Are Pre and Post Imaging Sufficient to Assess Intrafraction Prostate Motion?

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

Many publications have used pre and post imaging (x-ray localization of fiducials, cone-beam CT, ultrasound) to assess intrafraction prostate motion. The aim of this study is to determine if this method is a sufficient assessment of movement of the prostate during radiation delivery.

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Materials/Methods 

The Calypso^® 4D Localization System was used to continuously track the prostate isocenter (at a rate of 10 Hz) of 35 patients over 1157 total fractions, representing 195 hours of tracking information. Pre and post static imaging was simulated by sampling the beginning and end of each tracking session data set and generating a linear model trajectory between the two points. This linear trajectory was used to predict whether the prostate exceeded radial margins of 3, 5, 7 and 10 mm for more than 30 seconds. This prediction was then compared with the data as measured by the Calypso System. The ability for the pre and post imaging technique to correctly identify if the tracked prostate isocenter deviated from the specified margins was assessed by calculating the sensitivity and specificity for each individual patient, as well as for the total number of fractions.

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Results 

The sensitivity of pre and post imaging in determining motion greater than 3, 5, 7, or 10 mm for all fractions was 53%, 49%, 39% and 56%, respectively. The sensitivity was low, even for relatively large margins (7, 10 mm) and showed a considerably random distribution. The specificity of the pre and post imaging method in determining motion greater than 3, 5, 7, or 10 mm was 98%, 99%, 100% and 100%. There was large patient variability in the sensitivity, ranging from 12% to 100% in the 3 mm margin (Figure 1). Qualitatively, the poor sensitivity was secondary to the unpredictability of when a 30-second event occurred, and that the events often resolved by the end of the fraction.

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Conclusions 

Pre and post imaging is not a sensitive test of intrafraction prostate motion. Because this method does not accurately reflect the intrafraction movement of the prostate throughout the duration of radiation delivery, caution should be used when relying on pre and post imaging for clinical interventions aimed at intrafraction prostate motion.

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