SU-E-J-242: Post-Treatment Planning Tool for Estimating Dose Distribution Delivered to Spinal Radiosurgery Patients Based On Measured Intra-Fraction Positional Data.
PURPOSE
To demonstrate feasibility and utility of a planning tool for post treatment reconstruction of delivered dose distribution based on measured patient motion. The spinal cord dose limits in clinical use are based on planned dosimetric data which can significantly differ from delivered dose. This tool can be utilized to correlate clinical outcomes to delivered dose METHODS AND MATERIALS: ur previous studies have shown that without intrafraction motion management, patients can move up to 3mm. The current institutional protocol requires intrafraction motion management by stereoscopic imaging prior to each treatment field. Patients are repositioned when 1mm tolerance is exceeded. Dosimetric effects of 62 spinal radiosurgery patients were investigated by simulating patient motion. For each patient plan, three additional plans were created, where the planning isocenter was shifted towards the cord by 1mm, 2mm and 3mm, for a total of 248 plans. D95% was evaluated for the target volume and D0.035cc and V10Gy were evaluated for the spinal cord.


RESULTS
D95% decreased up to 3%, 9% and 15% for 1mm, 2mm and 3mm shifts correspondingly. These shifts resulted in D0.035cc exceeding 12Gy in 26%, 55% and 76% of patients, respectively, while previous work revealed that patients with a 1mm shift exceeded the 12Gy dose Dmax constraint 51% of the time. These respective shifts also resulted in 53%, 65% and 74% exceeding the constraint limit of 10Gy dose to 10% of the spinal cord outlined 6 mm above and below the target volume.


CONCLUSION
We have shown that delivered dose distributions can significantly differ from planned dose distributions. Spinal cord dose limits in clinical use are based on planned data, assuming no patient motion. This tool can be utilized to estimate delivered dose distribution and correlate clinical outcomes to delivered dose data.