Time Dependence of Intrafraction Patient Motion Assessed by Repeat Stereoscopic Imaging

doi:10.1016/j.ijrobp.2007.08.066

International Journal of Radiation OncologyBiologyPhysics

Volume 70, Issue 2, 1 February 2008, Pages 609-618

https://doi.org/10.1016/j.ijrobp.2007.08.066 Get rights and content

Purpose

To quantify intrafraction patient motion and its time dependence in immobilized intracranial and extracranial patients. The data can be used to optimize the intrafraction imaging frequency and consequent patient setup correction with an image guidance and tracking system, and to establish the required safety margins in the absence of such a system.

Method and Materials

The intrafraction motion of 32 intracranial patients, immobilized with a thermoplastic mask, and 11 supine- and 14 prone-treated extracranial spine patients, immobilized with a vacuum bag, were analyzed. The motion was recorded by an X-ray, stereoscopic, image-guidance system. For each group, we calculated separately the systematic (overall mean and SD) and the random displacement as a function of elapsed intrafraction time.

Results

The SD of the systematic intrafraction displacements increased linearly over time for all three patient groups. For intracranial-, supine-, and prone-treated patients, the SD increased to 0.8, 1.2, and 2.2 mm, respectively, in a period of 15 min. The random displacements for the prone-treated patients were significantly higher than for the other groups, namely 1.6 mm (1 SD), probably caused by respiratory motion.

Conclusions

Despite the applied immobilization devices, patients drift away from their initial position during a treatment fraction. These drifts are in general small if compared with conventional treatment margins, but will significantly contribute to the margin for high-precision radiation treatments with treatment times of 15 min or longer.

Introduction

With advanced in-room imaging techniques, the interfraction patient setup error could in principal be reduced to the millimeter or even submillimeter range 1, 2, 3, 4, 5. To guarantee that the patient does not move after the initial setup verification and correction, immobilization devices are used. In general, we prefer noninvasive over invasive devices for reasons of patient comfort and ease of use. These devices, such as vacuum bags and thermoplastic masks, cannot fully eliminate intrafraction movement. The magnitude and probability of patient movement from the initial setup to the moment of dose delivery will most likely increase when fraction times are extended by, for example, the use of intensity-modulated radiation therapy, noncoplanar treatment techniques, hypofractionation, and extensive image-guidance procedures. In such cases, intrafraction patient motion might have an impact on the required clinical target volume to planning target volume margin.

In this article, we present detailed measurements of intrafraction motion of the skull and the spine, the latter treated in supine or prone position. Intrafraction motion was assessed by analyzing stereoscopic kV X-ray images acquired on average every 1.7 min during CyberKnife (6) treatment fractions. Only a few studies on intrafraction patient motion have been published 4, 7, 8, 9, 10. In most of these studies, imaging data acquired at the start and the end of a treatment fraction, or just before and after delivery of a treatment beam, were analyzed. The relatively long durations (45–70 min) of CyberKnife treatments and that the patient position is updated regularly made it possible to quantify the time dependence of intrafraction motion. Murphy et al. also measured intrafraction motion with the CyberKnife image-guidance system, but only reported patient movements between consecutive image acquisitions taken at 1-min to 2-min intervals. The purpose of the current study was to quantify intrafraction patient movements over a longer period. Furthermore, we differentiated systematic drifts in patient motion from random displacements. The results can be used to optimize the imaging and correction frequency for CyberKnife treatments. For high-precision radiation treatments on conventional linear accelerators, the results can be used to incorporate intrafraction motion associated with longer treatment times in the margin calculation.

Section snippets

Patient group

The intrafraction motion data of 57 consecutive patients, all treated with the robotic CyberKnife therapy unit, were analyzed. The number of treatment fractions per patient ranged between 1 and 12. Thirty-two patients were treated for intracranial and head-and-neck tumors, the others for extracranial tumors. Fourteen of the latter patients were treated in a prone, and 11 in a supine orientation. The prone orientation was chosen for tumors located dorsally, because the CyberKnife has limited

Treatment characteristics

Table 1 summarizes the mean treatment time per fraction, the mean number of localizations per treatment fraction and per patient, and the localization frequency for the three patient groups. The initial setup and intermediate setups were not included in the reported values. Intermediate setups are setups between treatment parts, setups that follow couch corrections if the patient motion exceeded the limits of the robot correction, and setups that follow treatment interruptions for other reasons.

Accuracy of the imaging system

The overall accuracy of the system, which comprises the accuracy related to the acquisition of the planning CT scan, the patient or target position verification during the treatment, the motion compensation, and the actual dose delivery, has been measured on a regular basis. The measurements were performed with an anthropomorphic head phantom loaded with two orthogonal radiochromic films. The mean 3D overall targeting error (the displacements of the dose contours of the treatment plan and the

Conclusions

Despite applied immobilization devices, patients drift away from their initial setup during a treatment fraction. These drifts are, in general, small if compared with other error sources (e.g., example interfraction setup errors, delineation errors, geometrical errors from internal organ motion). The drifts are also small compared with conventional treatment margins, which lie in the order of 5–10 mm, but will significantly contribute to the margin for high-precision radiation treatments with

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III.

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Physics ContributionTime Dependence of Intrafraction Patient Motion Assessed by Repeat Stereoscopic Imaging

Purpose

Method and Materials

Results

Conclusions

Introduction

Section snippets

Patient group

Treatment characteristics

Accuracy of the imaging system

Conclusions

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Physics Contribution
Time Dependence of Intrafraction Patient Motion Assessed by Repeat Stereoscopic Imaging