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Computed tomography of renal metastases
Computed Tomography of Renal Metastases Francesco Ferrozzi, Davide Bova, and Fabio Campodonico Since the advent of CT, secondary neoplastic lesions of the kidneys have been detected with increasing frequency. After reviewing a large series of cases of renal metastases, we have been able to classify the CT findings into seven major categories that are discussed and illustrated in this article. The differential diagnoses between metastatic disease of the kidneys and other lesions such as renal infarctions, renal lymphoma, and primary malignancies are also considered.
Copyright© 1997 by W.B. Saunders Company
HE ROUTINE performance of CT in the staging and follow-up of malignancies has resulted in increased detection of secondary renal neoplasms. 1.2 Additionally, we have also seen relevant changes in both the known morphological and structural aspects of renal metastases. These changes are probably due to a number of factors, such as the implementation of highly sophisticated scanners, the effects of anticancer therapy, and improved knowledge of the behavior of the causative primaries. The latter two factors particularly appear to be of pivotal importance in the radiologist's approach to the staging and follow-up of neoplastic disease. Unusual morphological and structural features may be more reliably attributed to a primary malignancy; if such features are known, monitoring of structural changes within metastatic lesions during the course of radiation and chemotherapy may offer clues to proper tailoring of further therapy. 3,4 The differential diagnosis of noncystic renal lesions in patients with a history of malignancy has likewise become increasingly important. At the time of staging, differentiation of an incidental benign lesion from a solitary metastasis is a frequent query. Likewise, during follow-up, the ever more frequent finding of a second malignancy, whether synchronous or metachronous, needs to be weighed against the likelihood of secondary neoplastic diffusion. 2-4 Renal metastases are insidiously silent from the clinical and laboratory standpoint. 4,5 Thus CT, which is a very sensitiTe technique in characterizing anonymous renal masses, 1,6,7 becomes mandatory in our opinion. The diagnostic yield of this modality is further increased by an accurate evaluation of pathological features and specific routes of spread of the primary considered.
T
FREQUENCY AND PATHOGENESIS
The finding of secondary renal neoplastic involvement, especially from epithelial tumors, is a frequent occurrence at autopsy, ranging from 7% to 20%, according to different studies reported in the literature. 8,9 In decreasing order of frequency, the most common sites of origin of primary' malignancies are the lung, breast, stomach, pancreas, colon, kidney, and esophagus. Although melanomas metastasize to the kidneys in up to 37% of cases during their clinical course, they represent only 2% of the overall secondary renal neoplasms, due to their relatively rare occurrence) ,I°-13 The typical route of diffusion is generally hematogeneous, resulting in a preferential cortical involvement. The lymphatics have been advocated as the most likely route of propagation to the perirenal spaces; the intercostal, juxtavertebral, and paraaortic lymph nodes would represent the intermediate stations. Neoplastic extension to the kidneys by contiguity has also been reported. 9 The best-known and typical appearance of renal metastases is represented by multiple, bilateral small nodular lesions. 2,4,7 However, not infrequentlY, solitary larger lesions are detected, often arising from large bowel neoplasms. 3,6 A!though renal metastases usually occur in the midst of mu!tiorgan metastatic disease involving liver, lungs, adrenals, and retroperitoneal lymph nodes, 4 it is becoming increasingly frequent to apprec!ate isolated renal involvement as the only From the Istituto di Scienze Radiologiche, Universitgt degli Studi di Parma; Servizio di Radiodiagnostica, Ospedale Militare di Medicina LegaIe di Piacenza. Address reprint requests to Davide Bova, MD, Nuclear Medicine Department, Loyola University Medical Cente~ 2160 S First Ave, Maywood, IL 60153. Copyright © 1997 by W.B. Saunders Company 0887-2721/97/1802-000555.00/0
Seminars in Ultrasound, CT, and MRI, Vol 18, N o 2 (April), 1997: p p 115-121
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FERROZZI, BOVA, AND CAMPODONICO
Fig 1. Adenocarcinoma of the lung. Bilateral smal ! lesions, with homogeneous hypodensity after contrast medium administration.
finding of recurrent neoplastic disease, occasionally even after a long Period after the detection and treatment of the causative primary. 6,7,14 CLINICAL FINDINGS In the majority of cases, renal metastases are asymptomatic. Patients with renal metastasis report only minor and vague complaints, such as lumbar pain or discomfort. The finding of hematuria, most typically of the microscopic type, is occasional, whereas proteinuria is a truly exceptional occurrence.4,6,8,15 In one of the largest series reported in the literature, regarding 118 autoptic analyses of renal metastases, only 17% of patients showed serological signs of renal impairment and 24% of them hematuria. 16 A more blatant symptomatologic and biohumoral picture, with frankly inflammatory marld'ngs and pyonephr0sis, may a!ternatively be the result Of an extraparenchymal extension of the metastasis resulting in hydronephrosis secondary to neoplastic infiltration of the renal pelvis or proximal ureter. 3'!5'17
Fig 2. Adenocarcinoma of the sigmoid colon. CT shows bilateral necrotic masses of different size and contour, with evidence of patchy inhomogeneous contrast enhancement of the solid v!able portion of tumor, (Reprinted by permission of th e publisher from "CT of renal metastatic disease: A pictorial essay," Ferrozzi F, et al, Clinical Imaging, 19:60-64. Copyright. 1995 by Elsevier Science Inc.~1)
necrosis (Fig 3), and with irregular and illdefined borders displaying infiltrating characteristics.2,3,6,9,n 3. Lesions with involvement of the perirenal space, which may present in two ways: A bulging lesion that extends into and effaces the perirenal space (Fig 4) or an irregularly contoured mass with thin streaks that infiltrate the perirena! space, with or without thickening of the renal fascia (Figs 5 and 6). This picture is often seen in metastases from melanoma.6,n,18 4. Hemorrhagic lesions with mild contrast enhancement (Fig 7). Depending on whether the bleed is acute or chronic and, depending on the size of the nodule, the hemorrhagic metas-
CT FINDINGS
In addition to the classical find!ng of multiple, bilateral, hypodense small nodules, the review of a large series of renal metastases has enabled us to show an ample range of morphostructural appearances, collected in seven main categories: 1. Multiple hypodense lesions, generally small (10-15 mm) and bilateral (Figs 1 and 2}, showing only mild contrast enhancement? 2. Single, generally voluminous, lesions with fluid' or fluidlike content, due to colliquated
Fig 3. Aden0carcinoma of the colon. There is a large solitary mass of the left kidney exhibiting central massive necrosis. There is no gross alteration of the renal contour.
CT OF RENAL METASTASES
Fig 4. Small-cell lung cancer. There is a crescentic area of hypovascular, solid tissue occupying the perirenal space on the left that effaces the fat plane between the kidney and the lower pole of the spleen (arrow).
tasis may exhibit a variable appearance, ranging from an inhomogeneously hyperdense texture to a frankly pseudocystic-appearing mass (Fig 8). Hemorrhagic lesions, according to recent studies, appear to be more typical of hypervascular malignancies.S,9,19,20 5. Lesions with loci of calcification (Figs 9 and 10).21, 22
6. Single solid homogeneous and hypovascular lesions, of variable size, generally displaying well-defined and regular contours (Figs 11 and 12). Signs of extracapsular extension of the mass are not the rule. 6,8,9 7. Diffusely infiltrating hypodense lesions that massively involve globally enlarged kidney.
Fig 5, Gastric adenocarcinoma. Massive lymphangitic spread is noted bilaterally in the perirenal spaces, w i t h numerous streaks of soft tissue density of variable thickness and extension. Lymphangitic spread extensively involves the retroperitoneal space at this level.
117
Fig 6. Large-cell anaplastic carcinoma of the lung. A necrotic lesion with irregular contours is noted posteriorly in the right pararenal space, without any apparent extension through the fascia.
These lesions generally display only moderate enhancement after contrast medium administration (Fig 13). 42-9 A close association between the histological appearance of the causative primary tumor and the structural aspect of the metastases as seen on imaging is at times evident. For example, calcification histologically typical of such primary tumors as osteosarcoma and chondrosarcomas (Fig 10) and mucoid and papillary carcinomas often are reproduced in their metastatic loci. 21,22The most important cause is thought to be cellular differentiation toward a form that is osteogenic (orthoplastic or metaplastic calcifications, extensive and amorphous in appearance), secretory (mucoid calcification, often punctate or speckled), or papillary (with the typical psammoma bodies, Fig 9). 23 Analogous considerations are appropriate for hemorrhagic lesions, in which the hypervascular
Fig 7. Leiomyosarcoma of the thigh. CT scan shows a metastatic lesion in the left kidney exhibiting a discrete, well-defined focus of marked central hyperdensity due to recent hemorrhage (arrow).
118
Fig 8. Melanoma. Multiple hemorrhagic lesions of the left kidney are appreciated on this unenhanced CT scan, The most posterior lesion shows mottled foci of mild hyperdensity, attesting its subacute age, whereas the most anterior one appears cystic, due to its older age.
nature of the originating malignancy is reproduced in the secondary lesions as well, with a resultant tendency toward intralesional bleeding (Figs 7 and 8). Melanomas, leiomyosarcomas, malignant apudomas and pheochromocytomas, choriocarcinomas, and hemangiosarcomas are the neoplasms most commonly displaying these features. 9,11,2° However, the relative age of the hemorrhagic event can deeply influence the final appearance of these lesions on the various imaging modalities, CT in particular. 3,~9In the acute phase, these nodules tend to appear inhomogeneously hyperdense (Fig 7), whereas the older lesions appear pseudocystic, occasionally containing fluid-fluid or fluid-debris levels (Fig 8). When close to the parenchymal surface of the kidney, they may rupture through the
Fig 9. Papillary adenocarcinoma of the thyroid. This large exophytic nodule in the lower pole of the left kidney exhibits thick, peripheral ringlike calcification and hypodense content, the latter of which is secondary to colloid secretion by well-differentiated tumor cells.
FERROZZI, BOVA, AND CAMPQDONICO
Fig 10. Primary chondrosarcoma of the orbit. This inhomogeneous metastatic mass in the left kidney exhibits amorphous foci of coarse calcification throughout the lesion, the largest of which measures approximately 2 cm. There is extensive replacement of the normal renal parenchyma by tumoral tissue. This patient presented with concurrent neoplastic thrombosis of the inferior vena cava and multiple lymph node metastases.
capsule, causing perirenal fluid collections. 2° Documentation of even modest levels of contrast enhancement within the solid portion of the lesion becomes a key sign in discriminating against the typical hemorrhagic cyst. In all other solitary solid masses, only biopsy can reliably differentiate a metastasis from primary renal lesions. 5,9,24 DIFFERENTIAL DIAGNOSIS
In our experience, there are four fundamental interpretation problems encountered in CT analysis of renal metastases, which are discussed in the following sections.
Fig 11. Clear-cell adenocarcinoma, previously treated with left nephrectomy. Baseline CT (not shown here) demonstrated a mildly hyperdense nodule which, after intravenous administration of contrast medium, displays only weak enhancement. Contralateral recurrence of disease (arrow) was marked in this patient by simultaneous pulmonary and osseous lesions.
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retraction. Lack of evolution over a short follow-up period may be another helpful clue.
Differential diagnosis qf bilateral large lesions. Similar-looking nodular lymphomatous localizations generally appear more homogeneous in density, exhibit less contrast enhancement, and are typically accompanied by concurrent diffuse adenopathies. 27-29On the other hand, the rare bilateral renal cell carcinomas display early tendency toward colliquative necrosis. 2,4,7 However, neither of these diseases discloses a distinct pathognomonic appearance (Fig 2). Fig 12. Small-cell lung cancer. There is a small solid nodule located on the lateral aspect of the right kidney (arrow). This 1.4-cm mass appears homogenously hypodense and demonstrates only mild bulging of the renal contour with effacement of the cortex. A contralateral voluminous renal cyst is also concurrently noted.
Differential diagnosis between metastases and cystic or ischemic lesions, in the presence of multiple bilateral small lesions (Fig 1). Multiple renal cortical infarctions can indeed be found in neoplastic patients as a result of paraneoplastic hypercoagulability syndromes. 8,25,26 In these settings, we find it extremely helpful to use thinsection (3 to 4 mm) scanning to reduce partial volume averaging artifacts, as well as to accurately assess the presence of even minimal contrast enhancement that would exclude both cysts and infarctions as diagnostic possibilities. Additionally, cortical infarctions may also show signs of capsular
Fig 13. Malignant thymoma. Voluminous lesion massively infiltrating the right kidney, which appears markedly increased in size. The irregularly contoured mass displays only mild contrast enhancement. (Reprinted by permission of the publisher from "'CT of renal metastatic disease: A pictorial essay," Ferrozzi F, et al; Clinical Imaging, 19:60-64. Copyright 1995 by Elseier Science Inc..al)
Differential diagnosis of primary renal tumor versus solitary metastasis. Solitary hypovascular lesions, with homogeneous density, should trigger consideration of oncocytomas, lymphoma, myoid angiomyolipomas, and leiomyomas, in addition to the rarer renal cell carcinomas. In the presence of predominantly necrotic masses (Figs 2 and 3), a synchronous or metachronous renal cell carcinoma is the main differential consideration that should then trigger the search for the other well-known ancillary signs, such as hypervascularization and thrombosis of the renal vein and inferior vena cava. Lastly, the massively infiltrating patterns, with total or subtotal substitution of normal renal parenchyma (Fig 13), are seen again in carcinomas, lymphomas, and sarcomas. However, these patterns notwithstanding, proper diagnosis may be obtained only through biopsy. 2,4,7,24 Differential diagnosis of lesions involving the perirenal space. Lesions in the form of streaks of soft tissue of variable density and shape, with or without true nodular thickenings, are difficult to differentiate (Figs 4 through 6). Collateral venous circulation (renoazygous, gastrorenal, adrenolumbar, and renolumbar, gonadal, and ureteral venous plexa), particularly conspicuous in the event of renal vein thrombosis, is easily recognizable because of its typically striking vascular enhancement.6.11Ja,30 Also, deep and superficial collateral lymph circulation may occasionally resemble neoplastic involvement (Fig 5). Observation of the precise anatomic topography, such as strands of soft tissue converging toward nodal stations of the renal hilum, aorta and vena cava, left para-aortic or, more rarely, superior mesenteric chains, may be of help. Inflammatory changes, secondary to acute pancreatitis or retroperitoneal abscess/cellulitis, posttraumatic hemorrhage, or lymphomatous infiltrations
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FERROZZI, BOVA, AND CAMPODONICO
are other differential entities. History and clinical findings are additional diagnostic guides that can o c c a s i o n a l l y be helpful. 3,4,6,8,15-17 D i a p h r a g m a t i c p s e u d o t u m o r is a peculiar entity represented by a focal nodular h y p e r t r o p h y o f one o f the d i a p h r a g m a t i c crura. This finding may, on s o m e slices, simulate a s p a c e - o c c u p y i n g lesion within the perirenal space, but r e c o g n i t i o n o f the m u s c u l a r origin on the contiguous slices will generally r e s o l v e the doubt. CONCLUSIONS
The p o l y m o r p h i c , structural aspect of renal m e tastases has e x p a n d e d the s p e c t r u m o f differential diagnoses and m a d e the interpretation o f noncystic lesions e v e n m o r e arduous during staging and f o l l o w - u p o f m a l i g n a n t neoplasms. K n o w l e d g e o f the histopathologic, as well as b i o l o g i c and clinical
characteristics o f the primary tumor considered m a y be e x t r e m e l y helpful in the characterization o f the secondary renal lesions. 1,6-9,24 E m p l o y m e n t o f interventional techniques in order to classify histologically the u n d e t e r m i n e d lesions appears e v e r m o r e justified, o w i n g to the radically different o u t c o m e o f a solitary secondary lesion, as o p p o s e d to concurrent primary benign or m a l i g n a n t tumors, either at staging or at followup.4, 24 Albeit m o r e often part o f a larger picture o f disseminated malignancies, 4 renal metastases m a y occasionally be the only f o r m o f recurrence o f disease or the only f o r m of extraorgan extension o f the primary neoplasm. 6,jl In such case, a p r o m p t r e c o g n i t i o n o f their nature is crucial to a tailored therapy and, therefore, to a better patient survival.
REFERENCES 1. Mitnick JS, Bosniak MA, Rothberg M, et al: Metastatic neoplasm to the kidney studied by computed tomography and sonography. J Comput Assist Tomogr 9:43-49, 1985 2. Honda H, Coffman CE, Berbaum KS, et al: CT analysis of metastatic neoplasms to the kidney. Acta Radiol 33:39-44, 1992 3. Nishitani HM, Onitsuka H, Kawahira K, et al: Computed tomography of renal metastases. J Comput Assist Tomogr 8:727-730, 1984 4. Pagani JJ: Solid renal mass in the cancer patient: Second primary renal cell carcinoma versus renal metastasis. J Comput Assist Tomogr 7(3):444-448, 1983 5. Dalla-Palma L, Pozzi Mucelli RS, Zuiani C: Ecografia e tomografia computerizzata nella diagnostica dellc metastasi renali. Radiol Med 82:95-100, 1991 6. Choyke PL, White EM, Zeman RK, et al: Renal metastases: Clinico-pathologic and radiologic correlation. Radiology 162:359-363, 1987 7. Bhatt GM, Bernardino ME, Graham SD: CT diagnosis of renal metastases. J Comput Assist Tomogr 7:1032-1034, 1983 8. Pascal RR: Renal manifestation of extrarenal neoplasms. Human Pathol 22:7-17, 1980 9. Abrams HL, Spiro R, Goldstein N: Metastases in carcinoma. Analysis in 1000 autopsied cases. Cancer 3:75-85, 1950 10. Bruneton JN, Drouillard J, Laurent F, et al: Imagerie des mdtastases r6nales. J Radiol 69:639-643, 1988 11. Strunk H, Schweden F, Teifke A, et al: Nierenemetastasen in der Computertomographie. Rontgen-Blatter 41:140-144, 1988 12. Becker WE, Schellhammer P: Renal metastases from carcinoma of the lung. Br J Uro158:494-498, 1986 13. Hietala SO, Wablqvist L: Metastatic tumors to the kidney. A post-mortem, radiologic and clinical investigation. Acta Radio123:585-591, 1982 14. Ladefoged C, Bisgaard C, Petri J: Solitary renal metastasis 23 years after extirpation of a bronchial cystic adenoid carcinoma. Scand J Thorac Cardiovasc Sorg 18:245-248, 1984 15. Chippindale AJ, Bisset RA, Mamtora H: Two patients
with symptomatic renal metastases. Clinic Radiol 40:95-97, 1989 16. KlingerME: Secondary tumors of the genitourinary tract. J Urol 65:144-153, 1951 17. Issa MM, Cox JL, Jeffrey RB, et al: Diffuse infiltrating renal metastases presenting as pyonephrosis. J Compnt Assist Tomogr 15:175-177, 1991 18. Wilbur AC, Turk JN, Capek V: Perirenal metastases from lung cancer: CT diagnosis. J Comput Assist Tomogr 16:589591, 1992 19. Tipton A, Goldman SM, Fisbman EK, et al: Cystic renal melanoma: CT/ultrasound correlation. Urol Radiol 9:39-41, 1987 20. Mastrodomenico L, Korobkin M, Silverman PM, et al: Perinephric hemorrhage from metastatic carcinoma to the kidney. J Comput Assist Tomogr 7:727-729, 1983 21. Lhospitel S, Abeille JF, Kron PH, et al: Masses calcifi6es du rein. Apropos de 58 dossiers. J Radio169:365-376, 1988 22. Ferrozzi F, Rossi A: Aspects tomodensitom6triques des m6tastases a forme calcifiante. Apropos de 40 cas. J Radiol 72:305-312, 1991 23. Ayres R, Curry NS, Gordon L, et al: Renal metastases from osteogenic sarcoma. Urol Radiol 7:39-41, 1985 24. Chassagne R Perol MB, Doucet J, et al: Les M6tastases rdnales des cancers. Apropos de 9 cas, et revue de la litt6rature. Ann Mdd Interne 145:103-106, 1994 25. Wong WE, Moss AA, Federle MR et al: Renal infarction. CT diagnosis and correlation between CT findings and etiologies. Radiology 150:201-205, 1984 26. Joss R, Goldhirsch A, Brunner K: Komplikationen im Bereich der Niere und der ableitenden Harnwege im Verlauf yon Tumorkrankheiten. J Suisse Med 110:390-404, 1980 27. Bruneton JN, Drouillard J, Caramella E, et al: Lymphomes du rein. Int~r~t de l'echographie et de la scanographie. J Radiol 65:755-760, 1984
CT OF RENAL METASTASES
28. Osborne BM, Brenner M, Weitzner S, et al: Malignant lymphoma presenting as a renal mass. Four cases. Am J Surg Pathol 11:375-382, 1987 29. Jafri SZ, Bree RL, Amendola MA, et al: CT of renal and perirenal non-Hodgkin lymphoma. Am J Roentgenol 138:11011105, 1982
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30. Pollack HM, Banner MR Amendola MA: Other malignant neoplasms of the renal parenchyma. Semin Roentgenol 22:260-274, 1987 31. Ferrozzi F, Bova D, De Chiara F, et al: CT of renal metastatic disease. A pictorial essay. Clinical Imaging 19:60-64, 1995
Copyright© 1997 by W.B. Saunders Company
HE ROUTINE performance of CT in the staging and follow-up of malignancies has resulted in increased detection of secondary renal neoplasms. 1.2 Additionally, we have also seen relevant changes in both the known morphological and structural aspects of renal metastases. These changes are probably due to a number of factors, such as the implementation of highly sophisticated scanners, the effects of anticancer therapy, and improved knowledge of the behavior of the causative primaries. The latter two factors particularly appear to be of pivotal importance in the radiologist's approach to the staging and follow-up of neoplastic disease. Unusual morphological and structural features may be more reliably attributed to a primary malignancy; if such features are known, monitoring of structural changes within metastatic lesions during the course of radiation and chemotherapy may offer clues to proper tailoring of further therapy. 3,4 The differential diagnosis of noncystic renal lesions in patients with a history of malignancy has likewise become increasingly important. At the time of staging, differentiation of an incidental benign lesion from a solitary metastasis is a frequent query. Likewise, during follow-up, the ever more frequent finding of a second malignancy, whether synchronous or metachronous, needs to be weighed against the likelihood of secondary neoplastic diffusion. 2-4 Renal metastases are insidiously silent from the clinical and laboratory standpoint. 4,5 Thus CT, which is a very sensitiTe technique in characterizing anonymous renal masses, 1,6,7 becomes mandatory in our opinion. The diagnostic yield of this modality is further increased by an accurate evaluation of pathological features and specific routes of spread of the primary considered.
T
FREQUENCY AND PATHOGENESIS
The finding of secondary renal neoplastic involvement, especially from epithelial tumors, is a frequent occurrence at autopsy, ranging from 7% to 20%, according to different studies reported in the literature. 8,9 In decreasing order of frequency, the most common sites of origin of primary' malignancies are the lung, breast, stomach, pancreas, colon, kidney, and esophagus. Although melanomas metastasize to the kidneys in up to 37% of cases during their clinical course, they represent only 2% of the overall secondary renal neoplasms, due to their relatively rare occurrence) ,I°-13 The typical route of diffusion is generally hematogeneous, resulting in a preferential cortical involvement. The lymphatics have been advocated as the most likely route of propagation to the perirenal spaces; the intercostal, juxtavertebral, and paraaortic lymph nodes would represent the intermediate stations. Neoplastic extension to the kidneys by contiguity has also been reported. 9 The best-known and typical appearance of renal metastases is represented by multiple, bilateral small nodular lesions. 2,4,7 However, not infrequentlY, solitary larger lesions are detected, often arising from large bowel neoplasms. 3,6 A!though renal metastases usually occur in the midst of mu!tiorgan metastatic disease involving liver, lungs, adrenals, and retroperitoneal lymph nodes, 4 it is becoming increasingly frequent to apprec!ate isolated renal involvement as the only From the Istituto di Scienze Radiologiche, Universitgt degli Studi di Parma; Servizio di Radiodiagnostica, Ospedale Militare di Medicina LegaIe di Piacenza. Address reprint requests to Davide Bova, MD, Nuclear Medicine Department, Loyola University Medical Cente~ 2160 S First Ave, Maywood, IL 60153. Copyright © 1997 by W.B. Saunders Company 0887-2721/97/1802-000555.00/0
Seminars in Ultrasound, CT, and MRI, Vol 18, N o 2 (April), 1997: p p 115-121
115
116
FERROZZI, BOVA, AND CAMPODONICO
Fig 1. Adenocarcinoma of the lung. Bilateral smal ! lesions, with homogeneous hypodensity after contrast medium administration.
finding of recurrent neoplastic disease, occasionally even after a long Period after the detection and treatment of the causative primary. 6,7,14 CLINICAL FINDINGS In the majority of cases, renal metastases are asymptomatic. Patients with renal metastasis report only minor and vague complaints, such as lumbar pain or discomfort. The finding of hematuria, most typically of the microscopic type, is occasional, whereas proteinuria is a truly exceptional occurrence.4,6,8,15 In one of the largest series reported in the literature, regarding 118 autoptic analyses of renal metastases, only 17% of patients showed serological signs of renal impairment and 24% of them hematuria. 16 A more blatant symptomatologic and biohumoral picture, with frankly inflammatory marld'ngs and pyonephr0sis, may a!ternatively be the result Of an extraparenchymal extension of the metastasis resulting in hydronephrosis secondary to neoplastic infiltration of the renal pelvis or proximal ureter. 3'!5'17
Fig 2. Adenocarcinoma of the sigmoid colon. CT shows bilateral necrotic masses of different size and contour, with evidence of patchy inhomogeneous contrast enhancement of the solid v!able portion of tumor, (Reprinted by permission of th e publisher from "CT of renal metastatic disease: A pictorial essay," Ferrozzi F, et al, Clinical Imaging, 19:60-64. Copyright. 1995 by Elsevier Science Inc.~1)
necrosis (Fig 3), and with irregular and illdefined borders displaying infiltrating characteristics.2,3,6,9,n 3. Lesions with involvement of the perirenal space, which may present in two ways: A bulging lesion that extends into and effaces the perirenal space (Fig 4) or an irregularly contoured mass with thin streaks that infiltrate the perirena! space, with or without thickening of the renal fascia (Figs 5 and 6). This picture is often seen in metastases from melanoma.6,n,18 4. Hemorrhagic lesions with mild contrast enhancement (Fig 7). Depending on whether the bleed is acute or chronic and, depending on the size of the nodule, the hemorrhagic metas-
CT FINDINGS
In addition to the classical find!ng of multiple, bilateral, hypodense small nodules, the review of a large series of renal metastases has enabled us to show an ample range of morphostructural appearances, collected in seven main categories: 1. Multiple hypodense lesions, generally small (10-15 mm) and bilateral (Figs 1 and 2}, showing only mild contrast enhancement? 2. Single, generally voluminous, lesions with fluid' or fluidlike content, due to colliquated
Fig 3. Aden0carcinoma of the colon. There is a large solitary mass of the left kidney exhibiting central massive necrosis. There is no gross alteration of the renal contour.
CT OF RENAL METASTASES
Fig 4. Small-cell lung cancer. There is a crescentic area of hypovascular, solid tissue occupying the perirenal space on the left that effaces the fat plane between the kidney and the lower pole of the spleen (arrow).
tasis may exhibit a variable appearance, ranging from an inhomogeneously hyperdense texture to a frankly pseudocystic-appearing mass (Fig 8). Hemorrhagic lesions, according to recent studies, appear to be more typical of hypervascular malignancies.S,9,19,20 5. Lesions with loci of calcification (Figs 9 and 10).21, 22
6. Single solid homogeneous and hypovascular lesions, of variable size, generally displaying well-defined and regular contours (Figs 11 and 12). Signs of extracapsular extension of the mass are not the rule. 6,8,9 7. Diffusely infiltrating hypodense lesions that massively involve globally enlarged kidney.
Fig 5, Gastric adenocarcinoma. Massive lymphangitic spread is noted bilaterally in the perirenal spaces, w i t h numerous streaks of soft tissue density of variable thickness and extension. Lymphangitic spread extensively involves the retroperitoneal space at this level.
117
Fig 6. Large-cell anaplastic carcinoma of the lung. A necrotic lesion with irregular contours is noted posteriorly in the right pararenal space, without any apparent extension through the fascia.
These lesions generally display only moderate enhancement after contrast medium administration (Fig 13). 42-9 A close association between the histological appearance of the causative primary tumor and the structural aspect of the metastases as seen on imaging is at times evident. For example, calcification histologically typical of such primary tumors as osteosarcoma and chondrosarcomas (Fig 10) and mucoid and papillary carcinomas often are reproduced in their metastatic loci. 21,22The most important cause is thought to be cellular differentiation toward a form that is osteogenic (orthoplastic or metaplastic calcifications, extensive and amorphous in appearance), secretory (mucoid calcification, often punctate or speckled), or papillary (with the typical psammoma bodies, Fig 9). 23 Analogous considerations are appropriate for hemorrhagic lesions, in which the hypervascular
Fig 7. Leiomyosarcoma of the thigh. CT scan shows a metastatic lesion in the left kidney exhibiting a discrete, well-defined focus of marked central hyperdensity due to recent hemorrhage (arrow).
118
Fig 8. Melanoma. Multiple hemorrhagic lesions of the left kidney are appreciated on this unenhanced CT scan, The most posterior lesion shows mottled foci of mild hyperdensity, attesting its subacute age, whereas the most anterior one appears cystic, due to its older age.
nature of the originating malignancy is reproduced in the secondary lesions as well, with a resultant tendency toward intralesional bleeding (Figs 7 and 8). Melanomas, leiomyosarcomas, malignant apudomas and pheochromocytomas, choriocarcinomas, and hemangiosarcomas are the neoplasms most commonly displaying these features. 9,11,2° However, the relative age of the hemorrhagic event can deeply influence the final appearance of these lesions on the various imaging modalities, CT in particular. 3,~9In the acute phase, these nodules tend to appear inhomogeneously hyperdense (Fig 7), whereas the older lesions appear pseudocystic, occasionally containing fluid-fluid or fluid-debris levels (Fig 8). When close to the parenchymal surface of the kidney, they may rupture through the
Fig 9. Papillary adenocarcinoma of the thyroid. This large exophytic nodule in the lower pole of the left kidney exhibits thick, peripheral ringlike calcification and hypodense content, the latter of which is secondary to colloid secretion by well-differentiated tumor cells.
FERROZZI, BOVA, AND CAMPQDONICO
Fig 10. Primary chondrosarcoma of the orbit. This inhomogeneous metastatic mass in the left kidney exhibits amorphous foci of coarse calcification throughout the lesion, the largest of which measures approximately 2 cm. There is extensive replacement of the normal renal parenchyma by tumoral tissue. This patient presented with concurrent neoplastic thrombosis of the inferior vena cava and multiple lymph node metastases.
capsule, causing perirenal fluid collections. 2° Documentation of even modest levels of contrast enhancement within the solid portion of the lesion becomes a key sign in discriminating against the typical hemorrhagic cyst. In all other solitary solid masses, only biopsy can reliably differentiate a metastasis from primary renal lesions. 5,9,24 DIFFERENTIAL DIAGNOSIS
In our experience, there are four fundamental interpretation problems encountered in CT analysis of renal metastases, which are discussed in the following sections.
Fig 11. Clear-cell adenocarcinoma, previously treated with left nephrectomy. Baseline CT (not shown here) demonstrated a mildly hyperdense nodule which, after intravenous administration of contrast medium, displays only weak enhancement. Contralateral recurrence of disease (arrow) was marked in this patient by simultaneous pulmonary and osseous lesions.
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119
retraction. Lack of evolution over a short follow-up period may be another helpful clue.
Differential diagnosis qf bilateral large lesions. Similar-looking nodular lymphomatous localizations generally appear more homogeneous in density, exhibit less contrast enhancement, and are typically accompanied by concurrent diffuse adenopathies. 27-29On the other hand, the rare bilateral renal cell carcinomas display early tendency toward colliquative necrosis. 2,4,7 However, neither of these diseases discloses a distinct pathognomonic appearance (Fig 2). Fig 12. Small-cell lung cancer. There is a small solid nodule located on the lateral aspect of the right kidney (arrow). This 1.4-cm mass appears homogenously hypodense and demonstrates only mild bulging of the renal contour with effacement of the cortex. A contralateral voluminous renal cyst is also concurrently noted.
Differential diagnosis between metastases and cystic or ischemic lesions, in the presence of multiple bilateral small lesions (Fig 1). Multiple renal cortical infarctions can indeed be found in neoplastic patients as a result of paraneoplastic hypercoagulability syndromes. 8,25,26 In these settings, we find it extremely helpful to use thinsection (3 to 4 mm) scanning to reduce partial volume averaging artifacts, as well as to accurately assess the presence of even minimal contrast enhancement that would exclude both cysts and infarctions as diagnostic possibilities. Additionally, cortical infarctions may also show signs of capsular
Fig 13. Malignant thymoma. Voluminous lesion massively infiltrating the right kidney, which appears markedly increased in size. The irregularly contoured mass displays only mild contrast enhancement. (Reprinted by permission of the publisher from "'CT of renal metastatic disease: A pictorial essay," Ferrozzi F, et al; Clinical Imaging, 19:60-64. Copyright 1995 by Elseier Science Inc..al)
Differential diagnosis of primary renal tumor versus solitary metastasis. Solitary hypovascular lesions, with homogeneous density, should trigger consideration of oncocytomas, lymphoma, myoid angiomyolipomas, and leiomyomas, in addition to the rarer renal cell carcinomas. In the presence of predominantly necrotic masses (Figs 2 and 3), a synchronous or metachronous renal cell carcinoma is the main differential consideration that should then trigger the search for the other well-known ancillary signs, such as hypervascularization and thrombosis of the renal vein and inferior vena cava. Lastly, the massively infiltrating patterns, with total or subtotal substitution of normal renal parenchyma (Fig 13), are seen again in carcinomas, lymphomas, and sarcomas. However, these patterns notwithstanding, proper diagnosis may be obtained only through biopsy. 2,4,7,24 Differential diagnosis of lesions involving the perirenal space. Lesions in the form of streaks of soft tissue of variable density and shape, with or without true nodular thickenings, are difficult to differentiate (Figs 4 through 6). Collateral venous circulation (renoazygous, gastrorenal, adrenolumbar, and renolumbar, gonadal, and ureteral venous plexa), particularly conspicuous in the event of renal vein thrombosis, is easily recognizable because of its typically striking vascular enhancement.6.11Ja,30 Also, deep and superficial collateral lymph circulation may occasionally resemble neoplastic involvement (Fig 5). Observation of the precise anatomic topography, such as strands of soft tissue converging toward nodal stations of the renal hilum, aorta and vena cava, left para-aortic or, more rarely, superior mesenteric chains, may be of help. Inflammatory changes, secondary to acute pancreatitis or retroperitoneal abscess/cellulitis, posttraumatic hemorrhage, or lymphomatous infiltrations
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are other differential entities. History and clinical findings are additional diagnostic guides that can o c c a s i o n a l l y be helpful. 3,4,6,8,15-17 D i a p h r a g m a t i c p s e u d o t u m o r is a peculiar entity represented by a focal nodular h y p e r t r o p h y o f one o f the d i a p h r a g m a t i c crura. This finding may, on s o m e slices, simulate a s p a c e - o c c u p y i n g lesion within the perirenal space, but r e c o g n i t i o n o f the m u s c u l a r origin on the contiguous slices will generally r e s o l v e the doubt. CONCLUSIONS
The p o l y m o r p h i c , structural aspect of renal m e tastases has e x p a n d e d the s p e c t r u m o f differential diagnoses and m a d e the interpretation o f noncystic lesions e v e n m o r e arduous during staging and f o l l o w - u p o f m a l i g n a n t neoplasms. K n o w l e d g e o f the histopathologic, as well as b i o l o g i c and clinical
characteristics o f the primary tumor considered m a y be e x t r e m e l y helpful in the characterization o f the secondary renal lesions. 1,6-9,24 E m p l o y m e n t o f interventional techniques in order to classify histologically the u n d e t e r m i n e d lesions appears e v e r m o r e justified, o w i n g to the radically different o u t c o m e o f a solitary secondary lesion, as o p p o s e d to concurrent primary benign or m a l i g n a n t tumors, either at staging or at followup.4, 24 Albeit m o r e often part o f a larger picture o f disseminated malignancies, 4 renal metastases m a y occasionally be the only f o r m o f recurrence o f disease or the only f o r m of extraorgan extension o f the primary neoplasm. 6,jl In such case, a p r o m p t r e c o g n i t i o n o f their nature is crucial to a tailored therapy and, therefore, to a better patient survival.
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