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### 国際会議

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New prostate volume calculation formula to improve the specificity of PSA density.

__Akira Kimura__, Kiyoshi Hirasawa,Yuji Kurooka,and Kazuki Kawabe

Tokyo, Japan

[objective] 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 π/6) has a risk to increase false positives in BPH, because prolate ellipsoid has a tendency to underestimate the volume in BPH.We recently developed a new prostate volume calculation method using the full information obtained from biplane transrectal ultrasonogram (Int J Urol 1997;4:152-156).The method; 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 (V_{０}) and biplane planimetry (V_{１}).

[result] The averages of V_{０} were 32ml in cancer and 49ml in BPH, while those of V_{１} were 31ml and 52ml respectively. Accordingly, the averages of PSA/V_{０} were0.27 in cancer and 0.13 in BPH, while those of PSA/V_{１} 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/V_{０} were 74% and 65%, while those of PSA/V_{１} 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.