Gains from real-time tracking of prostate motion during external beam radiation therapy.

PURPOSE: To study the gains from real-time tracking of prostate motion and threshold-based intervention and the feasibility of margin reduction for external beam radiation therapy of prostate cancer. METHODS AND MATERIALS: Prostate intrafractional motion data from 775 randomly selected treatment fractions (105 prostate patients) were analyzed. Statistical distributions of prostate intrafractional displacement from baseline were used for treatment margin calculation together with other geometrical uncertainties for all patients and a subset of 7 patient who exhibited the largest intrafractional motion. Compared with treatment without any intrafractional intervention, potential reductions in treatment margins were evaluated for treatments with 5-mm and 3-mm threshold-based intervention and four-dimensional (4D) treatments with and without prostate rotation correction.
RESULTS: The percentage of time of prostate displacement from the baseline by 3 mm and 5 mm in any direction was 13.4% and 1.8%, respectively, for the general patient population. The ratios were 41% and 15% for the 7 selected patients. Reductions in the posterior margin were 0.2, 0.5, 1.3, and 3.1 mm from the original 7.7 mm, respectively, for 5-mm and 3-mm threshold-based treatments and 4D treatments with and without prostate rotation correction for all patients. They were 1.3, 1.9, 3.1 and 4.9 mm from the original 9.5 mm, corresponding to the 7 selected patients. The treatment margin reductions in other directions were even smaller.
CONCLUSIONS: Real-time motion tracking and threshold-based intrafractional intervention may play a significant roll in treatment margin reduction for a small fraction of patients but not for the general patient population. Four-dimensional treatments with prostate rotation correction can reduce the treatment margin more significantly.