- Email: [email protected]
Computed tomography appearance of the prostatic fossa following radical prostatectomy
OLDER
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patient with a level of 0.3 ng/ml. Criteria selected for evaluation were as follows: (1) smooth versus irregular margins of the prostatic b e d tissue; (2) homogeneity of density of the prostatic b e d tissue; (3) symmetry of residual seminal vesicles; (4) infiltration of fat around the seminal vesicles; (5) infiltration of perirectal fat; (6) margins of the levator ani muscle; and (7) overall rating regarding the probability of recurrent malignancy. Tissue volume (in cubic centimeters) in the prostatic b e d was determined using the formula, k × W x H x 0.52 [1], where L = the distance from the bladder base to the pelvic floor, W = the m a x i m u m transverse diameter of the soft tissue in the prostatic bed, and H = the m a x i m u m anteroposterior diameter of this soft tissue (Fig. 1). The readers were trained in making the measurements and were instructed about the meaning of the assessment criteria b y one of us prior to their reviewing the scans. Measurements were made from hard copies with calipers. All of the cases included in the study (n = 24) were interpreted independently by four board-certified radiologists specializing in CT scanning, urologic radiology, or both. This included three staff radiologists and one abdominal imaging fellow. The author w h o developed the diagnostic criteria was excluded from the readings. The cases were randomized for each reader, and readers were unaware of patients' names and diagnosis. Only the images including the prostatic b e d and lower pelvis were presented to the readers. Each reader independently interpreted the cases in a single sitting, determining the length, width, and height of the tissue in the
Vol. 2, No. 6, June 1995
prostatic b e d and evaluating the seven criteria listed earlier for each case, noting their decisions on a response sheet. Each criterion had six possible responses, ranging from definitely not present (1) to definitely present (6). One of us w h o was not a reader calculated the volume of tissue in the prostatic fossa from the measurements readers determined.
Analysis The graded responses on the individual data sheets were entered into a computer database. We generated receiver operating characteristic (ROC) curves for individual and pooled data using a Sun Microsystems (Mountain View, CA) computer with the ROCFIT program (developed by Charles Metz of the University of Chicago). This program estimates the binormal ROC curves from continuously distributed data using maximum-likelihood estimation. The areas generated underneath each ROC curve [6] were used to compare the diagnostic performance of each of the seven criteria described earlier. In the few cases in which ROC results under maximum-likelihood estimation were degenerate, the areas underneath the ROC curves were estimated using the trapezoidal rule [6]. The m e a n sensitivities and specificities for each criterion were calculated by designating graded responses 1-3 as negative and responses 4-6 as positive. From the individual data response sheets, w e calculated the m e a n prostatic volumes and standard deviations for normal and abnormal cases. We used an F ratio to determine that there was no statistically significant difference b e t w e e n the two calculated variances. As such, w e could assume equal variances and assess differences b e t w e e n the two m e a n volumes using a twosided, two-sample, unpaired t test. Significance was set at the .05 level.
RESULTS
FIGURE 1. Computed tomography scan below the bladder base and above the urogenital diaphragm. Soft tissue (ST) in prostatic fossa is representative of the tissue measured.
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For each of the seven criteria, the individual, pooled, and mean areas underneath the ROC curves are shown in Table 1. Data degeneracy using maximum-likelihood estimation occurred in only three of the 28 total individual ROC curves. The mean sensitivity and specificity of each criterion are also summarized in Table 1. Neither pooled nor mean ROC area exceeded .70 for any criterion. There was generally low specificity for all criteria studied. The distribution of residual prostatic volumes following prostatectomy as calculated for the measure-
Vol. 2, No. 6, June 1995
APPEARANCE OF PROSTATIC FOSSA AFTER PROSTATECTOMY
TABLE 1: ROC Areas, Sensitivity, and Specificity for Evaluated Criteria Reader
Area
Volume Analysisof Non-recurring vs Recurring ProstateAdenocarcinoma g
SD
Irregular margins ofpmstaticfossatissue(.66,.32) A .460 .122 B .435 .122 C .498 .141 D .589 .146 Pooled .480 .061 M .496 .034 (SE) Inhomogeneous density o f p r o s t a t i c f o s s a t i s s u e ( . 7 6 , . l l ) A .445 .134 B .276 .115 C .379 .191 D .284 .126 Pooled .394 .062 M .346 .040 (SE) Asymmetric residualseminalvesicle (.86,.16) A .629 .133 B .487 .124 C .750 .135 D .875 .084 Pooled .596 .062 M .685 .083 (SE) Infiltr~ion offataround seminalvesicle(.69,.43) A .720 .106 B .552 .123 C .733 .110 D .666 .113 Pooled .648 .057 M .668 .041 (SE) Infiltration ofperirectalfat(.71,.41) A .668 .115 B .387 •112 C .552 .127 D .784 .111 Pooled .564 .060 M .598 .085(SE) Irregular margins ofthelevatorani(•78,.91) A .586 .148 B .546 .123 C .510 a -D .343 -Pooled .514 .065 M .496 .053(SE) Overallr~ing(.75,.32) A .563 .120 B .439 .121 C .647 .115 D .577 a -Pooled .563 .060 M .557 .043 (SE) Numbers in parentheses are the sensitivity and specificity, respectively. ROC = receiver operating characteristic, SD = standard deviation, M = mean, SE = standard error. aDegenerate data set. Area underneath the ROC curve subsequently estimated by trapezoidal rule.
ments o f all the readers, for the n o r m a l a n d a b n o r m a l cases, is s h o w n using a b o x plot (Fig. 2). The m e a n prostatic v o l u m e for the positive cases w a s 15.01 c m 3
_=
m Negative Cases vs Positive Cases
FIGURE 2. Box plot showing volume distribution of negative versus positive cases. Black area represents 25th percentile and brackets the 75th percentile. White line represents the mean value.
(SD = 9.26); for the negative cases, it w a s 11.06 c m 3 (SD = 7.14). This difference w a s statistically significant, t(94) = 2.31, p < .05. DISCUSSION
Our results s h o w that after prostatectomy, there w a s a measurable a m o u n t of soft tissue immediately caudal to the bladder base. N o n e of the CT criteria w e evaluated w e r e useful in separating normal postoperative tissue from recurrent prostate cancer. Summers et al. [4] described the CT finding in 17 postprostatectomy patients thought to be free of recurrent disease, but he did not include a comparison of normal and abnormal cases• Our study e x p a n d e d on this concept b y comparing the CT findings of normal patients with those having locally recurrent disease. As can be seen from Table 1, n o n e of the criteria w e evaluated had ROC curve areas that w o u l d indicate an ability to discriminate b e t w e e n normal and abnormal cases• The presence, symmetry, and appearance of the fat surrounding the seminal vesicles gave the largest ROC curve areas in our study, but these areas are still unacceptably small for reliable detection of recurrent disease. Our finding is" consistent with prior studies o n preoperative patients with prostatic carcinoma that s h o w e d that asymmetry of the seminal vesicles was of n o value in determining the extent of tumor spread [7, 8]. The h o m o g e n e i t y and margins of the tissue in the prostatic b e d also p r o v e d to be of n o value in detecting carcinoma. The ROC areas u n d e r n e a t h the curves for 473
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Vol. 2, No. 6, June 1995
both of these were less than .50. Infiltration of the perirectal fat was only slightly better as a discriminatory factor with an ROC curve area of .59. Using all the criteria to determine benign versus malignant tissue gave a mean ROC for all of the readers of .55, roughly the equivalent of chance. No cases were consistently called definitely positive or definitely negative by all of the readers. The volume of tissue in the prostatic bed had a wide range for both the normal and abnormal cases. Although the mean values differed, volume was not a clinically important discriminating factor because of the large overlap of values. Many of our patients, both positive and negative, showed what appeared to be residual seminal vesicles (Fig. 3). It is often difficult to remove the entire seminal vesicle, and in most cases this tissue probably repre-
sented the apexes of the seminal vesicles. Summers et al. [4] showed, through a review of pathologic tissue, that this seminal vesiclelike tissue was present even when the seminal vesicles were completely removed and therefore did not always represent the seminal vesicles. During radical retropubic or perineal prostatectomy, the bladder base is anastomosed to the urethra near the urogenital diaphragm. The amount of tissue present between the bladder base and urogenital diaphragm will d e p e n d on the surgical details, including the amount of urethra, if any, cephalad to the pelvic floor. Retrograde urethrograms and cystograms performed on many postprostatectomy patients showed that the bladder neck is often not immediately adjacent to the pelvic floor and that there is a variable distance between the pelvic floor and bladder base, probably attributable to either an elongation of the b l a d d e r base or to residual urethra above the pelvic floor. REFERENCES
FIGURE 3, Residual seminal vesicles or seminal vesiclelike tissue immediately posterior to the bladder. There is slight asymmetry but no infiltration of fat in this positive case.
1. Wasserman NF, Kapoor DA, Hildebrandt WC, et al. Transrectal US in evaluation of patients after radical prostatectomy: I. Normal postoperative anatomy. Radiology1992;185:361-366. 2. Wasserman NF, Kapoor DA, Hildebrandt WC, et al. Transrectal US in evaluation of patients after radical prostatectomy: I1. Transrectal US and biopsy findings in the presence of residual and early recurrent prostatic cancer. Radiology1992;185:367-372. 3. Solomon CG, Flisak ME, Olson MC, Dudiak CM, Flanigan RC, Waters WB. Radical prostatectomy: transrectal sonographic evaluation to assess for local recurrence. Radiology1993:189:713-719. 4. Summers RM, Korobkin M, Quint LE, et al. Pelvic CT findings after radical prostatectomy. J ComputAssist Tomogr1993;17:767-771. 5, Lightner DJ, Lange PH, Reddy PK, Moore L. Prostate specific antigen and local recurrence after radical prostatectomy, J Uro11990;144:921-926. 6, Hanley JA, McNeil BJ. The meaning and use of the area under a receiver operating characteristic (ROC) curve. Radiology1982;143:29-36. 7. Larner J, Gay S, Grizos W, Fortier G, Malcomb R. Significance of CTscan-detected seminal vesicle enlargement in prostate cancer: a pilot study. Urology1993;41:259-261. 8. Platt JF, Bree RL, Schwab RE. The accuracy of CT in the staging of carcinoma of the prostate. AJR 1987;149:315-318.
Announcement The University of Michigan Medical School, Department of Radiology, is sponsoring the 9th Annual Symposium on Breast Disease: Diagnostic Imaging and Current Management With a Special Focus Session for Technologists on July 23-25, 1995, at the Grand Traverse Resort Village, Grand Traverse, MI. Fifteen credit hours of Category 1 of PRA will be awarded. The course director is Dorit D. Adler, MD, Associate Professor of Radiology, Associate Director, Division of Breast Imaging. To register or receive a brochure, contact the University of Michigan, Office of Continuing Education, (800) 9623555 or (313) 763-1400; fax (313) 936-1641 (reference Code S).
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E T AL.
patient with a level of 0.3 ng/ml. Criteria selected for evaluation were as follows: (1) smooth versus irregular margins of the prostatic b e d tissue; (2) homogeneity of density of the prostatic b e d tissue; (3) symmetry of residual seminal vesicles; (4) infiltration of fat around the seminal vesicles; (5) infiltration of perirectal fat; (6) margins of the levator ani muscle; and (7) overall rating regarding the probability of recurrent malignancy. Tissue volume (in cubic centimeters) in the prostatic b e d was determined using the formula, k × W x H x 0.52 [1], where L = the distance from the bladder base to the pelvic floor, W = the m a x i m u m transverse diameter of the soft tissue in the prostatic bed, and H = the m a x i m u m anteroposterior diameter of this soft tissue (Fig. 1). The readers were trained in making the measurements and were instructed about the meaning of the assessment criteria b y one of us prior to their reviewing the scans. Measurements were made from hard copies with calipers. All of the cases included in the study (n = 24) were interpreted independently by four board-certified radiologists specializing in CT scanning, urologic radiology, or both. This included three staff radiologists and one abdominal imaging fellow. The author w h o developed the diagnostic criteria was excluded from the readings. The cases were randomized for each reader, and readers were unaware of patients' names and diagnosis. Only the images including the prostatic b e d and lower pelvis were presented to the readers. Each reader independently interpreted the cases in a single sitting, determining the length, width, and height of the tissue in the
Vol. 2, No. 6, June 1995
prostatic b e d and evaluating the seven criteria listed earlier for each case, noting their decisions on a response sheet. Each criterion had six possible responses, ranging from definitely not present (1) to definitely present (6). One of us w h o was not a reader calculated the volume of tissue in the prostatic fossa from the measurements readers determined.
Analysis The graded responses on the individual data sheets were entered into a computer database. We generated receiver operating characteristic (ROC) curves for individual and pooled data using a Sun Microsystems (Mountain View, CA) computer with the ROCFIT program (developed by Charles Metz of the University of Chicago). This program estimates the binormal ROC curves from continuously distributed data using maximum-likelihood estimation. The areas generated underneath each ROC curve [6] were used to compare the diagnostic performance of each of the seven criteria described earlier. In the few cases in which ROC results under maximum-likelihood estimation were degenerate, the areas underneath the ROC curves were estimated using the trapezoidal rule [6]. The m e a n sensitivities and specificities for each criterion were calculated by designating graded responses 1-3 as negative and responses 4-6 as positive. From the individual data response sheets, w e calculated the m e a n prostatic volumes and standard deviations for normal and abnormal cases. We used an F ratio to determine that there was no statistically significant difference b e t w e e n the two calculated variances. As such, w e could assume equal variances and assess differences b e t w e e n the two m e a n volumes using a twosided, two-sample, unpaired t test. Significance was set at the .05 level.
RESULTS
FIGURE 1. Computed tomography scan below the bladder base and above the urogenital diaphragm. Soft tissue (ST) in prostatic fossa is representative of the tissue measured.
472
For each of the seven criteria, the individual, pooled, and mean areas underneath the ROC curves are shown in Table 1. Data degeneracy using maximum-likelihood estimation occurred in only three of the 28 total individual ROC curves. The mean sensitivity and specificity of each criterion are also summarized in Table 1. Neither pooled nor mean ROC area exceeded .70 for any criterion. There was generally low specificity for all criteria studied. The distribution of residual prostatic volumes following prostatectomy as calculated for the measure-
Vol. 2, No. 6, June 1995
APPEARANCE OF PROSTATIC FOSSA AFTER PROSTATECTOMY
TABLE 1: ROC Areas, Sensitivity, and Specificity for Evaluated Criteria Reader
Area
Volume Analysisof Non-recurring vs Recurring ProstateAdenocarcinoma g
SD
Irregular margins ofpmstaticfossatissue(.66,.32) A .460 .122 B .435 .122 C .498 .141 D .589 .146 Pooled .480 .061 M .496 .034 (SE) Inhomogeneous density o f p r o s t a t i c f o s s a t i s s u e ( . 7 6 , . l l ) A .445 .134 B .276 .115 C .379 .191 D .284 .126 Pooled .394 .062 M .346 .040 (SE) Asymmetric residualseminalvesicle (.86,.16) A .629 .133 B .487 .124 C .750 .135 D .875 .084 Pooled .596 .062 M .685 .083 (SE) Infiltr~ion offataround seminalvesicle(.69,.43) A .720 .106 B .552 .123 C .733 .110 D .666 .113 Pooled .648 .057 M .668 .041 (SE) Infiltration ofperirectalfat(.71,.41) A .668 .115 B .387 •112 C .552 .127 D .784 .111 Pooled .564 .060 M .598 .085(SE) Irregular margins ofthelevatorani(•78,.91) A .586 .148 B .546 .123 C .510 a -D .343 -Pooled .514 .065 M .496 .053(SE) Overallr~ing(.75,.32) A .563 .120 B .439 .121 C .647 .115 D .577 a -Pooled .563 .060 M .557 .043 (SE) Numbers in parentheses are the sensitivity and specificity, respectively. ROC = receiver operating characteristic, SD = standard deviation, M = mean, SE = standard error. aDegenerate data set. Area underneath the ROC curve subsequently estimated by trapezoidal rule.
ments o f all the readers, for the n o r m a l a n d a b n o r m a l cases, is s h o w n using a b o x plot (Fig. 2). The m e a n prostatic v o l u m e for the positive cases w a s 15.01 c m 3
_=
m Negative Cases vs Positive Cases
FIGURE 2. Box plot showing volume distribution of negative versus positive cases. Black area represents 25th percentile and brackets the 75th percentile. White line represents the mean value.
(SD = 9.26); for the negative cases, it w a s 11.06 c m 3 (SD = 7.14). This difference w a s statistically significant, t(94) = 2.31, p < .05. DISCUSSION
Our results s h o w that after prostatectomy, there w a s a measurable a m o u n t of soft tissue immediately caudal to the bladder base. N o n e of the CT criteria w e evaluated w e r e useful in separating normal postoperative tissue from recurrent prostate cancer. Summers et al. [4] described the CT finding in 17 postprostatectomy patients thought to be free of recurrent disease, but he did not include a comparison of normal and abnormal cases• Our study e x p a n d e d on this concept b y comparing the CT findings of normal patients with those having locally recurrent disease. As can be seen from Table 1, n o n e of the criteria w e evaluated had ROC curve areas that w o u l d indicate an ability to discriminate b e t w e e n normal and abnormal cases• The presence, symmetry, and appearance of the fat surrounding the seminal vesicles gave the largest ROC curve areas in our study, but these areas are still unacceptably small for reliable detection of recurrent disease. Our finding is" consistent with prior studies o n preoperative patients with prostatic carcinoma that s h o w e d that asymmetry of the seminal vesicles was of n o value in determining the extent of tumor spread [7, 8]. The h o m o g e n e i t y and margins of the tissue in the prostatic b e d also p r o v e d to be of n o value in detecting carcinoma. The ROC areas u n d e r n e a t h the curves for 473
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Vol. 2, No. 6, June 1995
both of these were less than .50. Infiltration of the perirectal fat was only slightly better as a discriminatory factor with an ROC curve area of .59. Using all the criteria to determine benign versus malignant tissue gave a mean ROC for all of the readers of .55, roughly the equivalent of chance. No cases were consistently called definitely positive or definitely negative by all of the readers. The volume of tissue in the prostatic bed had a wide range for both the normal and abnormal cases. Although the mean values differed, volume was not a clinically important discriminating factor because of the large overlap of values. Many of our patients, both positive and negative, showed what appeared to be residual seminal vesicles (Fig. 3). It is often difficult to remove the entire seminal vesicle, and in most cases this tissue probably repre-
sented the apexes of the seminal vesicles. Summers et al. [4] showed, through a review of pathologic tissue, that this seminal vesiclelike tissue was present even when the seminal vesicles were completely removed and therefore did not always represent the seminal vesicles. During radical retropubic or perineal prostatectomy, the bladder base is anastomosed to the urethra near the urogenital diaphragm. The amount of tissue present between the bladder base and urogenital diaphragm will d e p e n d on the surgical details, including the amount of urethra, if any, cephalad to the pelvic floor. Retrograde urethrograms and cystograms performed on many postprostatectomy patients showed that the bladder neck is often not immediately adjacent to the pelvic floor and that there is a variable distance between the pelvic floor and bladder base, probably attributable to either an elongation of the b l a d d e r base or to residual urethra above the pelvic floor. REFERENCES
FIGURE 3, Residual seminal vesicles or seminal vesiclelike tissue immediately posterior to the bladder. There is slight asymmetry but no infiltration of fat in this positive case.
1. Wasserman NF, Kapoor DA, Hildebrandt WC, et al. Transrectal US in evaluation of patients after radical prostatectomy: I. Normal postoperative anatomy. Radiology1992;185:361-366. 2. Wasserman NF, Kapoor DA, Hildebrandt WC, et al. Transrectal US in evaluation of patients after radical prostatectomy: I1. Transrectal US and biopsy findings in the presence of residual and early recurrent prostatic cancer. Radiology1992;185:367-372. 3. Solomon CG, Flisak ME, Olson MC, Dudiak CM, Flanigan RC, Waters WB. Radical prostatectomy: transrectal sonographic evaluation to assess for local recurrence. Radiology1993:189:713-719. 4. Summers RM, Korobkin M, Quint LE, et al. Pelvic CT findings after radical prostatectomy. J ComputAssist Tomogr1993;17:767-771. 5, Lightner DJ, Lange PH, Reddy PK, Moore L. Prostate specific antigen and local recurrence after radical prostatectomy, J Uro11990;144:921-926. 6, Hanley JA, McNeil BJ. The meaning and use of the area under a receiver operating characteristic (ROC) curve. Radiology1982;143:29-36. 7. Larner J, Gay S, Grizos W, Fortier G, Malcomb R. Significance of CTscan-detected seminal vesicle enlargement in prostate cancer: a pilot study. Urology1993;41:259-261. 8. Platt JF, Bree RL, Schwab RE. The accuracy of CT in the staging of carcinoma of the prostate. AJR 1987;149:315-318.
Announcement The University of Michigan Medical School, Department of Radiology, is sponsoring the 9th Annual Symposium on Breast Disease: Diagnostic Imaging and Current Management With a Special Focus Session for Technologists on July 23-25, 1995, at the Grand Traverse Resort Village, Grand Traverse, MI. Fifteen credit hours of Category 1 of PRA will be awarded. The course director is Dorit D. Adler, MD, Associate Professor of Radiology, Associate Director, Division of Breast Imaging. To register or receive a brochure, contact the University of Michigan, Office of Continuing Education, (800) 9623555 or (313) 763-1400; fax (313) 936-1641 (reference Code S).
474