Comparison of correction protocols for image-guided radiation therapy

In radiation therapy, patient positioning uncertainty and organ motion require that a volume larger than the actual tumour (named the planning target volume [PTV]) be irradiated to ensure that the tumour receives the prescribed dose. Image-guided patient positioning can correct targeting errors, thus reducing the uncertainty in the position of the tumour, and therefore, the size of the PTV. Positioning uncertainties are reduced with frequent imaging, but require increasing the overall time of a treatment session. We compare conventional 'off-line' error correction protocols to our 'on-line' correction protocol for prostate cancer patients. Simulations indicate that our on-line protocol leads to much smaller residual uncertainty than off-line protocols, leading to much smaller PTV margins. Our on-line protocol allows the irradiation of substantially smaller volumes than conventional off-line protocols, leading to reduced normal tissue complications. Introducing an "intervention threshold" has retained some of the efficiency of the off-line strategy.