases with the use of 8 MU half protocol, the doses of organs located anterior, such as intestine, thyroid and bladder, increases. It is observed that both the contrast resolution and the spatial resolution of the 8 MU half protocol is better than the 8 MU protocol. It also gives information about position in a shorter time.

Conclusion After obtain all this information about MV CBCT protocols, we figure out that the choice of CBCT protocol should be done after treatment planning by considering of the doses and location of the critical organs. than the others as expected. EP-1512 Comparison between dose transmission detector and 3d dosimetry for lung SBRT treatments. F.R. Giglioli 1 , E. Gallio 1 , C. Fiandra 2 , O. Hammad 3 , R. Ragona 2 1 A.O.U. Città della Salute e della Scienza- Department of Medical Physics- Torino- Italy, Medical Physics, Torino, Italy 2 University of Turin- Radiotherapy Unit - Torino- Italy, Department of Oncology, Torino, Italy 3 International Center for Theoretical Physics- Trieste- Italy, ICTP, Trieste, Italy

Purpose or Objective