Organ-specific modulation complexity score for the evaluation of dose delivery

The purpose of this study was to correlate the modulation complexity score (MCS) with organ location and to predict potential dose errors for organs before beam delivery for intensity-modulated radiation therapy (IMRT) dosimetry. Sixteen head and neck cancer patients treated with IMRT were selected. Distribution of the relative dose error on each beam was performed using forward projection to the planned dose to compute the predicted dose after doing per-beam quality assurance. Original organ-specific modulation complexity score (oMCS) was created based on a modified MLC, which depended on organ location. First, MCS was calculated based on the change in leaf position between adjacent MLC leaves. Second, the segment edge map (SEM) calculated from the intensity map for each beam was applied to the calculation volume. The oMCS with segment edge (oMCS(edge)) was derived from the product of oMCS and SEM. The correlation between the dose errors (planned and predicted) and oMCS(edge) values was evaluated for the target and organs at risk. We have also expanded the original MCS concept to oMCS(edge) including the organ location. We observed a moderate correlation between the dose errors and oMCS(edge) for all organs and volumes of interest except the gross tumor volume, brain stem, and spinal cord. In other organs, a moderate improvement in sensitivity was observed on the SEM, which was correlated with dose errors. Although the implementation of oMCS(edge) would be impractical for normal clinical settings, it is expected that oMCS(edge) would help a treatment planner to judge whether or not the treatment plan would be acceptably delivered.