Observations on prostate intrafraction motion and the effect of reduced treatment time using volumetric modulated arc therapy.

PURPOSE

Intensity modulated radiation therapy (IMRT) has allowed accurate delivery of prostate radiotherapy; Volumetric modulated arc therapy (VMAT) offers an advancement of this technique with possible dosimetric advantages and delivery in a shorter time than standard IMRT. We hypothesize that treatment duration is a controllable factor associated with intrafraction target motion.

METHODS

Included patients were treated for localized prostate cancer using IMRT or VMAT (RapidArc, Varian Medical Systems, Palo Alto, CA). Continuous motion data were monitored simultaneously using electromagnetic transponders (Calypso 4D Localization System, Calypso Medical Technologies, Inc, Seattle, WA). Displacements were recorded in the RL (right-left), SI (superior-inferior), and AP (anterior-posterior) directions at 10/second (Hz). Daily motion was reported as the mean (R̄̄) and 95th percentile (R95) displacement value for the entire session. Time effect was assessed by measuring daily displacement variables (R̄̄, R95) after each successive minute of treatment.

RESULTS

Thirty-seven patients were included, accounting for 1332 treatment sessions. Mean session time was 7.4 minutes (range, 0.5-37.2; interquartile range, 4.8-9.2). R̄̄ (0.06, 0.08, 0.11, 0.18) and R95 (0.14, 0.18, 0.23, 0.36) values (RL, SI, AP, 3-dimensional [3D], respectively) were evaluated for the entire cohort. Regression analysis showed treatment time to be the strongest predictor of observed displacements (P < .001 AP, SI, 3D; P < 0.05 RL). Ninety-five displacements increased continuously from 0.05 cm, 0.09 cm, 0.12 cm, and 0.16 cm after 1 minute to 0.21 cm, 0.20 cm, 0.29 cm, and 0.47 after 10 minutes (RL, SI, AP, and 3D). Mean session time for VMAT was 4.6 minutes compared to 8.4 minutes for IMRT (difference = 3.8 min, P < .0001); VMAT was associated with reduced motion for both (difference = 0.02, 0.03, 0.05, 0.07 cm) and (0.03, 0.04, 0.11, 0.12 cm) displacements.

CONCLUSION

Our study is unique in exploring the role of session duration on intrafraction motion in the setting of electromagnetic transponders as well as VMAT. Our main results demonstrate that observed intrafraction prostate motion during radiotherapy is greater with increasing session time. Additionally, VMAT, due to shorter treatment sessions, resulted in significant reduction (30%-40%) in intrafraction displacements.