COMPUTED TOMOGRAPHY DOSE INDEX MEASUREMENT FOR Hi-ART MEGAVOLTAGE HELICAL CT.

On-line megavoltage computed tomography (MVCT) images are used to verify patient daily set-up in Hi-ART helical TomoTherapy unit. To evaluate the patient dose from MVCT scanning in image guidance, weighted computed tomography (CT) dose index (CTDI) was measured with PTW TM30009 CT pencil chamber in head and body phantoms for slice thicknesses of 2, 4 and 6 mm with different scan lengths. Dose length products (DLPs) were subsequently calculated. The CTDI and DLP were compared with XVI kV CBCT and Brilliance simulator CT for routine clinical protocols. It was shown that CTDI and DLP had close relationship with the slice thickness and the scan length. The dose distribution in the transversal plane was very inhomogeneous due to the attenuation of the couch. Patient dose from MVCT was lower than XVI CBCT for the head scan, while larger for body scan. CTDI, which is measured easily and reproducibly, can be used to assess the patient dose in MVCT. Regular measurement should be performed in QA & QC programmes. Appropriate slice thickness and scan range should be chosen to reduce the patient dose.