- Email: [email protected]
Is Ultrasound Imaging Inferior to Computed Tomography or Magnetic Resonance Imaging in Evaluating Renal Mass Size?
Ambulatory and Office Urology Is Ultrasound Imaging Inferior to Computed Tomography or Magnetic Resonance Imaging in Evaluating Renal Mass Size? Phillip Mucksavage, Parvati Ramchandani, S. Bruce Malkowicz, and Thomas J. Guzzo OBJECTIVE
METHODS
RESULTS
CONCLUSION
To evaluate whether ultrasonography was inferior in detecting the size of a renal mass preoperatively because of the increased attention on the harmful effects of ionizing radiation in medical imaging. A prospectively maintained database was reviewed of all patients who underwent renal ultrasonography before definitive therapy for the renal mass. Every patient who underwent ultrasound imaging also underwent computed tomography (CT) or magnetic resonance imaging (MRI), or both, before treatment. The size of the largest tumor identified per imaging modality was compared among the modalities using correlation and analysis of variance. A total of 116 patients underwent ultrasound imaging before therapy. Of these patients, 80 also underwent MRI, 66 underwent CT, and 38 underwent all 3 modalities before treatment. The average pathologic tumor size for the entire cohort was 4.45 cm (range 1-13). The size differences between CT and MRI compared with ultrasound were small (⬍3.5%). Compared with MRI and CT, ultrasound was also well correlated (P ⬍ .001 and P ⬍ .001). In patients who underwent all 3 imaging modalities, no difference was found in the average tumor size (P ⫽ .896). Ultrasound imaging does not appear to be inferior to CT and MRI in the imaging of renal masses. This is useful in reducing the costs and levels of radiation exposure for the long-term follow-up of patients receiving active surveillance. UROLOGY 79: 28 –31, 2012. © 2012 Elsevier Inc.
A
ctive surveillance is becoming an acceptable option for small renal masses in selected patient populations.1 Cross-sectional imaging is essential for accurate diagnosis and staging of renal masses; however, the increased use of ionizing radiation from CT is not without consequences. Epidemiologic evidence suggests that undergoing 2-3 abdominal CT scans annually can result in an increased risk of cancer, and 1.5%2.0% of all cancers in the United States are attributed to the radiation from CT studies.2-4 Although no accepted guidelines are available on the timing of cross-sectional imaging, patients could be subjected to ⱖ2 annually. Magnetic resonance imaging (MRI) is an alternative to CT; however, it is expensive and often takes much longer to complete. Low-dose CT holds promise in monitoring patients with stones5; however, its benefits for the follow-up and diagnosis of renal masses have not been established. Ultrasonography is often considered inferior
From the Department of Urology, University of California, Irvine, School of Medicine, Orange, California; Department of Radiology and Division of Urology, Department of Surgery, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania Reprint requests: Phillip Mucksavage, M.D., Department of Urology, University of California, Irvine, School of Medicine, 101 The City Drive, Building 55, Room 308, Orange, CA 92868. E-mail: [email protected] Submitted: May 4, 2011, accepted (with revisions): September 4, 2011
28
© 2012 Elsevier Inc. All Rights Reserved
to MRI and CT, because it might not adequately image important staging characteristics essential to renal masses. It, however, is the least expensive modality, does not use ionizing radiation, and is very quick to perform. The growth of a tumor, as determined by the largest measured diameter, has been used in the decision to proceed to treatment for a patient on an active surveillance protocol. We examined whether ultrasonography was inferior in determining the size of the renal masses in a contemporary series before therapy at a high-volume tertiary care center.
MATERIAL AND METHODS After institutional review board approval, a retrospective review of a prospectively maintained database consisting of 776 patients who underwent radical or partial nephrectomy from 1994 to 2007 at a single institution for an enhancing renal mass was performed. The clinical and pathologic characteristics of each patient were captured and analyzed according to the preoperative imaging modality. All patients included in the analysis had their imaging reviewed by dedicated genitourinary radiologists. The clinical size of the tumor was defined as the largest reported diameter of the mass. In patients with multiple ipsilateral tumors, only the diameter of the largest tumor was included in the present analysis. 0090-4295/12/$36.00 doi:10.1016/j.urology.2011.09.036
78 (67.2) 38 (32.8)
also highly correlated (r ⫽ .749, P ⬍ .0001; Fig. 1C). A body mass index ⬍30 kg/m2 had slightly lower correlation coefficients for CT (n ⫽ 42, r ⫽ .589, P ⫽ .002) and MRI (n ⫽ 57, r ⫽ .828, P ⬍ .0001) than a body mass index ⬎30 kg/m2 (CT, n ⫽ 24, r ⫽ .816, P ⬍ .0001; MRI, n ⫽ 23, r ⫽ .838, P ⬍ .0001).
28.7 19.2-50.9
COMMENT
Table 1. Characteristics of patients who underwent ultrasonography before treatment (n ⫽ 116) Category Gender (n) Male Female Body mass index (kg/m2) Mean Range Pathologic size (cm) Mean Range Surgery (n) Radical nephrectomy Partial nephrectomy Histologic type (n) Clear cell Papillary Other malignancy Imaging modality (n) MRI and ultrasound CT and ultrasound MRI and CT MRI, CT, and ultrasound
Value
4.45 1-13 79 (68.10) 37 (31.90) 69 (59.5) 19 (16.38) 28 (24.1) 80 (69.0) 66 (56.9) 38 (32.8) 38 (32.8)
Data in parentheses are percentages.
An analysis was performed for each imaging modality using a paired Student t test and Pearson’s correlation. All statistical analyses were performed using Stata software, version 9.0 (StataCorp, College Station, TX) and GraphPad Prism, version 4.03 (GraphPad Software, La Jolla, CA).
RESULTS The clinical characteristics of the patients who underwent ultrasound imaging before radical or partial nephrectomy are summarized in Table 1. Overall, most were men (67.2%), and all renal masses were malignant tumors. Most people underwent MRI and ultrasonography (69%) and 38 patients (32.8%) underwent all 3 modalities. The differences in renal mass size according to CT or MRI are listed in Table 2. On average, ultrasonography overestimated the size of the renal mass compared with MRI and CT by 0.16 cm (or approximately 3.5%) for both modalities, which was not significant. The average size difference between CT and MRI was smaller (0.03 cm or 0.7%). Patients who underwent all 3 modalities were also compared, and no significant difference was found in the tumor size compared across all modalities (P ⫽ .896). The average size measured between each modality comparing tumors ⬍4 cm was also not significantly different (P ⫽ .128); however, the percentage of differences in size was greater (ultrasonography/MRI of 9.67% and ultrasonography/CT of 4.23%). A scatter plot of the largest renal mass size reported by CT or MRI and compared with ultrasound size is shown in Figure 1. A high correlation between the ultrasound size and size reported from CT (r ⫽ .713, P ⬍ .0001) or MRI (r ⫽ .836, P ⬍ .0001). As a comparison, the relationship between the CT and MRI tumor size was UROLOGY 79 (1), 2012
With increased public awareness of the effect of ionizing radiation due to medical imaging, methods to eliminate or reduce radiation exposure are warranted. However, more patients are opting for less-invasive methods, such as active surveillance and thermal ablation, for the treatment or monitoring of a small renal mass.6 Because most tumors on active surveillance protocols are small (⬍3 cm), the risk of a rapidly developing advanced tumor is low.7 These masses might, therefore, be followed up for long periods; thus, using modalities that are readily available and cost-effective are essential. The estimation of renal tumor size and growth rate is essential to current active surveillance protocols. Although CT and MRI are generally preferable to ultrasonography, few comparative data suggest ultrasonography is inferior in predicting renal mass size compared with CT or MRI. In our study, the differences in renal mass size were small (⬍4%) and not statistically significantly different from those found using CT or MRI. The ultrasound size also correlated well with the CT and MRI size for a broad range of sizes. Although no accepted protocols for active surveillance are available, changes in tumor size have been used as a method to initiate delayed intervention. Chawla et al7 performed a meta-analysis of 234 renal mass with an average size of 2.60 cm and found a growth rate of 0.28 cm/y. This is an average 10.7% increase in the largest diameter of the tumor measured, far greater than the ⬍4% difference we found in estimating the tumor size with ultrasonography. However, the growth rate might not always estimate the risk of malignancy because malignant tumors, in a small comparative series, appeared to have the same rate of zero net growth as benign masses.8 It is generally accepted that CT and MRI can much more accurately predict extra renal invasion, the presence or lymph nodes, and evaluation of the retroperitoneum compared with ultrasonography9-11; however, the renal tumor size or growth appears to be 1 of the most important factors in determining delayed intervention in active surveillance protocols. Because most tumors on active surveillance are small, the likelihood of transformation from a Stage T1a to T3a or greater is small, and ⬍2% of patients on an active surveillance protocol have developed metastatic disease.7 Therefore, ultrasonography could be used to estimate the tumor size as an adjunct to routine CT or MRI. Our study had some limitations, not including the retrospective nature of our report. The difference in time 29
Table 2. Comparison of reported size according to imaging modality Imaging
US Size (cm)
CT Size (cm)
CT/US (n ⫽ 66) MRI/US (n ⫽ 80) CT/MRI (n ⫽ 38) CT/MRI/US (n ⫽ 116)
4.73 ⫾ 2.41 4.65 ⫾ 2.48
4.57 ⫾ 2.50
4.47 ⫾ 2.04
4.26 ⫾ 1.91 4.27 ⫾ 1.91
MRI Size (cm) 4.49 ⫾ 2.31 4.29 ⫾ 2.04 4.29 ⫾ 2.04
Difference (cm)
Percentage of Difference (%)
0.16 (⫺7.3-8.0) 0.16 (⫺4.7-6.2) 0.03 (⫺3.6-5.4)
3.50 3.56 0.70
P Value .466* .303* .918* .896†
US, ultrasonography; CT, computed tomography; MRI, magnetic resonance imaging. Percentage of difference compared with CT or MRI size. * P value determined by paired Student t test. † P value determined by one-way analysis of variance.
Figure 1. Scatter plots of reported tumor sizes according to respective imaging modalities. All modalities had significant correlations with ultrasound size.
between the scans was not examined owing to the limited availability of the data. This was not likely to be a major limiting factor because most renal masses are slow growing. The study was also limited by the lack of standardization of obtaining and reviewing each imaging modality, a product of the retrospective nature. Finally, we did not examine other factors that are important when evaluating renal masses, such as invasion of the perirenal or 30
sinus fat, lymph node involvement, and so forth. We did not examine these factors, because it has been clearly established that CT and MRI are far superior at evaluating these parameters, and it was beyond the scope of the present study. Instead, we hoped to alleviate some of the fears of using ultrasonography as an adjunct to CT and MRI in estimating the tumor size. Long-term evaluation of ultrasonography compared with CT and MRI would be UROLOGY 79 (1), 2012
necessary to determine whether ultrasonography can be used as the primary imaging modality of choice in active surveillance protocols. The present study also did not examine the differences between tumor size and radiographic size. This concept has been evaluated in a number of retrospective and prospective studies, which have found that radiographic imaging slightly overestimates the final pathologic size when using CT scanning.12-14 Because these differences are generally very small (⬍1.0 cm), they generally do not affect pathologic staging in most cases or oncologic outcomes. The present study did not examine these differences, because it intended to assuage the inherent fears about using ultrasonography to follow small renal masses. In the present study, we found that ultrasonography was not inferior in detecting the size of renal masses compared with MRI and CT. This is important, given that neither MRI nor ultrasonography require ionizing radiation for imaging. Furthermore, the lack of inferiority of ultrasonography allows for healthcare cost containment, while reducing radiation exposure. In active surveillance protocols, our results also suggest the radiographic tumor size can be meaningfully compared among different imaging modalities, and ultrasonography can be a useful tool for monitoring the size of renal masses.
CONCLUSIONS Ultrasonography does not appear to be inferior in the assessment of renal mass size compared with MRI or CT. This might be beneficial for long-term active surveillance protocols to contain healthcare costs and reduce radiation exposure. References 1. Campbell SC, Novick AC, Belldegrun A, et al. Guideline for management of the clinical T1 renal mass. J Urol. 2009;182:12711279. 2. Fazel R, Krumholz HM, Wang YF, et al. Exposure to low-dose ionizing radiation from medical imaging procedures. N Engl J Med. 2009;361:849-857. 3. Smith-Bindman R, Lipson J, Marcus R, et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med. 2009;169:2078-2086. 4. Brenner DJ, Hall EJ. Computed tomography—an increasing source of radiation exposure. N Engl J Med. 2007;357:2277-2284. 5. Niemann T, Kollmann T, Bongartz G. Diagnostic performance of low-dose CT for the detection of urolithiasis: a meta-analysis. AJR Am J Roentgenol. 2008;191:396-401. 6. Weight CJ, Fergany AF, Gunn PW, et al. The impact of minimally invasive techniques on open partial nephrectomy: a 10-year single institutional experience. J Urol. 2008;180:84-88. 7. Chawla SN, Crispen PL, Hanlon AL, et al. The natural history of observed enhancing renal masses: meta-analysis and review of the world literature. J Urol. 2006;175:425-431. 8. Kunkle DA, Crispen PL, Chen DYT, et al. Enhancing renal masses with zero net growth during active surveillance. J Urol. 2007;177: 849-853.
UROLOGY 79 (1), 2012
9. Levine E, Maklad NF, Rosenthal SJ, et al. Comparison of computed-tomography and ultrasound in abdominal staging of renal cancer. Urology. 1980;16:317-322. 10. Reznek RH. Imaging in the staging of renal cell carcinoma. Eur Radiol. 1996;6:120-128. 11. Jamis-Dow CA, Choyke PL, Jennings SB, et al. Small (⬍ or ⫽ 3-cm) renal masses: detection with CT versus US and pathologic correlation. Radiology. 1996;198:785-788. 12. Herr HW. Radiographic vs surgical size of renal tumours after partial nephrectomy. BJU Int. 2000;85:19-21. 13. Herr HW, Lee CT, Sharma S, et al. Radiographic versus pathologic size of renal tumors: implications for partial nephrectomy. Urology. 2001;58:157-160. 14. Kurta JM, Thompson RH, Kundu S, et al. Contemporary imaging of patients with a renal mass: does size on computed tomography equal pathological size? BJU Int. 2009;103:24-27.
EDITORIAL COMMENT The authors have attempted to address a very timely and clinically relevant question—whether ultrasonography is as good as either computed tomography (CT) or magnetic resonance imaging in determining the size of renal masses. The limitations and drawbacks of these cross-sectional techniques include the frequent need for intravenous contrast injection, dependence on adequate renal function to administer contrast, and generally greater costs compared with ultrasonography. In addition, significant concern has recently been raised regarding the radiation dose associated with abdominal CT and the increased risk of cancer development. Thus, the ability of ultrasonography to accurately assess the size of a mass within the kidney suggests that it might be able to substitute for CT and/or magnetic resonance imaging. This is particularly useful in patients pursuing active surveillance for small renal masses, which is increasingly recognized as a reasonable management strategy for select patients. Additional study is needed to determine how accurate ultrasonography is in serial measurements over time (ie, longitudinal follow-up) and whether the determination of volume is as feasible and accurate as the measurement of a single dimension. In addition, one must be cautious in applying these findings to the typical candidate for surveillance, likely with a smaller tumor size (⬍3 cm). In this study, the mean tumor diameter was approximately 4.5 cm with evidence of greater differences in measurements for the group with masses ⬍4 cm. Improvements in ultrasound technology and techniques (eg, contrast enhancement) might permit better characterization of masses and tumor stage. Although the urologist is traditionally most comfortable interpreting cross-sectional images, particularly CT, we must question the established paradigms and evolve to incorporate comparable modalities with less morbidity for the patient and reduced cost for the healthcare system. In the case of imaging for renal masses, the appropriate modality—including ultrasonography and low-dose CT—should be carefully considered, along with the frequency and actual need for such imaging. Maxwell V. Meng, M.D., Department of Urology University of California, San Francisco, School of Medicine, San Francisco, California doi:10.1016/j.urology.2011.10.003 UROLOGY 79: 31, 2012. © 2012 Elsevier Inc.
31
METHODS
RESULTS
CONCLUSION
To evaluate whether ultrasonography was inferior in detecting the size of a renal mass preoperatively because of the increased attention on the harmful effects of ionizing radiation in medical imaging. A prospectively maintained database was reviewed of all patients who underwent renal ultrasonography before definitive therapy for the renal mass. Every patient who underwent ultrasound imaging also underwent computed tomography (CT) or magnetic resonance imaging (MRI), or both, before treatment. The size of the largest tumor identified per imaging modality was compared among the modalities using correlation and analysis of variance. A total of 116 patients underwent ultrasound imaging before therapy. Of these patients, 80 also underwent MRI, 66 underwent CT, and 38 underwent all 3 modalities before treatment. The average pathologic tumor size for the entire cohort was 4.45 cm (range 1-13). The size differences between CT and MRI compared with ultrasound were small (⬍3.5%). Compared with MRI and CT, ultrasound was also well correlated (P ⬍ .001 and P ⬍ .001). In patients who underwent all 3 imaging modalities, no difference was found in the average tumor size (P ⫽ .896). Ultrasound imaging does not appear to be inferior to CT and MRI in the imaging of renal masses. This is useful in reducing the costs and levels of radiation exposure for the long-term follow-up of patients receiving active surveillance. UROLOGY 79: 28 –31, 2012. © 2012 Elsevier Inc.
A
ctive surveillance is becoming an acceptable option for small renal masses in selected patient populations.1 Cross-sectional imaging is essential for accurate diagnosis and staging of renal masses; however, the increased use of ionizing radiation from CT is not without consequences. Epidemiologic evidence suggests that undergoing 2-3 abdominal CT scans annually can result in an increased risk of cancer, and 1.5%2.0% of all cancers in the United States are attributed to the radiation from CT studies.2-4 Although no accepted guidelines are available on the timing of cross-sectional imaging, patients could be subjected to ⱖ2 annually. Magnetic resonance imaging (MRI) is an alternative to CT; however, it is expensive and often takes much longer to complete. Low-dose CT holds promise in monitoring patients with stones5; however, its benefits for the follow-up and diagnosis of renal masses have not been established. Ultrasonography is often considered inferior
From the Department of Urology, University of California, Irvine, School of Medicine, Orange, California; Department of Radiology and Division of Urology, Department of Surgery, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania Reprint requests: Phillip Mucksavage, M.D., Department of Urology, University of California, Irvine, School of Medicine, 101 The City Drive, Building 55, Room 308, Orange, CA 92868. E-mail: [email protected] Submitted: May 4, 2011, accepted (with revisions): September 4, 2011
28
© 2012 Elsevier Inc. All Rights Reserved
to MRI and CT, because it might not adequately image important staging characteristics essential to renal masses. It, however, is the least expensive modality, does not use ionizing radiation, and is very quick to perform. The growth of a tumor, as determined by the largest measured diameter, has been used in the decision to proceed to treatment for a patient on an active surveillance protocol. We examined whether ultrasonography was inferior in determining the size of the renal masses in a contemporary series before therapy at a high-volume tertiary care center.
MATERIAL AND METHODS After institutional review board approval, a retrospective review of a prospectively maintained database consisting of 776 patients who underwent radical or partial nephrectomy from 1994 to 2007 at a single institution for an enhancing renal mass was performed. The clinical and pathologic characteristics of each patient were captured and analyzed according to the preoperative imaging modality. All patients included in the analysis had their imaging reviewed by dedicated genitourinary radiologists. The clinical size of the tumor was defined as the largest reported diameter of the mass. In patients with multiple ipsilateral tumors, only the diameter of the largest tumor was included in the present analysis. 0090-4295/12/$36.00 doi:10.1016/j.urology.2011.09.036
78 (67.2) 38 (32.8)
also highly correlated (r ⫽ .749, P ⬍ .0001; Fig. 1C). A body mass index ⬍30 kg/m2 had slightly lower correlation coefficients for CT (n ⫽ 42, r ⫽ .589, P ⫽ .002) and MRI (n ⫽ 57, r ⫽ .828, P ⬍ .0001) than a body mass index ⬎30 kg/m2 (CT, n ⫽ 24, r ⫽ .816, P ⬍ .0001; MRI, n ⫽ 23, r ⫽ .838, P ⬍ .0001).
28.7 19.2-50.9
COMMENT
Table 1. Characteristics of patients who underwent ultrasonography before treatment (n ⫽ 116) Category Gender (n) Male Female Body mass index (kg/m2) Mean Range Pathologic size (cm) Mean Range Surgery (n) Radical nephrectomy Partial nephrectomy Histologic type (n) Clear cell Papillary Other malignancy Imaging modality (n) MRI and ultrasound CT and ultrasound MRI and CT MRI, CT, and ultrasound
Value
4.45 1-13 79 (68.10) 37 (31.90) 69 (59.5) 19 (16.38) 28 (24.1) 80 (69.0) 66 (56.9) 38 (32.8) 38 (32.8)
Data in parentheses are percentages.
An analysis was performed for each imaging modality using a paired Student t test and Pearson’s correlation. All statistical analyses were performed using Stata software, version 9.0 (StataCorp, College Station, TX) and GraphPad Prism, version 4.03 (GraphPad Software, La Jolla, CA).
RESULTS The clinical characteristics of the patients who underwent ultrasound imaging before radical or partial nephrectomy are summarized in Table 1. Overall, most were men (67.2%), and all renal masses were malignant tumors. Most people underwent MRI and ultrasonography (69%) and 38 patients (32.8%) underwent all 3 modalities. The differences in renal mass size according to CT or MRI are listed in Table 2. On average, ultrasonography overestimated the size of the renal mass compared with MRI and CT by 0.16 cm (or approximately 3.5%) for both modalities, which was not significant. The average size difference between CT and MRI was smaller (0.03 cm or 0.7%). Patients who underwent all 3 modalities were also compared, and no significant difference was found in the tumor size compared across all modalities (P ⫽ .896). The average size measured between each modality comparing tumors ⬍4 cm was also not significantly different (P ⫽ .128); however, the percentage of differences in size was greater (ultrasonography/MRI of 9.67% and ultrasonography/CT of 4.23%). A scatter plot of the largest renal mass size reported by CT or MRI and compared with ultrasound size is shown in Figure 1. A high correlation between the ultrasound size and size reported from CT (r ⫽ .713, P ⬍ .0001) or MRI (r ⫽ .836, P ⬍ .0001). As a comparison, the relationship between the CT and MRI tumor size was UROLOGY 79 (1), 2012
With increased public awareness of the effect of ionizing radiation due to medical imaging, methods to eliminate or reduce radiation exposure are warranted. However, more patients are opting for less-invasive methods, such as active surveillance and thermal ablation, for the treatment or monitoring of a small renal mass.6 Because most tumors on active surveillance protocols are small (⬍3 cm), the risk of a rapidly developing advanced tumor is low.7 These masses might, therefore, be followed up for long periods; thus, using modalities that are readily available and cost-effective are essential. The estimation of renal tumor size and growth rate is essential to current active surveillance protocols. Although CT and MRI are generally preferable to ultrasonography, few comparative data suggest ultrasonography is inferior in predicting renal mass size compared with CT or MRI. In our study, the differences in renal mass size were small (⬍4%) and not statistically significantly different from those found using CT or MRI. The ultrasound size also correlated well with the CT and MRI size for a broad range of sizes. Although no accepted protocols for active surveillance are available, changes in tumor size have been used as a method to initiate delayed intervention. Chawla et al7 performed a meta-analysis of 234 renal mass with an average size of 2.60 cm and found a growth rate of 0.28 cm/y. This is an average 10.7% increase in the largest diameter of the tumor measured, far greater than the ⬍4% difference we found in estimating the tumor size with ultrasonography. However, the growth rate might not always estimate the risk of malignancy because malignant tumors, in a small comparative series, appeared to have the same rate of zero net growth as benign masses.8 It is generally accepted that CT and MRI can much more accurately predict extra renal invasion, the presence or lymph nodes, and evaluation of the retroperitoneum compared with ultrasonography9-11; however, the renal tumor size or growth appears to be 1 of the most important factors in determining delayed intervention in active surveillance protocols. Because most tumors on active surveillance are small, the likelihood of transformation from a Stage T1a to T3a or greater is small, and ⬍2% of patients on an active surveillance protocol have developed metastatic disease.7 Therefore, ultrasonography could be used to estimate the tumor size as an adjunct to routine CT or MRI. Our study had some limitations, not including the retrospective nature of our report. The difference in time 29
Table 2. Comparison of reported size according to imaging modality Imaging
US Size (cm)
CT Size (cm)
CT/US (n ⫽ 66) MRI/US (n ⫽ 80) CT/MRI (n ⫽ 38) CT/MRI/US (n ⫽ 116)
4.73 ⫾ 2.41 4.65 ⫾ 2.48
4.57 ⫾ 2.50
4.47 ⫾ 2.04
4.26 ⫾ 1.91 4.27 ⫾ 1.91
MRI Size (cm) 4.49 ⫾ 2.31 4.29 ⫾ 2.04 4.29 ⫾ 2.04
Difference (cm)
Percentage of Difference (%)
0.16 (⫺7.3-8.0) 0.16 (⫺4.7-6.2) 0.03 (⫺3.6-5.4)
3.50 3.56 0.70
P Value .466* .303* .918* .896†
US, ultrasonography; CT, computed tomography; MRI, magnetic resonance imaging. Percentage of difference compared with CT or MRI size. * P value determined by paired Student t test. † P value determined by one-way analysis of variance.
Figure 1. Scatter plots of reported tumor sizes according to respective imaging modalities. All modalities had significant correlations with ultrasound size.
between the scans was not examined owing to the limited availability of the data. This was not likely to be a major limiting factor because most renal masses are slow growing. The study was also limited by the lack of standardization of obtaining and reviewing each imaging modality, a product of the retrospective nature. Finally, we did not examine other factors that are important when evaluating renal masses, such as invasion of the perirenal or 30
sinus fat, lymph node involvement, and so forth. We did not examine these factors, because it has been clearly established that CT and MRI are far superior at evaluating these parameters, and it was beyond the scope of the present study. Instead, we hoped to alleviate some of the fears of using ultrasonography as an adjunct to CT and MRI in estimating the tumor size. Long-term evaluation of ultrasonography compared with CT and MRI would be UROLOGY 79 (1), 2012
necessary to determine whether ultrasonography can be used as the primary imaging modality of choice in active surveillance protocols. The present study also did not examine the differences between tumor size and radiographic size. This concept has been evaluated in a number of retrospective and prospective studies, which have found that radiographic imaging slightly overestimates the final pathologic size when using CT scanning.12-14 Because these differences are generally very small (⬍1.0 cm), they generally do not affect pathologic staging in most cases or oncologic outcomes. The present study did not examine these differences, because it intended to assuage the inherent fears about using ultrasonography to follow small renal masses. In the present study, we found that ultrasonography was not inferior in detecting the size of renal masses compared with MRI and CT. This is important, given that neither MRI nor ultrasonography require ionizing radiation for imaging. Furthermore, the lack of inferiority of ultrasonography allows for healthcare cost containment, while reducing radiation exposure. In active surveillance protocols, our results also suggest the radiographic tumor size can be meaningfully compared among different imaging modalities, and ultrasonography can be a useful tool for monitoring the size of renal masses.
CONCLUSIONS Ultrasonography does not appear to be inferior in the assessment of renal mass size compared with MRI or CT. This might be beneficial for long-term active surveillance protocols to contain healthcare costs and reduce radiation exposure. References 1. Campbell SC, Novick AC, Belldegrun A, et al. Guideline for management of the clinical T1 renal mass. J Urol. 2009;182:12711279. 2. Fazel R, Krumholz HM, Wang YF, et al. Exposure to low-dose ionizing radiation from medical imaging procedures. N Engl J Med. 2009;361:849-857. 3. Smith-Bindman R, Lipson J, Marcus R, et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med. 2009;169:2078-2086. 4. Brenner DJ, Hall EJ. Computed tomography—an increasing source of radiation exposure. N Engl J Med. 2007;357:2277-2284. 5. Niemann T, Kollmann T, Bongartz G. Diagnostic performance of low-dose CT for the detection of urolithiasis: a meta-analysis. AJR Am J Roentgenol. 2008;191:396-401. 6. Weight CJ, Fergany AF, Gunn PW, et al. The impact of minimally invasive techniques on open partial nephrectomy: a 10-year single institutional experience. J Urol. 2008;180:84-88. 7. Chawla SN, Crispen PL, Hanlon AL, et al. The natural history of observed enhancing renal masses: meta-analysis and review of the world literature. J Urol. 2006;175:425-431. 8. Kunkle DA, Crispen PL, Chen DYT, et al. Enhancing renal masses with zero net growth during active surveillance. J Urol. 2007;177: 849-853.
UROLOGY 79 (1), 2012
9. Levine E, Maklad NF, Rosenthal SJ, et al. Comparison of computed-tomography and ultrasound in abdominal staging of renal cancer. Urology. 1980;16:317-322. 10. Reznek RH. Imaging in the staging of renal cell carcinoma. Eur Radiol. 1996;6:120-128. 11. Jamis-Dow CA, Choyke PL, Jennings SB, et al. Small (⬍ or ⫽ 3-cm) renal masses: detection with CT versus US and pathologic correlation. Radiology. 1996;198:785-788. 12. Herr HW. Radiographic vs surgical size of renal tumours after partial nephrectomy. BJU Int. 2000;85:19-21. 13. Herr HW, Lee CT, Sharma S, et al. Radiographic versus pathologic size of renal tumors: implications for partial nephrectomy. Urology. 2001;58:157-160. 14. Kurta JM, Thompson RH, Kundu S, et al. Contemporary imaging of patients with a renal mass: does size on computed tomography equal pathological size? BJU Int. 2009;103:24-27.
EDITORIAL COMMENT The authors have attempted to address a very timely and clinically relevant question—whether ultrasonography is as good as either computed tomography (CT) or magnetic resonance imaging in determining the size of renal masses. The limitations and drawbacks of these cross-sectional techniques include the frequent need for intravenous contrast injection, dependence on adequate renal function to administer contrast, and generally greater costs compared with ultrasonography. In addition, significant concern has recently been raised regarding the radiation dose associated with abdominal CT and the increased risk of cancer development. Thus, the ability of ultrasonography to accurately assess the size of a mass within the kidney suggests that it might be able to substitute for CT and/or magnetic resonance imaging. This is particularly useful in patients pursuing active surveillance for small renal masses, which is increasingly recognized as a reasonable management strategy for select patients. Additional study is needed to determine how accurate ultrasonography is in serial measurements over time (ie, longitudinal follow-up) and whether the determination of volume is as feasible and accurate as the measurement of a single dimension. In addition, one must be cautious in applying these findings to the typical candidate for surveillance, likely with a smaller tumor size (⬍3 cm). In this study, the mean tumor diameter was approximately 4.5 cm with evidence of greater differences in measurements for the group with masses ⬍4 cm. Improvements in ultrasound technology and techniques (eg, contrast enhancement) might permit better characterization of masses and tumor stage. Although the urologist is traditionally most comfortable interpreting cross-sectional images, particularly CT, we must question the established paradigms and evolve to incorporate comparable modalities with less morbidity for the patient and reduced cost for the healthcare system. In the case of imaging for renal masses, the appropriate modality—including ultrasonography and low-dose CT—should be carefully considered, along with the frequency and actual need for such imaging. Maxwell V. Meng, M.D., Department of Urology University of California, San Francisco, School of Medicine, San Francisco, California doi:10.1016/j.urology.2011.10.003 UROLOGY 79: 31, 2012. © 2012 Elsevier Inc.
31