Clinical investigation
Prostate
Comparison of ultrasound and implanted seed marker prostate localization methods: Implications for image-guided radiotherapy

Presented as a poster discussion at the 47th Annual Meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO), Oct. 19, 2005, Denver, CO.
https://doi.org/10.1016/j.ijrobp.2006.01.008Get rights and content

Purpose: To analyze two methods of image-guided radiotherapy (IGRT) for external beam radiotherapy of prostate cancer.

Methods and Materials: The prostate was localized by ultrasound (US) in lateral (left/right), vertical (anteroposterior), and longitudinal (superior/inferior) dimensions and then by fiducial seed marker (SM) kV X-ray. Assuming initial setup to skin marks as the origin, the mean suggested shifts (for all dimensions) were hypothesized to be similar and within 1 mm of the origin. The three-dimensional distance discrepancy between suggested SM and US shift points was calculated. We hypothesized a mean discrepancy >5 mm as clinically significant.

Results: From 40 patients, 1019 US/SM measurements were obtained. Lateral, vertical, and longitudinal dimensional comparisons reveal statistically significant differences in mean shifts (p < 0.0001 for all). US dimensional shifts reveal significantly greater variability. The US three-dimensional vector is greater and more variable than the SM vector (p < 0.0001). The mean US/SM three-dimensional distance discrepancy is 8.8 mm (significantly >5 mm, p < 0.0001).

Conclusions: Ultrasound and SM methods suggest different shifts. US data reveal greater systematic/random error vs. SM data. The US data suggest larger PTV expansion margins (∼9 mm) are necessary for US IGRT vs. SM IGRT (∼3 mm). The hypotheses that US and SM methods suggest similar shifts and that the mean US/SM three-dimensional distance discrepancy is ≤5 mm are rejected.

Introduction

Relative to its location at time of simulation, the prostate gland is known to move. This is a problem for very conformal external beam radiotherapy. A system to locate prostate position helps negate this problem. Various systems exist. Two with widespread use are ultrasound (US) localization of the prostate and X-ray localization of the prostate using implanted seed markers as fiducials.

Although publications have appeared regarding various prostate localization methods, it is not clear from previous works what the three-dimensional (3D) relationship of US and seed marker (SM) localization methods is (1, 2). We sought to better define this relationship. To clarify, after patient setup to skin marks, US localization could suggest a shift to one point in space (e.g., PU), whereas a simultaneous assessment of prostate position by seed markers might suggest a shift to a different point in space (e.g., PS). It would be of value to get a sense of the usual distance between PU and PS, especially if this distance were large. However, if US/SM localization methods are quite agreeable, a large sample of PUs and PSs should be relatively equally distributed about one another and about the origin, the origin being the initially measured (i.e., simulated) prostate position. This notion follows from the central limit theorem (3).

At the MIMA Cancer Center, we began using ultrasound prostate localization in November 2003. We began kilovoltage (kV) X-ray image-guided radiotherapy (IGRT) in mid-2004. We have analyzed the relationship between the two methods and report that herein.

Section snippets

Patients

A nonrandomized cohort of 40 patients was analyzed across 1019 radiotherapy fractions. All patients signed written consent for therapy.

We began ultrasound for prostate localization in November 2003. In June 2004, we obtained and began using a kV X-ray IGRT system (and began implanting metallic fiducials in the prostate gland). We continued measurement of prostate position via ultrasound, however, for several months. The 40 patients retrospectively analyzed in this study were treated between

Results

A consecutive cohort of 40 patients across 1,019 separate daily radiotherapy fractions was retrospectively analyzed. From the cohort, 1,019 daily US measurements were obtained, and a total of 1,019 simultaneous SM measurements were obtained (6,114 X/Y/Z coordinates recorded in all).

The data from all SM and US shifts are given in Table 1. All US vs. SM dimensional measurements reveal significant differences. SM shifts are not significantly more than 1 mm from the origin in any dimension, but US

The US and SM data

We are aware of two other US vs. SM studies. Van den Heuvel et al. studied 15 patients across 275 fractions (1). Langen et al. obtained 92 measurements from 10 patients (2). Our study appears to generally reproduce previous findings, but this is the first study to compare SonArray and ExacTrac measurements. Also, only Langen et al. have simultaneously compared SM and US measurements. As in the prior studies, we find US and SM measurements to be dissimilar. We also find that there is significant

Conclusions

The methods used in this study measure consistently different prostate gland locations. US and SM X/Y/Z measurements differ significantly. US lateral, longitudinal, and 3D vector shifts are significantly more variable than SM shifts suggesting greater random error with the US method. There is greater systematic error with the US method. The data suggest larger PTV margins (∼9 mm) are necessary with US IGRT vs. SM IGRT (∼3 mm). The hypotheses that SM and US shift measurements are similar and

References (29)

Cited by (134)

  • Comparison of 2 transabdominal ultrasound image guidance techniques for prostate and prostatic fossa radiation therapy

    2017, Practical Radiation Oncology
    Citation Excerpt :

    However, Robinson et al13 reported poor correlation between 3D US and CT, but also stated that this may have been due to use of inexperienced US users. Several studies have apparently attributed all discrepancy in localization to the US modality, not taking CT localization inaccuracies and temporal changes between image acquisitions into account.13‐16 A recent review paper by Fontanarosa et al17 presents summaries of multiple IGRT modalities in comparison to 3D US, and describes how US technology is implemented in the radiation therapy workflow.

  • Intensity Modulated Radiotherapy and Image Guidance

    2016, Prostate Cancer: Science and Clinical Practice: Second Edition
  • Prostate Cancer

    2015, Clinical Radiation Oncology
View all citing articles on Scopus

Todd J. Scarbrough is a paid speaker for Varian Medical Systems; Joseph Y. Ting is a paid speaker for Varian Medical Systems.

View full text