SU-F-J-48: Effect of Scan Length On Magnitude of Imaging Dose in KV CBCT.

PURPOSE: To study effect of scan length on magnitude of imaging dose deposition in Varian kV CBCT for head & neck and pelvis CBCT.

METHODS: To study effect of scan length we measured imaging dose at depth of 8 cm for head and neck Cone Beam Computed Tomography (CBCT) acquisition (X ray beam energy is used 100kV and 200 degree of gantry rotation) and at 16 cm depth for pelvis CBCT acquisition (X ray beam energy used is 125 kV and 360 degree of gantry rotation) in specially designed phantom. We used farmer chamber which was calibrated in kV X ray range for measurements .Dose was measured with default field size, and reducing field size along y direction to 10 cm and 5 cm.

RESULTS: As the energy of the beam decreases the scattered radiation increases and this contributes significantly to the dose deposited in the patient. By reducing the scan length to 10 Cm from default 20.6 cm we found a dose reduction of 14% for head and neck CBCT protocol and a reduction of 26% for pelvis CBCT protocol. Similarly for a scan length of 5cm compared to default the dose reduction in head and neck CBCT protocol is 36% while in the pelvis CBCT protocol the dose reduction is 50%.

CONCLUSION: By limiting the scan length we can control the scatter radiation generated and hence the dose to the patient. However the variation in dose reduction for same length used in two protocols is because of the scan geometry. The pelvis CBCT protocol uses a full rotation and head and neck CBCT protocol uses partial rotation.