PROSTATE VOLUME CALCULATED BY BIPLANE PLANIMETRY IMPROVES THE ACCURACY OF PSA DENSITY.
A. Kimura, M. Yoshida, I. Saito, and T. Kitamura, Tokyo, Japan
Aim: PSA density is the quotient of PSA divided by transrectal ultrasound determined prostate volume. A common volume equation; prolate ellipsoid (height times width times length times pi/6) has a risk to increase false positives in BPH, because prolate ellipsoid has a tendency to underestimate the volume in BPH. Recently, we proposed a new method of calculation that we termed "biplane planimetry". By this new method, the prostatic contours of both cross- and sagittal sections are traced. Based on the cross and sagittal contours, a non-ellipsoidal model is created. The model is composed of sequentially arranged copies of the cross-section, which are reduced so that the anteroposterior diameters of the copies fit the contour of the sagittal section. Biplane planimetry calculates the volume accurately in BPH as well as in cancer. The usefulness of prolate ellipsoid and biplane planimetry as denominator in PSA density was compared.
Method: In nineteen patients with prostatic cancer and twenty patients with BPH having PSA values of 4 to 10 ng/ml, prostatic volumes were calculated both by prolate ellipsoid (Ve) and biplane planimetry (Vb).
Results: The averages of Ve were 32ml in cancer and 49ml in BPH, while those of Vb were 31ml and 52ml respectively. Accordingly, the averages of PSA/Ve were 0.27 in cancer and 0.13 in BPH, while those of PSA/Vb were 0.28 and 0.12 respectively. Using a PSA density cutoff of 0.15 as recommended in the literature, the sensitivity and specificity of PSA/Ve were 74% and 65%, while those of PSA/Vb were 79% and 75%.
Conclusion: Because biplane planimetry does not underestimate the volume of BPH, the false positive rate of PSA density by biplane planimetry is smaller than that by prolate ellipsoid.
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