New prostate volume calculation formula to improve the specificity of PSA density.
Akira Kimura
Tokyo Kyosai Hospital
Kiyoshi Hirasawa, Yuji Kurooka, Kazuki Kawabe
University of Tokyo,Tokyo, Japan
PSA density is the quotient of PSA divided by prostatic volume.For the calculation of PSA density, volume measured by US needs not to be the same with real prostatic volume. Requirements of volume measurent are constant error rate regardless of prostatic shape, and reproducibility. For volume calculation, prolate ellipsoid is the most popular method. From the cross and sagittal section, width, height, and length are measured. Height times width times length times π/6 gives the volume of prolate ellipsoid having the three axes. Prolate ellipsoid has a tendency to underestimate the volume in BPH, because rectal surface of the prostate in BPH is pressed by ultrasonic probe. Recently, we proposed a new calculation method which 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 (height; H) of the copies fit the contour of the sagittal section (Fig.1). The areas of the copies are reduced in proportion with the square of the reduced rates of the height (H2), and so the formula for biplane planimetry is given as :
l x Amax x Σ(Hi/Hmax)2
where l is a stepped interval of the arrangement of copies, Amax is the area of the maximum cross section, Hmax is the height of the maximum cross section, and Hi are the heights measured at certain intervals in the sagittal section where the reduced copies are arranged.
Using transrectal sonograms of 100 patients, we previously compared the accuracy of prolate ellipsoid and biplane planimetry. Step-section planimetry was used as the gold standard. Prolate ellipsoid underestimated the volume by 10% in cancer and by 14% in BPH, while the biplane planimetry underestimated the volume by 7% both in cancer and BPH. Biplane planimetry had the same error rates in BPH and in cancer.
Underestimation of volume only in BPH leads overestimation of PSA density in BPH. Therefore, the prolate ellipsoid has a risk to increase false positives of PSA density.
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 (V０) and biplane planimetry (V１). For those whose PSA levels were measured several times, values measured just before the US guide biopsy were aplied.
result
The averages of V０ were 32ml in cancer and 49ml in BPH, while those of V１ were 31ml and 52ml respectively. Accordingly, the averages of PSA/V０ were0.27 in cancer and 0.13 in BPH, while those of PSA/V１ were 0.28 and 0.12 respectively(Fig.2). Using a PSA density cutoff of 0.15 as recommended in the literature, the sensitivity and specificity of PSA/V０ were 74% and 65%, while those of PSA/V１ were 79% and 75%.
Receiver operator characteristic curves were constructed for PSA, PSA densities by prolate ellipsoid and by biplane planimetry(Fig.3). The ROC curve of biplane planimetry came closer to the left and top boundaries than that of prolate ellipsoid. Areas under the curves were 0.64, 0.87, and 0.89 respectively.
discussion
As 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.
Biplane planimetry is not available in ultrasonic consoles now used. So, in this study, only the cross section area was measured by the equipment. To measure the heights in the sagittal section, a transparent sheet with measures was overlaid. Using the microsoft software : Exel, heights(Hi) and Σ(Hi/Hmax)2 were culculated. It was a little bid cumbersome, so we could not get biplane planimetry prospectively. So, this study is a retrospective one. I hope biplane planimetry will be incorporated in the console so that the volume by this method can be calculated during examination.
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.
