Development of a pragmatic methodology for measurement of dose for KV CBCT in radiotherapy: A pilot study

Imaging modalities such as on-board kilovoltage cone beam computed tomography (kV CBCT) on a linear accelerator (linac) are utilised daily to verify positioning in the multi-week fractionation schedule of a cancer patient undergoing treatment. There is a need for optimisation of radiological protection in kV CBCT imaging protocols to avoid unnecessarily high doses to normal tissues surrounding the target. Traditionally, a 100 mm ionisation chamber and a cylindrical PMMA phantom are used in measuring CT doses, but these are not available in many radiotherapy centres. This study explores an alternative setup using a 0.6 cc Farmer-type ionisation chamber and a 30 cm × 30 cm × 30 cm cubically shaped phantom comprising slabs of water equivalent material typically used in radiotherapy centres. The weighted dose index values determined from the alternative setup are compared with those determined from the conventional cylindrical CT phantoms. The results indicate that the alternative setup using equipment more accessible in radiotherapy facilities is a viable alternative to the use of a CT phantom to quantify kV CBCT radiation dose from linac-based image-guided radiotherapy equipment that can be used for optimisation processes. This should allow most radiotherapy centres all over the world to engage in meaningful dose measurement and optimisation for CBCT.