23
Urologic Tumors
Eugene C. Lin and Abass Alavi

Renal Cell Carcinoma:1 Renal Masses

Clinical Indication: C

1. Solid lesions. PET imaging is of limited value in solid lesions demonstrated by conventional imaging techniques because resection is usually necessary.
   
2. Indeterminate renal cysts
   
   
   
   
   1. A positive PET scan in an indeterminate renal cyst (Fig. 23.1) is very specific for malignancy, and further diagnostic tests such as cyst aspiration can be avoided before resection.²
      
   2. However, a negative PET scan does not completely rule out malignancy.
   
   
3. Renal metastases. Limited data suggest that PET can detect renal metastases.³ Primary and metastatic renal tumors have a similar degree of fluorodeoxyglucose (FDG) uptake.⁴

Accuracy and Comparison with Other Modalities5

FDG PET is specific for the diagnosis of malignancy in renal masses, but sensitivity will vary depending upon lesion size and location (Table 23.1).

Pearls and Pitfalls

1. Necessity of diuresis. Diuresis is extremely important if PET is performed to evaluate a renal mass.
   
   
   
   
   

   
   

   Fig. 23.1 Cystic renal cell carcinoma. Coronal positron emission tomography/computed tomography demonstrates focal areas of peripheral uptake in a large left upper pole renal cyst, consistent with cystic renal cell carcinoma. However, the lack of fluorodeoxyglucose in a complex renal cyst would not exclude renal cell carcinoma.

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   Table 23.1 Sensitivity and Specificity of Positron Emission Tomography (PET) versus Computed Tomography (CT) in the Detection of Renal Tumors

   	Sensitivity %	Specificity %
   PET	60	100
   CT	92	100

   
   
   
   
   1. False-negative results can result from the presence of urinary activity adjacent to and obscuring the lesion (Fig. 23.2).
      
   2. False-positive results can occur from focal collections of urine that mimic a lesion.
   
   
2. Adjacent lesions. Lesions outside but adjacent to the kidney can sometimes appear to be exophytic renal masses on PET (see Fig. 7.5, p. 82).
   
   
   
   
   • Correlation with anatomical imaging is necessary before a diagnosis of an exophytic renal mass is made.
   
   
3. Degree of uptake. FDG uptake greater than that of renal parenchyma is noted in the majority of renal cell carcinomas, but the degree of uptake is sometimes only minimally more than the surrounding tissues and may be difficult to differentiate from normal parenchyma (Fig. 23.3).
   
   
   
   
   

   
   

   Fig. 23.2 Renal mass or collecting system? Coronal positron emission tomography scan demonstrates focal activity in the upper pole of the right kidney (arrow). Note the similarity of appearance to renal collecting system activity (arrowhead). In this case, it is difficult to determine whether this is uptake in a renal mass or stasis in the upper pole collecting system. The uptake was secondary to a renal cell carcinoma.

   
   
   
   

   
   

   Fig. 23.3 Subtle renal cell carcinoma. Coronal positron emission tomography scan demonstrates a subtle medial right upper pole renal mass (arrow). Note that this is only slightly more intense than normal renal parenchyma and can only be diagnosed by the contour deformity. Most renal cell carcinomas are more intense than this.

   
   
4. Oncocytomas. Oncocytomas are usually isointense with the adjacent renal parenchyma, although uptake can occasionally be in the range of carcinoma.^6,7
   
5. Angiomyolipomas. The spectrum of uptake in angiomyolipomas has not been reported, but there has been one reported case of false-positive uptake in an angiomyolipoma.⁸
   
6. Inflammatory lesions. Inflammatory lesions such as xanthogranulomatous pyelonephritis can have false-positive increased uptake.⁹

Staging and Restaging

Clinical Indication: C

1. PET is primarily useful in
   
   
   
   
   1. Identifying distant metastases (Fig. 23.4)
      
   2. Evaluating indeterminate lesions seen on anatomical imaging techniques
      
   3. Solitary metastasis being considered for resection
      
      
      
      
      

      
      

      Fig. 23.4 Metastatic renal cell carcinoma. Coronal positron emission tomography scan demonstrates a left renal cell carcinoma (arrow) metastatic to the bone, liver, abdominal nodes, and mediastinum (arrowheads).

   
   
2. PET is unlikely to be helpful in tumors with a low histological grade and limited local stage (≤ T2).6
   
3. Restaging. PET is most helpful in further evaluating patients with indeterminate lesions seen on anatomical imaging modalities.

Accuracy and Comparison with Other Modalities

1. Staging and PET. Sensitivity 64%, specificity 100%10
   
   
   
   
   1. PET is insensitive but specific.
      
   2. PET is less sensitive than computed tomography (CT) in all anatomical sites but is more specific in the lungs and bone marrow.⁵
      
   3. PET can sometimes detect bone marrow metastases from renal cell carcinoma, which are not detected on a bone scan.
   
   
2. Restaging and PET. Sensitivity 71%, specificity 75%¹¹

Pitfalls

Testicular Cancer

Staging

Clinical Indication: C

The accuracy of PET in detected testicular neoplasms is unknown, but PET can sometimes detect unsuspected testicular neoplasms (Fig. 23.5). PET is valuable in staging stage II testicular germ cell tumors (Fig. 23.6), but may not be of additional value in patients with stage I tumors.¹²

Accuracy and Comparison with Other Modalities

1. PET. Sensitivity 82%, specificity 94%13
   
2. The primary value of PET compared with CT is in reducing false-positive results.

Pitfalls

1. Both PET and CT will miss disease in small retroperitoneal nodes (≤ 1 cm).
   
2. PET cannot detect mature teratoma.
   
   
   
   
   

   
   

   Fig. 23.5 Testicular tumor. Focal testicular uptake is noted (arrow) on a coronal positron emission tomography scan. This was an incidentally detected right testicular seminoma.

   
   
   
   

   
   

   Fig. 23.6 Metastatic testicular cancer. Coronal positron emission tomography scan demonstrates uptake in a metastatic retroperitoneal node (arrowhead). The patient is status post right orchiectomy (arrow) for testicular cancer.

Recurrence

Clinical Indication: B

1. Histology. It is important to know whether the primary tumor was a seminoma or NSGCT. Mature teratoma does not have increased uptake. Because mature teratoma is present in more than 40% of resected masses in NSGCT, it is a major source of false-negative results (mature teratoma is benign but is removed due to risk of malignant transformation). In seminomas only 4% of residual lesions are mature teratoma. Thus, FDG PET has a greater role in evaluating tumor recurrence in seminoma than in NSGCT (Table 23.2).^14,15
   
2. Lesion size. The management of postchemotherapy masses in patients with seminoma is controversial. Lesions < 3 cm are usually followed by imaging. Lesions > 3 cm are more likely to contain residual tumor, and management ranges from observation to resection.
   

   
   

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   Table 23.2 Sensitivity and Specificity of Positron Emission Tomography (PET) versus Computed Tomography (CT) in the Detection of Recurrent Seminomas

   	Sensitivity %	Specificity %
   PET	80	100
   CT	70	74

   
   
   
   
   • PET is very effective in differentiating residual tumor from fibrosis in lesions ≥ 3 cm (Fig. 23.7).^14–16
   
   
3. Elevated tumor markers
   
   
   
   
   1. PET is helpful in patients with elevated tumor markers regardless of whether a residual mass is seen on CT.
      
   2. In patients with elevated markers and a residual mass, the negative predictive value is low (50%), but PET is often the first modality to identify the recurrence on follow-up studies.¹⁷

Accuracy and Comparison with Other Modalities

1. Seminomas15
   
   
   
   
   1. PET is more accurate than CT, primarily due to the higher specificity of PET.
      
   2. However, there have been reported cases of residual masses, some > 3 cm, with false-positive uptake secondary to necrosis or inflammation.¹⁸
      
      
      
      
      

      
      

      Fig. 23.7 Seminoma with retroperitoneal mass. Axial positron emission tomography/computed tomography in a patient with seminoma demonstrates a large retroperitoneal mass. The right aspect has fluorodeoxyglucose (FDG) uptake consistent with tumor; the left aspect is cystic-appearing without FDG uptake.

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