RPM tracing for the detection of changes in lung tumor position: In response to Alderliesten et al. Radiother Oncol 2012;105(2):155–60

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Without implanted fiducials, conventional treatment platforms do not routinely permit (near) real-time monitoring of the target position during treatment. We therefore read with interest the rigorous analysis by Alderliesten et al. [1], concerning surface imaging for monitoring lung tumor and patient position during stereotactic body radiotherapy (SBRT). They clearly illustrate the challenges of developing high-fidelity external surrogates for lung tumor position [2,3]: the technique was not useful in males and even in females the area under the ROC curve was no higher than 0.7, a 4mm threshold for a ROI based on both sides of the patient had a false positive rate of 0.31, and the difference between CBCT-derived tumor position and surface monitoring could be several millimeters.