Establishing diagnostic reference levels and optimization of radiation dose for head and neck and pelvic computed tomography protocols in radiation therapy planning

Study Procedure

Before initiating scan, the participant will be explained about the study and written informed consent will be obtained. The age and sex of the participants will be recorded, the following parameters will be collected from CT console for CT examination of head and neck and pelvic such as Tube potential, Tube current, Scan time, Rotation time, Slice thickness, Slice beam collimation, Pitch, Total slices, Field of view, start couch level and end couch level, Reconstruction algorithm that will be used.

The patients will be in supine position on flat couch and immobilization device fixed to particular area of scan with reference markers, head first into the gantry. Centre the table height such that the external auditory meatus is at the center of the gantry. Scanning will be performed using present Onco scan protocol.

CT dose quantities

The two dosimetric quantities namely CTDIvol and DLP will be recorded from dose info for each patient at the end of scan from CT console. CTDIvol will be given in milligray (mGy) and describes the average dose for a volume in either sequential or a helical sequence. DLP will be given in milligray centimeters (mGy.cm) and it is combination of CTDIvol and the scan length that described the total radiation dose received by the patients undergoing CT scans.

Effective dose will be calculated using formula:

E= k x DLP

E= effective dose, K= conversion factor, DLP= Dose Length Product

Phase II: Study Procedure

Optimizing CT radiation dose with respect to CT acquisition parameters would ideally accomplished by conducting the studies based on observer. Initially this can be performed by acquiring CT image of Phantom with different combination of kV and mAs. To find balance between CT image quality and radiation dose, the scans will be acquiring with a Catphan 503 at specified parameters like

Rotation time – 0.5 s/1sec

Slice thickness – 2.5 mm/ 3mm / 5mm

Pitch – 0.813

mAs and kV – all combinations available in CT scanner

After CT acquisition the data will be reconstructed using different algorithms. The reconstructed images will be reviewed by experienced Oncologists and Medical Physicists. By using optimal parameters obtained by analysis of phantom data will be used to create protocols based on optimum parameters.