Comparison of transabdominal ultrasound and electromagnetic transponders for prostate localization

Abstract

The aim of this study is to compare two methodologies of prostate localization in a large cohort of patients. Daily prostate localization using B-mode ultrasound has been performed at the Nebraska Medical Center since 2000. More recently, a technology using electromagnetic transponders implanted within the prostate was introduced into our clinic (Calypso(R)). With each technology, patients were localized initially using skin marks. Localization error distributions were determined from offsets between the initial setup positions and those determined by ultrasound or Calypso. Ultrasound localization data was summarized from 16619 imaging sessions spanning 7 years; Calypso localization data consists of 1524 fractions in 41 prostate patients treated in the course of a clinical trial at five institutions and 640 localizations from the first 16 patients treated with our clinical system. Ultrasound and Calypso patients treated between March and September 2007 at the Nebraska Medical Center were analyzed and compared, allowing a single institutional comparison of the two technologies. In this group of patients, the isocenter determined by ultrasound-based localization is on average 5.3 mm posterior to that determined by Calypso, while the systematic and random errors and PTV margins calculated from the ultrasound localizations were 3 - 4 times smaller than those calculated from the Calypso localizations. Our study finds that there are systematic differences between Calypso and ultrasound for prostate localization.

Figure 1

Axial and sagittal ultrasound images shown superimposed with bladder, prostate, seminal vesicles, and rectum contours from the treatment planning CT.

Figure 2

The Calypso console (left) with touch screen monitor and transmitter / receiver array, which generates a resonant signal in $8\text{mm}\text{long}\times 2\text{mm}$ diameter transponders (top‐right) implanted in the right and left base and the apex of the prostate (bottom‐right). The implantation is done under the guidance of ultrasound in a manner similar to a needle biopsy.

Figure 3

Distribution of setup errors relative to skin marks in 16,619 ultrasound localization procedures performed from 2000 through 2007.

Figure 4

Distribution of setup errors relative to skin marks in 1524 Calypso localization procedures performed during a multi‐institutional clinical trial.

Figure 5

Moving average of the 3D vector shift for patients localized with BAT in chronological order from 2000 through September 2007. For clarity, a 40 and 200 point moving average is shown in the red and black lines, respectively. Note the change that occurred in March 2004 due to a modification in the ultrasound localization procedure.

Figure 6

Distribution of setup errors relative to skin marks in 10,913 ultrasound localization procedures performed from 2000 through 2004.

Figure 7

Distribution of setup errors relative to skin marks in 5706 ultrasound localization procedures performed from 2005 through 2007.

Figure 8

Distribution of setup errors relative to skin marks in 829 ultrasound localization procedures performed between March and September, 2007.

Figure 9

Distribution of setup errors relative to skin marks in 640 Calypso localization procedures performed between March and September, 2007.