The Investigation of OAR Dose Associated by KV-CBCT Imaging in IGRT: A Dosimetric Study
I Catan1*, A Canbolat2, (1) MESI A.S., Turkey, ,(2)Special Defne Hospital , Turkey,SU-E-J-159 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose:kV-CBCT can provide an accurate target placement and patient positioning with reducing OARs doses. However some OARs such as testis and lenses which are not in the field of treatment may have an addtional doses due to being in imaging field. In this study, additional doses result from kV-CBCT were examined for these OARs.
Methods:Rando phantom was used. Particular holes were created for placing TLDs. For this purpose three slices of phantom were replaced with the home-made designed molded paraffin slices, one; in the head, the others; in the pelvic region. TLDs were calibrated with kV-CBCT at Elekta Synergy. Correction factors were applied to readings. Commonly used imaging protocols of pelvic region M10/M20 and for head&neck S10/S20. M10/M20 contain a 360° rotation, 120kV, 40ms-25mA, with 650 frames per scan. S10/S20 contain a 200°, 100kV, 10ms-10mA, with 334 frames.
The changes in OAR doses were investigated using various exposure parameters, i.e. mAs. For this purpose, M10/M20 were measured with 40mA and S10/S20 with 25mA. These values are in the available range of the X-ray tube. Three TLDs were used in each measurement points and exposed 10 times to be able to have reliable single imaging doses.
Results:Testis average doses in M10/M20 were 33.2±0.3/32.8±0.4mGy respectively. As the mA decreased to 25, the doses were reduced to 22.5±0.5/21.8±0.4mGy. Reduction of mA caused approximately %32 lower dose. Similar situation was observed for lenses (Table.1). S20/M20 had lower dose than S10/M10 due to window of FOV is located with off-axis and the spectrum has less effective high energy photons.
Conclusion:The kV-CBCT is known an accurate guidance technique, but a high level of dose per fraction is given to the OARs. TPS cannot take into account this additional doses. Due to biological dose effects, these should be monitored more carefully.