Purpose: To evaluate dosimetric consequences of intra-fraction interventions for the motion management of SBRT prostate patients.
Methods: Intrafraction monitoring at our institution consists of taking MV- KV pairs every 20 degrees; intervention is triggered when motion above 0.15 cm is observed for two consecutive image pairs. This monitoring system output contains the residual motion after correction (corrected). The natural uncorrected motion (uncorrected) has been reconstructed by subtracting the couch shifts from the corrected motion. We have identified 11 fractions where the patient average 3D uncorrected motion was greater than the PTV margin of 0.3cm. To simulate the dosimetric impact of this motion, an inhouse program, MoDoC, splits the VMAT beam into 177 static beams. Each beam is shifted in the opposite direction of the motion data. Finally, the plans for both corrected and uncorrected motions are calculated and CTV D95 and minimum dose were compared (paired T-test).
Results: The mean average 3d natural motion for this cohort was 0.54 ± 0.25cm. The mean CTV D95% was 91.8 ±9 and 98.1 ± 2 (p ₌ 0.04) and minimum CTV dose was 78±15 and 91.2 ± 5 (p ₌ 0.015) for uncorrected and corrected respectively.
Conclusion: We demonstrated that our homegrown system permits the evaluation of clinical significance of intra-fraction interventions. Such system can be used to retrospectively evaluate intervention strategies, and perform patient-specific evaluations, possibly resulting in more accurate dose delivery while reducing unnecessary interventions.