Measurements of CTDI100 and CTDIvol for MVCT Scans On a TomoTherapy System
E Chao*, Accuray Incorporated, Madison, WISU-E-J-41 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
To measure the computed tomography dose index (CTDI) for MVCT scans on the TomoTherapy™ System.
The CTDI₁₀₀ and CTDIvol were measured using a Standard Imaging A17 ion chamber inserted into a standard CTDI phantom (32 cm diameter and 16 cm diameter acrylic cylinders for body or head CTDI measurements, respectively). Dose measurements were performed using both a 'Fine' pitch helical scan with a scan range of ±2 cm, as well as an equivalent axial scan generated using a calibration procedure. The two methods yielded identical results, indicating the scattered radiation is well contained within the A17 ion chamber's active length. The A17 ion chamber has a non-standard active length for CTDI measurements of 11.0 cm. However, since the scattered dose is well contained within the A17's active length, the non-standard length can be accounted for in the CTDI calculation. Finally, the CTDI₁₀₀ calculation requires a choice of a nominal axial slice thickness that is not well defined for the TomoTherapy™ System since axial scans are not an available imaging mode. A 4 mm nominal slice thickness was chosen. For the CTDIvol calculations, this choice of nominal slice thickness results in a convenient pitch of 1, 2, and 3 for the Fine, Normal, and Coarse imaging modes, respectively.
On the 32 cm body phantom, the center CTDI₁₀₀ was 2.18 cGy and the average of the peripheral measurements was 2.72 cGy. The volume CTDI values are calculated to be CTDIvol (Fine) = 2.54 cGy, CTDIvol (Normal) = 1.27 cGy, and CTDIvol (Coarse) = 0.85 cGy. On the 16cm head phantom, the volume CTDI values are calculated to be CTDIvol (Fine) = 3.48 cGy, CTDIvol (Normal) = 1.74 cGy, and CTDIvol (Coarse) = 1.16 cGy.
Conclusion: The CTDI values described here provide a reference measured on one representative TomoTherapy™ System.