Purpose: To assess the dosimetric consequence of intrafraction prostate motion on helical tomotherapy plans.
Methods and materials: An electromagnetic tracking device was used to measure real-time prostate motion for 515 fractions (16 patients). Motion tracks were used to retrospectively recalculate dose distributions using a four-dimensional calculation engine. The minimum dose (D(min)), maximum dose (D(max)), and dose to 95% of the volume (D(95%)) were calculated for target volumes and compared with respective values from the treatment plan. The dosimetric effect was evaluated for each fraction. For each patient, the running cumulative effect was assessed throughout the course of treatment. Calculations were repeated assuming a time delay between initial patient setup and start of treatment.
Results: Averaged over all fractions, the mean change in target D(95%) was <1% (SD, 3-4%). Reductions in target D(95%) of up to 20% were seen in individual fractions. Changes in prostate D(95%) were similar in frequency and magnitude to D(95%) changes in the planning target volume. The cumulative effect on target D(95%) was approximately 1% (SD, 1%). The average cumulative effect after five fractions was 1% (SD, 1.5%).
Conclusions: In general, the dosimetric effect of observed prostate motion on target D(95%)was small. Infrequently severe D(95%) degradations were observed for individual fractions, but their effect on the cumulative dose distribution was quickly reduced with minimal fractionation.