Analysis of couch position tolerance limits to detect mistakes in patient setup

Abstract

This work investigates the use of the tolerance limits on the treatment couch position to detect mistakes in patient positioning and warn users of possible treatment errors. Computer controlled radiotherapy systems use the position of the treatment couch as a surrogate for patient position, and a tolerance limit is applied against a planned position. When the couch is out of tolerance, a warning is sent to a user to indicate a possible mistake in setup. A tight tolerance may catch all positioning mistakes while at the same time sending too many warnings; a loose tolerance will not catch all mistakes. We developed a statistical model of the absolute position for the three translational axes of the couch. The couch position for any fraction is considered a random variable xi. The ideal planned couch position xp is unknown before a patient starts treatment and must be estimated from the daily positions of xi. As such, xp is also a random variable. The tolerance, tol, is applied to the difference between the daily and planned position, di=xi−xp. The di is a linear combination of random variables and therefore the density of di is the convolution of distributions of xi and xp. Tolerance limits are based on the standard deviation of di such that couch positions that are more than two standard deviations away are considered out of tolerance. Using this framework, we investigated two methods of setting xp and tolerance limits. The first, called first day acquire (FDA), is to take couch position on the first day as the planned position. The second is to use the cumulative average (CumA) over previous fractions as the planned position. The standard deviation of di shrinks as more samples are used to determine xp and, as a result, the tolerance limit shrinks as a function of fraction number when a CumA technique is used. The metrics of sensitivity and specificity were used to characterize the performance of the two methods to correctly identify a couch position as in‐ or out‐of‐tolerance. These two methods were tested using simulated and real patient data. Five clinical sites with different indexed immobilization were tested. These were whole brain, head and neck, breast, thorax, and prostate. Analysis of the head and neck data shows that it is reasonable to model the daily couch position as a random variable in this treatment site. Using an average couch position for xp increased the sensitivity of the couch interlock and reduced the chances of acquiring a couch position that was a statistical outlier. Analysis of variation in couch position for different sites allowed the tolerance limit to be set specifically for a site and immobilization device. The CumA technique was able to increase the sensitivity of detecting out‐of‐tolerance positions while shrinking tolerance limits for a treatment course. Making better use of the software interlock on the couch positions could have a positive impact on patient safety and reduce mistakes in treatment delivery.PACS number: 87.55.Ne, 87.55.Qr, 87.55.tg, 87.55.tm

A self‐checking treatment couch coordinate calculation system in radiotherapy

“…Additionally, their workflow is not efficient, as it consists of several steps requiring human involvement in decision‐making. Several other groups reported efforts to detect setup mistakes by tightening the site‐specific tolerances for the couch coordinates. These methods, commonly known as tolerance table approaches, do not calculate the table coordinates in advance.…”

Reducing patient posture variability using the predicted couch position

“…Data on couch position variations for multiple immobilization devices have been published by Hadley et al 2 Data for head and neck immobilization have been published by Vakaet et al 7 Engelsman et al 8 use the couch positions to determine the interfractional patient motion for head and neck immobilization devices. Most studies on interfractional patient motion are based on imaging 9-11 and do not report the variation in couch position.…”

“…Hadley et al 2 describe a method in which the cumulative average over previous fractions is taken as the planned couch position. However, this may still differ from the couch position during planning computed tomography (CT).…”

“…The verification of the treated couch position is used to prevent errors in radiation delivery. [1][2][3] Tolerance limits are used to detect errors in patient setup. Regarding patient safety, one can consider the verification of the couch position as one of the defensive layers in the Swiss cheese model by Reason.…”

Site‐specific tolerance tables and indexing device to improve patient setup reproducibility

“…Patton et al (7) reviewed all the treatment errors from one institution and found inappropriate tolerance tables to be one type of error related to the R&V system. Hadley et al (8) compared the sensitivity and specificity between two baselines to calculate tolerance: a first day position and a cumulative average over previous fractions. They found that it was better to use the average as a baseline instead of the first day's couch parameters.…”