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Minimizing IGRT Imaging Exposures: KV Radiograph Vs. KV-CBCT Vs. MV Portal Images


G Ding

G Ding1*, P Munro2, (1) Vanderbilt University , NASHVILLE, TN, (2) Varian Medical Systems, iLab GmbH, Switzerland

SU-E-J-39 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
To compare the IGRT doses from MV, kV and CBCT images.
Methods:
kV imaging systems integrated into Varian Trilogy and TrueBeam accelerators were modeled using BEAMnrc/DOSXYZnrc Monte Carlo codes and the dose to calibration phantoms for a variety of kV beams (kVp, bow-tie filters, etc.) were calculated. The doses to the same phantoms and kV beams were then measured experimentally using calibrated ion-chambers. The "calibrated" Monte Carlo kV beams were used to calculate dose to CT images of patients. Organ doses were analyzed using DVHs.
Results:
The doses to the prostate are 0.015 and 2.2cGy using AP kV and MV images; are 0.06 and 2.3cGy using lateral kV and MV images; and, are 1.7 cGy using CBCT images. For head and neck images, the doses to the eye are 0.08 and 0.001cGy using AP and PA kV images; are 2.3 and 1.8cGy using AP and PA MV images; are 0.001 and 2.4cGy for lateral kV and MV images; and, are 0.2 cGy for CBCT images. For kV radiographs, organ doses can be further reduced, by over 30%, by using bow-tie filters. CBCT doses to the prostate are 1.6 and 0.9cGy for OBI and TrueBeam pelvis scans; a >40% dose reduction for the same image quality. For OBI CBCT head scans the doses to the eye and brain stem are 0.2 and 2.8cGy, respectively.
Conclusions:
Due to the low penetration of kV beams, selecting beam angles so that the sensitive organs are near the beam exit, and/or using bow-tie filters, can substantially reduce organ doses when using kV radiographs. For daily positioning of pediatric brain patient with a set of orthogonal kV images, a CBCT scan, or a set of orthogonal MV images, the doses to the eyes are 0.1, 0.2, and 4.7 cGy, respectively.


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