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Genitourinary Imaging |
1 From the Departments of Medical Physics (K.L.Z., A.S.D., S.K., M.M., J.A.K.), Radiology (K.L.Z., S.E., H.H., J.A.K.), Pathology (K.S., V.E.R.), Urology (M.W.K., P.T.S.), Epidemiology and Biostatistics (M.W.K.), and Medicine (J.A.K.), Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021. Received October 28, 2002; revision requested December 12; revision received January 15, 2003; accepted February 28. Supported by National Institutes of Health grants R21 CA 84258-01 and 7-R01 CA76423. Address correspondence to K.L.Z. (e-mail: zakiank@mskcc.org).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Findings in 40 patients who underwent combined endorectal MR imaging and hydrogen 1 MR spectroscopic imaging before radical prostatectomy and who had TZ tumor identified subsequently at step-section pathologic analysis were retrospectively reviewed. Within this population, a subset of 16 patients whose TZ tumor had a largest diameter of 1 cm or greater and was included in the MR spectroscopic imaging excitation volume was identified. In these 16 patients, the ratios of choline-containing compounds (Cho) and creatine/phosphocreatine (Cr) to citrate (Cit) (ie, [Cho + Cr]/Cit), Cho/Cr, and Cho/Cit were compared in tumor and control tissues. The presence of only Cho and the absence of all metabolites were also assessed.
RESULTS: The mean values of (Cho + Cr)/Cit, Cho/Cr, and Cho/Cit were different between TZ cancer and control tissues (P = .001, P = .003, and P = .001, respectively; Wilcoxon signed rank test). Nine (56%) of 16 patients had at least one tumor voxel in which Cho comprised the only detectable peak, while no control voxels showed only Cho (P = .008, McNemar test). The percentage of voxels in which no metabolites were detected did not differ between tumor and control tissues (P = .134, McNemar test).
CONCLUSION: TZ cancer has a metabolic profile that is different from that of benign TZ tissue; however, the broad range of metabolite ratios observed in TZ cancer precludes the use of a single ratio to differentiate TZ cancer from benign TZ tissue.
© RSNA, 2003
Index terms: Magnetic resonance (MR), spectroscopy, 844.12145 • Magnetic resonance (MR), tissue characterization, 844.12145 • Prostate, hyperplasia, 844.316 • Prostate, MR, 844.12145 • Prostate, neoplasms, 844.32
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Proton MR spectroscopic imaging has been successfully applied to the diagnosis of cancer in the PZ on the basis of the elevation of choline-containing compounds (Cho) and the reduction of citrate (Cit) in cancerous tissue compared with normal, healthy PZ tissue (10,14–17). The ratio (Cho + Cr)/Cit, where Cr is creatine/phosphocreatine, has been used to aid in the diagnosis of cancer, and the combination of endorectal MR imaging and MR spectroscopic imaging has shown high sensitivity and specificity for detecting tumors of the PZ (10). The goal of the current study was to determine whether cancers in the TZ possess a unique metabolic pattern by which they may be identified at proton MR spectroscopic imaging.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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In the imaging portion of our study, transverse T1-weighted MR images were obtained from the aortic bifurcation to below the prostatic apex by using a spin-echo pulse sequence with the following parameters: repetition time msec/echo time msec, 700/14; field of view, 24 cm; section thickness, 5 mm; gap, 1 mm; and number of signals acquired, two. T2-weighted MR images in the transverse and coronal planes were acquired by using a fast spin-echo sequence with the following parameters: 4,000/102; echo train length, 12; field of view, 14 cm; section thickness, 3 mm; gap, 0 mm; and number of signals acquired, four. MR spectroscopic imaging consisted of point-resolved spectroscopy (PRESS) voxel excitation (22) with band-selective inversion with gradient dephasing (BASING) water and lipid suppression (18) and spatial encoding by chemical shift imaging (23) at 6.25-mm resolution in all three dimensions. Timing parameters were 1,000/130 and imaging time, 17 minutes.
Data processing was performed at a Sun Ultra 10 workstation (Sun Microsystems, Mountain View, Calif) and included 2-Hz Lorentzian spectral apodization, four-dimensional Fourier transform, and automated frequency, phase, and baseline correction (21). Spectral data were zero filled to 3.1-mm resolution in the superior-inferior dimension and overlaid on corresponding transverse T2-weighted MR images. Peak areas were calculated by using numeric integration. To provide a noise measurement, we calculated the SD of the MR signal intensity in a region of the spectrum containing only noise. Metabolite peak areas were then normalized with respect to the noise SD to yield an approximate signal-to-noise ratio.
Pathologic Examination and Final Patient Group
Prostatectomy was followed by specimen fixation in 10% formalin for 36 hours. The distal 5-mm portion of the apex was amputated and coned. The remainder of the gland was serially sectioned from the apex to the base at 3–4-mm intervals and entirely submitted for paraffin embedding as a "whole mount." The seminal vesicles were amputated and embedded separately. After paraffin embedding was performed, microsections were cut at 3-mm intervals, placed on glass slides, and stained with hematoxylin-eosin. Cancer foci were outlined in ink by a pathologist (K.S. or V.E.R.) on slides of the whole-mount, apical, and seminal vesicle slices so that the foci could be grossly seen and photographed. These photographs constituted the tumor maps. For the purposes of this study, tumors considered to arise in the TZ and tumors of PZ origin with a large component invading the TZ were included. For each patient, the Gleason scores for both the entire radical prostatectomy specimen and the TZ tumor, the pathologic stage, the zonal origin of the TZ tumor, and the presence or absence of synchronous PZ tumors were tabulated.
Forty patients with histopathologically detected TZ cancer were identified in the total population of 130 patients. To ensure that more than 90% of at least one MR spectroscopic imaging voxel included tumor, we restricted the patient population to those 20 patients in whom the pathology step-section map showed a TZ tumor of 1 cm or greater in largest in-plane diameter. Data from four of these patients were eliminated from MR spectroscopic imaging analysis because most or all of the tumor was far anterior and was not included in the PRESS excitation volume; this yielded 16 patients with TZ tumors for quantitative analysis. Age, presurgical prostate-specific antigen (PSA) level, and clinical stage were recorded for each patient.
Quantitative Analysis of MR Spectroscopic Imaging Data
A radiologist (H.H.) with over 10 years of experience interpreting endorectal MR images of the prostate matched the whole-mount pathology step sections with the most closely corresponding MR images and identified the tumor region of interest in the images. Only MR spectroscopic imaging voxels with more than 90% of their volume contained within the tumor region of interest were considered "tumor" voxels. The metabolites studied were those resonating at approximately 3.22 parts per million (ppm) (ie, Cho, a peak that can have contributions from free choline, phosphocholine, glycerophosphocholine, ethanolamine, phosphoethanolamine, spermine, and taurine and that is dominated by the tri-methyl protons of choline and phosphocholine [15,24]), 3.02 ppm (ie, Cr), and 2.62 ppm (ie, Cit).
The normalized peak areas for Cho, Cr, and Cit for each tumor voxel were tabulated and used to calculate the following ratios: (Cho + Cr)/Cit, Cho/Cr, and Cho/Cit. For a voxel to be considered diagnostic in this study, at least one of the metabolites (Cho, Cr, or Cit) was required to have a signal-to-noise ratio of 2:1 or higher. In addition, any metabolite with a signal-to-noise ratio less than 1 was simply assigned the noise value. For each patient, the mean value of each ratio in the tumor region of interest was calculated. "Benign control" voxels were chosen randomly from the set of TZ MR spectroscopic imaging voxels that did not contain tumor and were not adjacent to tumor or the urethra. Whenever possible, the number of control voxels was made equal to the number of tumor voxels. For some patients with very large TZ tumors, there were fewer control voxels than tumor voxels. The number of voxels in tumor regions ranged from one to 47 in the 16 patients, while the number of control voxels ranged from one to 25. In addition to metabolite ratios, the number of voxels in which only Cho was detectable (with a signal-to-noise ratio of 2:1 or higher) was assessed. Finally, in both tumor and control regions, the percentage of voxels that contained no detectable metabolites was tabulated.
Statistical Analysis
Wilcoxon signed rank tests were used to compare tumor with control tissue for each of the following variables: (Cho + Cr)/Cit, Cho/Cr, and Cho/Cit. In each case, cancer tissue was paired with normal tissue for each patient. The significance of detecting just Cho in tumor voxels versus control voxels and the significance of detecting no metabolites were assessed with the McNemar test (25). A P value of less than .05 was considered to indicate a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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In vitro MR studies of prostate tissue extracts have been performed on transurethral and radical prostatectomy specimens (16,29–33). Proton MR has revealed, on average, elevated Cit levels in BPH tissue as compared with human prostate cancer in unspecified zones (29,30,32) and human prostate cancer in the PZ (33). However, the Cit level in BPH has been found to positively correlate with the percentage of glandular tissue in the specimen, and some stromal BPH tissue has been observed to contain levels of Cit comparable to those in cancer tissue (33). Kurhanewicz et al (16,30) and Cornel et al (32) reported higher levels of Cho in cancer than in BPH, while Schiebler et al (33) found no difference between choline content in cancer and in BPH. Cancer has also been shown to have lower levels of phosphoethanolamine than BPH (16,30,32). In a study involving phosphorus 31 (31P) MR spectroscopy, a lower ratio of phosphoethanolamine to phosphocholine was observed in human prostate cancer grown in nude mice and in a high-grade human tumor as compared with that in BPH (31,32). On the basis of results of extract studies, Cit content and the ratios of Cit to total choline, Cit to lactate, phosphocholine to total creatine, choline to total creatine, and phosphoethanolamine to phosphocholine have been suggested as in vivo markers for differentiating BPH from prostate cancer (30–32).
Improved hardware has permitted in vivo MR studies that have provided information on prostate cancer, BPH, and normal tissue (15,31,34–36). Results of an early 31P MR study showed that prostate glands from patients with BPH had elevated phosphomonester/nucleotide triphosphate levels compared with normal prostates (31). Localized proton MR techniques such as PRESS (22) and STEAM (stimulated-echo acquisition mode) (37), more recently combined with chemical shift imaging (23), have been used for further characterizing zonal anatomy in normal tissue and in disease (15,16,35,36). Healthy young volunteers with no BPH have been found to have a significantly lower Cit content in the central zone of the prostate than in the PZ (35,36). Results of 1H MR spectroscopic imaging have shown that glandular BPH nodules have Cit levels comparable to those in normal, healthy PZ tissue, while stromal BPH regions have reduced Cit levels that are in some cases similar to those in PZ cancers identified at MR imaging (15). Choline in BPH has been found to be elevated compared with choline in normal PZ tissue (15,16). Compared with levels in cancers of the PZ, mean Cho, Cr, and (Cho + Cr)/Cit levels in BPH have been found to differ significantly; however, overlap was substantial (15).
The present study was focused on TZ tumors that were identified at step-section histopathologic examination and were spatially correlated with T2-weighted MR images and MR spectroscopic images. Although previous prostate cancer extract studies were not specifically focused on TZ tumors, the trend of reduced Cit and elevated Cho levels seen in those studies agrees with the results of this in vivo study (29,30,32,33). In addition, the difference in (Cho + Cr)/Cit values between TZ cancer and BPH observed in the present study is similar to that observed between PZ cancers and BPH in a previous in vivo study (15). We suspect that the difference between TZ tumor and BPH metabolite ratios arises from both a reduction of Cit and an elevation of Cho in the tumor. It has been suggested that tumor cells, as compared with normal prostatic epithelium, have increased oxidation of Cit (38). Cho are constituents of membrane synthesis and degradation pathways and have been found to be elevated in many cancers (39–45). One possible reason for this elevation is increased cell proliferation (46–48).
Although one might have expected a wide range of (Cho + Cr)/Cit and Cho/Cit values in BPH voxels owing to variations in Cit levels, tumor ratios in the present study showed even greater variability. It has been shown in vitro that prostate tumor tissue samples containing no normal epithelial cells had no detectable Cit, while tumor samples containing nonmalignant epithelial cells contained substantial amounts of Cit (32). In our study, because the presence of normal or nonmalignant cells within a tumor voxel could not be ruled out, Cit may have been present to some degree. Also, although data were restricted to larger tumors that filled more than 90% of the voxel, some volume averaging of tumor with benign glandular tissue may have occurred. The level of Cho may vary with tumor grade (49). However, the small number of patients in the present study did not permit stratification on the basis of tumor grade.
The relatively close grouping of (Cho + Cr)/Cit and Cho/Cit in benign voxels that was observed in this study could have been caused in part by the elimination of voxels with no detectable metabolites—presumably those in stromal regions. This would have biased the benign voxel (Cho + Cr)/Cit and Cho/Cit ratios toward lower values (higher Cit). The Cho/Cr ratios in tumor and control voxels showed large variations and overlapped considerably, possibly due to the low signal-to-noise ratio of the Cr peak. In addition, Cho and Cr are not always well resolved, adding to the uncertainty.
In the PZ, (Cho + Cr)/Cit values in benign tissue and cancer tissue have been shown to not only be statistically different but also to have very little overlap (15,50). However, in the cases of TZ cancer evaluated in our study, there was substantial overlap between metabolite ratios in tumor and those in benign tissue, and choosing a cutoff value of (Cho + Cr)/Cit or Cho/Cit for differentiating cancer would result in some cancers going undetected. On the basis of the data in this study, the metabolic characteristic most likely to indicate TZ cancer is the presence of at least one voxel in which Cho is the only detectable metabolite peak.
While demonstrating the proton metabolic characteristics of TZ tumors, the results of this study also illuminated the technical difficulties involved in evaluating the TZ with MR spectroscopic imaging. For high-resolution MR spectroscopic imaging (6.25-mm in-plane resolution), an endorectal probe is used to achieve adequate signal-to-noise ratio within a clinically feasible imaging time. However, the anterior portion of the prostate gland, which is most distant from the probe, can still have relatively low signal-to-noise ratio on such images. Indeed, the signal-to-noise threshold level used in the present study was lower than that used previously in studies of the PZ (8,51). However, because this is the current technological limit, the lower threshold and the possibility of increased scatter in the data were accepted. Coverage of the anterior gland at MR spectroscopic imaging was purposefully restricted because (a) the PZ was the main region of clinical interest and (b) excitation of anterior periprostatic lipids can result in lipid contamination artifacts in MR spectroscopic imaging data. Therefore, tumors that were very anterior were in some cases partially or completely excluded from the excitation volume. Finally, the TZ is immediately adjacent to the urethra, which can contain glycerophosphocholine in seminal fluid (52). Glycerophosphocholine resonates in the choline region, making detection of TZ cancer adjacent to the urethra difficult.
Despite the difficulties in assessing the TZ, the benefits could be substantial. Currently, the PZ is targeted at most biopsies. It is not uncommon for a patient to be strongly suspected of having prostate cancer on the basis of a rising PSA level and yet have negative biopsy results (sometimes after multiple biopsy attempts) because the cancer is in the TZ. In fact, it has been suggested that patients with multiple negative biopsy results and an elevated and/or rising PSA level should undergo biopsies in which the TZ is specifically targeted (53). Even when the TZ is specifically targeted at biopsy, smaller tumors (<2 cm3) may be missed (28). Accurate imaging guidance could help improve positive yield for TZ biopsies. TZ tumors are more likely to be organ confined, have a lower Gleason score, and have higher biochemical cure rates than same-volume PZ tumors (13,54). Even quite large TZ tumors (7–86 cm3) are more likely to remain organ confined (55). Thus, if patients with single or dominant TZ tumors can be given a confident diagnosis with use of an imaging modality, they may be candidates for alternative therapies or watchful waiting, depending on other clinical considerations.
In the present study, we assessed only the MR spectroscopic imaging parameters of TZ cancers. Features indicating the presence of TZ tumors on T2-weighted MR images have recently been reported (56,57). In addition, dynamic contrast material–enhanced MR imaging may show promise for differentiating cancer from BPH (57,58). Recent technical improvements, including outer volume saturation pulses (59) and the ability to prescribe oblique MR spectroscopic imaging volumes, should permit the assessment of the entire TZ. With currently available technology, it is important that the TZ not be overlooked during prostate staging evaluation with MR imaging/MR spectroscopic imaging.
In summary, although a trend toward elevation of Cho and reduction in or lack of Cit was observed in TZ tumors when they were compared with BPH, the broad range of metabolite ratios observed precludes the use of a single ratio to differentiate TZ cancer from benign TZ tissue. A prospective study is now needed to ascertain the value of combined MR imaging/MR spectroscopic imaging in the detection of TZ tumors.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, K.L.Z.; study concepts, K.L.Z., H.H., S.E., P.T.S., V.E.R., J.A.K.; study design, K.L.Z., H.H., P.T.S., J.A.K., V.E.R.; literature research, K.L.Z.; clinical studies, V.E.R., K.S., P.T.S., H.H., S.E., K.Z., A.S.D., M.M.; data acquisition, A.S.D., M.M.; data analysis/interpretation, K.L.Z., S.E., S.K., K.S., M.W.K.; statistical analysis, M.W.K.; manuscript preparation, S.K., A.S.D., M.M., K.L.Z., H.H., S.E.; manuscript definition of intellectual content, K.L.Z., H.H.; manuscript editing, H.H., S.E., J.A.K., V.E.R., P.T.S., M.W.K., S.K., A.S.D.; manuscript revision/review, all authors; manuscript final version approval, H.H., J.A.K., S.E., K.L.Z.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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