A comparison of kV and MV imaging in head and neck image guided radiotherapy

Abstract 

Purpose

To compare and assess kV and MV imaging modalities and their role in image guided radiotherapy (IGRT) for head and neck cancer patients.

Method

Twelve patients receiving radical radiotherapy to the head and neck were analysed in this study. Six patients undertook MV daily online intervention and a further six patients undertook kV daily online intervention. Pre-intervention field placement data were collected from three separate observers' image match analysis for each patient. The radiotherapy collective involved in the daily online image match analysis formed the fourth observer in the study. The primary end point was to establish the difference in inter- and intra-observer variance between kV and MV imaging modalities.

Results

The range of the standard deviations of systematic set-up error for MV imaging calculated was 1.47–2.33mm (MV) and 1.61–1.64mm (kV) for the right–left (RL), 2.10–2.17mm (MV) and 1.53–1.84mm (kV) for the cranio–caudal (CC) and 1.43–1.63mm (MV) and 1.02–1.11mm (kV) for the anterior–posterior (AP). The mean inter-observer variance was 0.21mm (MV) and 0.41mm (kV) for the RL, 0.53mm (MV) and 0.55mm (kV) for the CC and 0.23mm (MV) and 0.16mm (kV) for the AP direction. Intra-observer mean variance was in the order of 0.60mm (MV) and 0.16mm (kV) for the RL, 1.41mm (MV) and 0.05mm (kV) for the CC and 1.41mm (MV) and 0.08mm (kV) for the AP.

Discussion

The data in this study suggest both inter- and intra-observer consistency across kV and MV imaging modalities were comparable. However, it is felt that the improved clarity and quality of kV imaging allows all observers to analyse images in a consistent manner, identifying and acting on potential field placement moves.

Conclusion

The introduction of kV imaging has maintained the high levels of inter- and intra-observer consistency achieved with MV imaging. This in turn further enables positive verification outcomes and supports the implementation of potential reductions in action thresholds. The increased quality, clarity and field of view offered by kV imaging have established it as the method of choice for head and neck IGRT at Radiation Oncology Queensland.

Keywords: Image guided radiotherapy, Kilovoltage imaging, Megavoltage imaging, Electronic portal imaging, Set-up error, Stabilisation