An automated method to establish patient specific asymmetric intrafraction motion monitoring tolerances for prostate radiotherapy.

[PURPOSE] Intrafraction motion management is recognized as a critical component of radiotherapy. While clinical trials often recommend its use, they rarely define motion management tolerances. Consequently, institutions set tolerance levels independently, often without considering patient specific anatomy, dose distributions or direction of motion. This study introduces an automated method that can be used to derive patient specific, asymmetric intrafraction motion monitoring tolerances based on individualized treatment plans.

[METHODS] Treatment plans for 20 prostate cancer patients receiving a simultaneous integrated boost to dominant intraprostatic lesions (DIL) were retrospectively analyzed. Referred to as the isocentre translation method, patient movement was simulated by recalculating plans with the isocentre shifted by 2, 4 and 5 mm in six different directions. The magnitude and direction that resulted in violations of dose constraints for organ at risk (OAR) rectum, bladder and urethra was determined assuming a systematic shift for all fractions. A second automated approach, the contour translation method, used Eclipse Scripting API to estimate directional tolerances via contour translation and Boolean operations, avoiding dose recalculation. Results for this method were validated against the isocentre translation method and compared for efficiency by assessing processing time.

[RESULTS] The magnitude and direction of motion to cause OAR constraint violations were organ and patient specific. Urethral violations were most sensitive to shifts toward the DIL, whereas bladder and rectum constraints were primarily affected by anterior/superior and posterior shifts, respectively. The contour translation method was demonstrated to be equivalent to the isocentre translation method within an equivalence margin of 0.5 mm. The contour translation method significantly reduced processing time (4 min 40s vs. 47 min 52s per patient).

[CONCLUSION] The direction and extent of motion impacting OAR constraints vary by patient and organ, supporting the need for personalized intrafraction motion monitoring tolerances. The proposed contour translation method provides a practical, efficient process that is able to facilitate individualized motion management in clinical workflows.