Preoperative planning for renal cell carcinoma: benefits of 64-slice CT imaging

REVIEW ARTICLE

Preoperative planning for renal cell carcinoma – benefits of 64–slice CT imaging

Manjiri Dighe^I; Thomas Takayama^II; William H. Bush Jr^I

^IDepartment of Radiology, University of Washington Medical Center, Seattle, Washington, USA

^IIDepartment of Surgery, University of Washington Medical Center, Seattle, Washington, USA

^{Correspondence}

ABSTRACT

Surgery is the primary form of treatment in localized renal cell carcinoma. Adrenal–sparing nephrectomy, laparoscopic nephrectomy and nephron–sparing partial nephrectomy are growing trends for more limited surgical resection. Accurate preoperative imaging is essential for planning the surgical approach. Multislice CT and MR are regarded as the most efficient modalities for imaging renal neoplasms. Development of faster CT systems like 64–slice CT with improved resolution and capability to achieve isotropic reformats have significantly enhanced the role of CT in imaging of renal neoplasms.This review article describes the present state, technique and benefits of 64–slice CT scanning in preoperative planning for RCC.

Key words: renal cell carcinoma; imaging, three–dimensional; surgery

INTRODUCTION

Renal cell carcinoma (RCC) is the fifth most common cancer in adults and accounts for 85% of all malignant tumors of the kidney (1). It represents 3% of all male and 2% of female cancers. The incidence of RCC is 6.0 – 8.0 per 100,000 of the population (2). Surgery is the primary form of treatment in localized renal cell carcinoma. In addition, adrenal–sparing nephrectomy, laparoscopic nephrectomy and nephron–sparing partial nephrectomy are growing trends for more limited surgical resection (3). The aims of preoperative imaging are to characterize the lesion, identify accessory vessels, define adjacent organ extension, identify lymph nodal or visceral metastases, reliably predict the presence and extent of any thrombus in the renal vein, vena cava and heart and evaluate the other kidney.

Multislice CT and MR are regarded as the most effective modalities for imaging renal neoplasms. Development of faster CT systems like the 64–slice CT scanner with improved resolution and capability to achieve isotropic reformats have significantly enhanced the role of CT in preoperative imaging of renal neoplasms.

IMAGING TECHNIQUE AND PROTOCOLS WITH 64 SLICE MULTIDETECTOR TECHNOLOGY

Helical CT has dramatically refined the diagnostic evaluation of renal pathologic conditions by allowing image acquisition through the entire kidney during various phases of contrast enhancement after the administration of a single bolus of intravenous contrast material (4–7). Multi–detector 64–slice CT technology takes advantage of significantly reduced image acquisition time for multi–phasic, thin–slice image acquisitions of the kidneys in a single breath–hold with superior contrast bolus utilization.

A precontrast phase provides a baseline from which to measure the enhancement within the lesion after the administration of intravenous contrast material. Calcifications are also seen in 30% of cases of RCC. The enhancement characteristic is important in distinguishing hyperdense cysts from solid tumors (Figures–1A and ^1B). Because most renal cell carcinomas have a rich vascular supply, they enhance significantly after administration of intravascular contrast material. Enhancement values of more than 15–20 HU are considered suspicious for malignancy (8); more variation in enhancement is found with small masses due to the dense opacification of adjacent normal parenchyma.

Generally, dual phase imaging with a nephrographic phase and a delayed phase is used for evaluating suspected renal neoplasms and depicting perinephric extension and vascular anatomy. For dual phase imaging, 150 ml of non–ionic lower osmolality contrast media (320 mg ml^–1) is injected at 3 cc/sec and images are acquired following delays of approximately 90 seconds and 6 minutes for the nephrographic and pyelographic phases, respectively. In some institutions, an initial scan is also performed in the corticomedullary phase. In the corticomedullary phase, the contrast material is primarily in the cortical capillaries, peritubular spaces, and cortical tubular lamina and has not yet filtered through the more distal renal tubules (9). In the nephrographic phase, the contrast material filters through the glomeruli into the loops of Henle and collecting tubules (9). The renal parenchyma enhances homogenously during this phase, due to tubular accumulation. Renal tumors enhance less than the surrounding parenchyma in this phase, because they lack normal glomerular–tubular function and hence this phase is most valuable for detecting renal masses and characterizing indeterminate lesions (10). The contrast reaches the collecting system during the delayed or the excretory phase and, as a result, the attenuation of the nephrogram progressively decreases (Figure–1C). Macari & Bosniak (11) have suggested that measurement of the washout of contrast material from a lesion at 15 minutes allows differentiation between hyperdense cysts and renal neoplasms. In their study, there was no change in the attenuation of high–density cysts between the initial contrast–enhanced CT scan and the 15–minute–delayed images. In comparison, all lesions that proved to be neoplasms at surgery or follow–up studies showed a decrease in attenuation or "de–enhancement" of at least 15 HU at delayed CT, which was attributed to the washout of contrast material from the vascular bed of the tumor (11).

Figure-1D

Contiguous thin sections are obtained using protocols dependent on machine manufacturer to enable overlapping reconstruction. At our institution, multiphase CT scans are obtained on GE Lightspeed VCT^TM 64 slice scanner at 0.625 mm thickness at a pitch of 1.375:1. These slices are reconstructed at 2.5 mm thickness in the axial plane and coronal reformats are obtained at 5 mm intervals. Sagittal reformats and 3D reconstruction are performed on a case specific basis. Cardiac gated studies are performed for patients with IVC extension (see Figure–4). Multi–phase CT of the abdomen is performed from top of the liver to 1 cm below the lowest kidney. A 15 minute scanogram is performed at the end of the exam to assess the ureters. The protocol used at our institution is illustrated in Table–1.

With the rapid acquisition of multiple simultaneous slices and thin–section reformatted coronal and/or sagittal images, the 64–slice CT scanner provides excellent detail in demonstrating local extension to the bowel and retroperitoneum (see Figure–2, ³, ⁵, ⁶ and ⁷). Multiplanar three–dimensional reconstruction techniques including volume rendering, maximum intensity projection and shaded surface display provide comprehensive information about the relationships and possible involvement of vascular structures in renal neoplasms.

TECHNICAL CONSIDERATIONS

Standard oral contrast media for pre–operative assessment of renal neoplasm and vasculature is unnecessary and may, in fact, obscure intravascular contrast. The radiation dose in our CT scanners is regulated using the Smart mA^TM feature, in which the mAs used is regulated according to the amount of tissue (bone/soft tissue) that has to be traversed in x, y and z axes. For example, the mAs used in the mid–abdomen is less than the mAs used in the pelvis due to the bony structures in the pelvis. The radiation dose given also depends on the patient's size with more dose required for larger patients and less for smaller patients. This is regulated according to the body thickness as illustrated in Table–2. Maximum mAs to be used can also be set in this Auto–mA^TM feature. Figure–1D illustrates the average dose generated during each scan and the total dose report.

IMAGING AND SURGICAL PLANNING

Surgical planning is usually enhanced by the information obtained from imaging studies: extent of the tumor and its location in the kidney; proximity to the renal collecting system and renal vessels; presence of fat planes between the tumor and other structures (for e.g., liver, colon and posterior abdominal wall muscles); tumor extension into the renal vein or IVC and the level of this extension. The presence of intrabdominal metastasis, the function of the contralateral kidney, and the appearance of the adrenal glands are also reliably assessed (12), Tables–3 and ⁴.

CONCLUSION

64 slice CT imaging provides an improvement in the preoperative analysis of renal masses. The reformatted images in multiple planes and cardiac–gated sequences can offer additional help in surgical planning and reformatted images provide detailed information about arterial and venous collaterals and invasion of adjacent organs.

CONFLICT OF INTEREST

None declared.

Accepted:

January 9, 2007

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Publication Dates

• Publication in this collection
  29 Aug 2007
• Date of issue
  June 2007

History

• Received
  09 Jan 2007
• Accepted
  09 Jan 2007