Field Placement Correction Using MV IGRT. Is Postintervention Imaging Necessary?

Purpose: Online image-guided radiation therapy (IGRT) is used for all radical pelvic patients at Radiation Oncology Queensland. One linear accelerator is equipped with megavoltage electronic portal imaging. Daily imaging on this linear accelerator introduces a dose burden that must be accounted for in the planning process. This dose burden is further complicated by postintervention images taken to verify field placement corrections. Analysis of setup errors and number and management of isocenter shifts was also used to identify an appropriate dose burden to be applied.

Method: The IGRT data of 50 radical pelvic patients were retrospectively collected and analysed, and the number of isocenter moves made was assessed. Statistical analysis of systematic and random errors, both preintervention and postintervention, was undertaken. Inclusive in this analysis was the number of times postintervention images revealed an error in manually entered isocenter shifts. The imaging dose used was also investigated.

Results: Online IGRT was able to reduce the setup error to <2 mm for all orthogonal planes. Postintervention imaging was shown to be necessary to assess field placement, because manual errors in field placement were found to occur. The generic dose burden in use was found to be excessive.

Conclusion: Daily IGRT is now considered an essential tool in modern radiation therapy. Postintervention imaging is required to ensure correct isocenter placement on linear accelerators where the process is manual. The current estimate of the worst-case scenario dose burden may be reduced to either incorporate a "population" dose or a more realistic absolute maximum dose. Any removal of a quality assurance process such as this requires evidence, consultation, and careful consideration.