Measurement of Prostate Volume with MRI (A Guide for the Perplexed): Biproximate Method with Analysis of Precision and Accuracy

Abstract

To review the anatomic basis of prostate boundary selection on T2-weighted magnetic resonance imaging (MRI). To introduce an alternative 3D ellipsoid measuring technique that maximizes precision, report the intra-and inter-observer reliability, and to advocate it's use for research involving multiple observers. We demonstrate prostate boundary anatomy using gross pathology and MRI examples. This provides background for selecting key boundary marks when measuring prostate volume. An alternative ellipsoid volume method is then proposed using these boundaries in an attempt to improve inter-observer precision. An IRB approved retrospective study of 140 patients with elevated serum prostate specific antigen levels and/or abnormal digital rectal examinations was done with T2-weighted MRI applying a new (Biproximate) technique. Measurements were made by 2 examiners, correlated with each other for inter-observer precision and correlated with an expert observer for accuracy. Correlation statistics, linear regression analysis, and tests of means were applied using p ≤ 0.05 as the threshold for significance. Inter-observer correlation (precision) was 0.95 between observers. Correlation between these observers and the expert (accuracy) was 0.94 and 0.97 respectively. Intraobserver correlation for the expert was 0.98. Means for inter-rater reliability and accuracy were all the same (p = 0.001). We conclude that using more precise reproducible landmarks with biproximate technique, precision and accuracy of total prostate volume is found to be demonstrated.

Comparison of a Commercially Available Prostate Segmentation Application to Traditional Prolate and Biproximate Ellipsoid Methods for Prostate Volume Measurement

“…Due to significant differences in the way the specimens were processed in these experiments, no true standard can be asssumed. A more robust discussion of this topic has been previously reported [3,11]. We should be more interested in measurements made in situ in the living blood -perfused patient for guidance in medical or surgical management.…”

“…The final matrix of volumetrics included raw unadjusted DynaCAD, (RCAD) manually adjusted DynaCAD (ACAD), traditional ellipsoid (TE) and biproximate ellipsoid (BE) data. Detailed description of the biproximate technique has been described [11]. Determination of whether a method over-or underestimated prostatic volume compared to the ACAD was calculated by dividing the difference in the 2 means by the larger of the 2 means and multiplying by 100 to derive the percent of change.…”

“…The purpose of this report is to compare DynaCAD ® performance to the traditional ellipsoid (TE) and recently published biproximate ellipsoid (BE) methods. The latter was developed for greater inter-observer consistency (precision) using MRI [11]. Can we rely on unadjusted (fully automatic) or manually adjusted DynaCAD ® for measurement of prostatic volume?…”

Peripheral Zone Enlargement: a newly described entity on MRI

“…Total prostate volumes (TPV) were measured using the biproximate prolate ellipsoid model (length × width × anteroposterior diameter × 0.52). Length measurement in the mid-sagittal plane used a line drawn at the vesico-prostatic junction (the vesico-prostatic line [VPL]) [5], and length measurement was made perpendicular to this line from the apex. Any prostatic tissue above the VPL was measured separately as a perpendicular from the proximal margin of the tissue to the VPL and added to the measurement below the VPL to derive total length of the prostate.…”

“…Any prostatic tissue above the VPL was measured separately as a perpendicular from the proximal margin of the tissue to the VPL and added to the measurement below the VPL to derive total length of the prostate. This procedure had been previously demonstrated to have inter-observer reliability and validated against the traditional method of measuring length from the proximal prostatic margin to the apex [5]. The traditional prolate ellipsoid measurement boundaries and formula was used to calculate the TZ volume (TZV).…”