Analysis of prostate patient setup and tracking data: potential intervention strategies

Abstract

Purpose: To evaluate the setup, interfraction, and intrafraction organ motion error distributions and simulate intrafraction intervention strategies for prostate radiotherapy.

Methods and materials: A total of 17 patients underwent treatment setup and were monitored using the Calypso system during radiotherapy. On average, the prostate tracking measurements were performed for 8 min/fraction for 28 fractions for each patient. For both patient couch shift data and intrafraction organ motion data, the systematic and random errors were obtained from the patient population. The planning target volume margins were calculated using the van Herk formula. Two intervention strategies were simulated using the tracking data: the deviation threshold and period. The related planning target volume margins, time costs, and prostate position "fluctuation" were presented.

Results: The required treatment margin for the left-right, superoinferior, and anteroposterior axes was 8.4, 10.8, and 14.7 mm for skin mark-only setup and 1.3, 2.3, and 2.8 mm using the on-line setup correction, respectively. Prostate motion significantly correlated among the superoinferior and anteroposterior directions. Of the 17 patients, 14 had prostate motion within 5 mm of the initial setup position for ≥91.6% of the total tracking time. The treatment margin decreased to 1.1, 1.8, and 2.3 mm with a 3-mm threshold correction and to 0.5, 1.0, and 1.5 mm with an every-2-min correction in the left-right, superoinferior, and anteroposterior directions, respectively. The periodic corrections significantly increase the treatment time and increased the number of instances when the setup correction was made during transient excursions.

Conclusions: The residual systematic and random error due to intrafraction prostate motion is small after on-line setup correction. Threshold-based and time-based intervention strategies both reduced the planning target volume margins. The time-based strategies increased the treatment time and the in-fraction position fluctuation.

Conflict of interest statement

Conflict of Interest:

None.

Copyright © 2011 Elsevier Inc. All rights reserved.

Figures

Figure 1

Cumulative probability intrafraction prostate motion along each axis and in 3D distance.

Figure 2

Motion correlations among the three different axes for all the treatment sessions of the patient population.

Figure 3

Time fraction of prostate 3D displacement greater than 3, 5, 7 and 10 mm as a function of elapsed time since initial interfraction error adjustment. The time fraction displaced was estimated for 1 minute time bins.

Figure 4

3D Displacement of isocenter from planned location for each patient, averaged over all measurements and fractions, each intervention technique.

Figure 5

The quadratic mean of fraction standard deviation for every patient of the original tracking data (bar symbols) and its ratios of each intervention technique to the original tracking data (hollow symbols).