Use of Motion Management to Model Unmanaged Motion: Dosimetric Consequences of Unmanaged Prostate Motion Assessed Using Real-Time Radiotherapy Adaptation Data

This case report presents a novel application of intrafraction prostate motion data from the Radixact Synchrony real-time radiotherapy treatment adaptation system (Accuray Inc., Sunnyvale, USA) to evaluate the dosimetric consequences if a specific patient treatment had been delivered without the use of real-time adaptation, i.e., if the prostate motion had been unmanaged. The patient included in this study was selected from a dataset of 50 completed prostate Radixact Synchrony treatments for which intrafraction motion had been collected. This patient was identified as having the highest proportion of fractions where the target motion exhibited continuous drift, a form of motion that is particularly likely to degrade treatment accuracy, especially when combined with the continuous superior motion of the Radixact treatment couch. Target trajectory data from the Synchrony real-time adaptation system were collected for this patient using the Delivery Analysis software (Accuray Inc.) and used, via a novel application of a deformation vector field that took couch motion interplay into account, to perform several voxel-wise deformations of the patient's planned treatment dose distribution to cover various intrafraction motion scenarios for two treatment regimens. For the standard hypofractionated prostate radiotherapy treatment regimen used in this patient's clinical treatment, 60 Gy in 20 fractions, this study showed that target coverage would have been compromised for this patient had the Synchrony system not been used to adapt the radiation beam to account for the observed prostate motions. Evidently, this patient benefited from the use of real-time target tracking even though an ultra-hypofractionated stereotactic body radiotherapy (SBRT) regimen was not used in this case. By rescaling the patient's hypofractionated dose distribution to model an ultra-hypofractionated SBRT treatment regimen, this study also suggests that a substantial and clinically unacceptable target underdosage might have occurred if this patient had been treated with SBRT and 3 mm margins without the use of the Synchrony system. This result points to an even greater benefit from Synchrony real-time adaptation for SBRT prostate treatments.