Clinical rationale for in vivo portal dosimetry in magnetic resonance guided online adaptive radiotherapy

BACKGROUND AND PURPOSE: In magnetic resonance guided online adaptive radiotherapy, the patient model used for plan adaptation and dose calculation is created online under stringent time constraints. This study investigated the ability of in vivo portal dosimetry to detect deviations between the online patient model used for plan adaptation and the actual anatomy of the patient during delivery. MATERIALS AND METHODS: Portal images acquired during treatment were used to reconstruct the delivered dose corresponding to online adapted plans of 42 prostate and 20 rectal cancer patients. The reconstructed dose distributions were compared with the dose distributions calculated online by the treatment planning system by γ-analysis and by the difference in median dose to the high-dose volume. RESULTS: Out of 245 prostate and 145 rectal cancer adapted plans, deviations were detected in 5 prostate and in 17 rectal adapted plans corresponding to 3 prostate and 6 rectal patients, respectively. For all but one of the alerts, deviations were explained due to discrepancies observed between the patient model used for plan adaptation and online magnetic resonance images. A single workflow incident in which the supporting arm of the anterior receive coil was accidentally moved in the treatment field was also detected. CONCLUSION: There is need for independent end-to-end checks in magnetic resonance guided online adaptive workflows including the verification of the online patient model. In vivo portal dosimetry can be used for such purpose as it can detect both patient related deviations and workflow incidents.