Rationale and Objectives
The objective of this study was to evaluate and compare contrast-enhanced subharmonic and harmonic ultrasound as tools for characterizing solid renal masses and monitoring their response to cryoablation therapy.
Materials and Methods
Sixteen patients undergoing percutaneous ablation of a renal mass provided informed consent to undergo ultrasound examinations the morning before and approximately 4 months after cryoablation. Ultrasound contrast parameters during pretreatment imaging were compared to biopsy results obtained during ablation ( n = 13). Posttreatment changes were evaluated by a radiologist and compared to contrast-enhanced magnetic resonance imaging (MRI)/computed tomography (CT) follow-up.
Results
All masses initially showed heterogeneous enhancement with both subharmonic and harmonic ultrasound. Early contrast washout in the mass relative to the cortex was observed in 6 of 9 malignant and 0 of 4 benign lesions in subharmonic mode and 8 of 9 malignant and 1 of 4 benign lesions in harmonic imaging. In cases where the lesion was adequately visualized at follow-up ( n = 12), subharmonic and harmonic ultrasound showed accuracies of 83% and 75%, respectively, in predicting treatment outcome. Although harmonic imaging showed less overall error, no significant differences ( P > .29) in ablation cavity volumes were observed between MRI/CT and either contrast-imaging mode.
Conclusions
Subharmonic and harmonic contrast-enhanced ultrasound may be a safe and accurate imaging alternative for characterizing renal masses and evaluating their response to cryoablation therapy. Although subharmonic imaging was more accurate in detecting effective cryoablation, harmonic imaging was superior in quantifying ablation cavity volumes.
Renal cell carcinoma (RCC) is becoming increasingly common with 63,920 new cases and 13,860 deaths reported in the United States in 2014 . Detection rates of RCC have doubled over the past 50 years, with nearly half of reported cases being detected incidentally during magnetic resonance imaging (MRI) or computed tomography (CT) imaging . Although surgical extirpation remains the standard of care, ablative therapy is playing a larger role in the treatment of poor surgical candidates and also can preserve renal function . Ablation of RCC can be performed laparoscopically or percutaneously. However, percutaneous ablation has been associated with shorter anesthesia times, fewer probes, shorter hospital stays, lower hospital charges, and quicker recovery relative to laparoscopic approaches . Of the thermal ablation modalities, cryoablation is often preferred because it creates an ice ball during ablation which can be tracked by CT guidance during the session. Cryoablation is also significantly less painful than heat-based techniques such as microwave and radiofrequency ablation, thereby avoiding the use of general anesthesia in many patients .
Protocols for cryoablation follow-up imaging vary but mainly rely on contrast-enhanced CT or MRI 1–6 months after procedure to evaluate effective ablation based on a lack of vascularity within the mass and a reduction in tumor volume . At our institution, contrast-enhanced CT or MRI after 3–4 months represents the clinical standard, as earlier imaging has been shown to result in false positives because of artifacts associated with postablation inflammation . Although well validated in evaluating cryoablation outcomes, CT and MRI contrast materials have been associated with nephrotoxicity (iodinated contrast) and nephrogenic systemic fibrosis (gadolinium-based contrast) and are not well suited for a patient population with compromised renal function. Thus, an imaging technique which could effectively evaluate cryoablation outcomes without the associated nephrotoxicity would be beneficial in this patient population.
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Materials and methods
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Ablation Volume=43π×Length2×Width2×Height2 Ablation Volume
=
4
3
π
×
Length
2
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Width
2
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Height
2
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Results
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Discussion
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Acknowledgment
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