Non-Contrast Perfusion Using Arterial Spin Labeled MR Imaging for Assessment of Therapy Response in Metastatic Renal Cell Carcinoma

The incidence of kidney cancer has steadily increased over the past three to four decades and is among the 10 most frequently diagnosed cancers in the US. Approximately 63,990 new cases of kidney cancer are estimated in 2017 and the prognosis has been historically poor. The current 5-year survival rates are estimated at 74% overall, decreasing to 53% among patients with locally advanced diseases.

The most common form of kidney cancer, renal cell carcinoma (RCC), occurs in 90% of all kidney cancers. Among patients with localized RCC who are treated with nephrectomy, approximately one quarter have relapses in distant sites. Among patients with metastatic RCC, the 5-year survival rates are approximately 8%. With better understanding of the pathogenesis of the most common type of RCC, clear-cell renal cell carcinoma (ccRCC), newer treatment options with new agents are being developed to increase survival rates.

The high cost and potential risks associated with human trials for the newly developed experimental therapies have emphasized the need for sensitive monitoring of tumor response. Imaging approaches can play an important role in the evaluation and selection of potential new therapies with non-invasive longitudinal monitoring of treatment response. Currently, the radiological assessment of treatment outcomes predominantly relies on morphological (i.e. size) changes using the Response Evaluation Criteria in Solid Tumors (RECIST) and other similar scores. This is a major limiting factor as the effects of many therapeutic agents at the microscopic level precede the eventual changes in tumor size. One such tumor property that has gained increased attention is angiogenesis, which has been shown to support tumor proliferation and infiltration. Increasing numbers of clinical trials have begun targeting tumor vascular supplies by directly inhibiting angiogenesis (e.g. antiangiogenic therapy). Such clinical trials and the eventual clinical use of these therapies would be greatly assisted by the availability of robust imaging indicators of angiogenesis (i.e. tissue perfusion).

Positron Emission Tomography (PET) using 15O-labeled water (15O-PET) is considered the gold standard for non-invasive measurement of tissue perfusion. However, the use of 15O-PET requires a cyclotron in close proximity to PET to produce short lived 15O-water (half life 2.4 min), limiting its applicability in clinical settings. Alternative imaging techniques include ultrasound using microbubbles, perfusion computed tomography (CT) using iodinated contrast agent and perfusion MRI using gadolinium based contrast agents. All of these techniques require exogenous agents, restricting their use in longitudinal monitoring of treatment response.

ASL-MRI has recently emerged as a quantitative imaging (QI) method to measure perfusion (or capillary blood flow) without the administration of exogenous contrast agents. ASL magnetically "labels" the highly permeable water in the blood as a tracer and measures their accumulation in the tissue of interest, without injecting any exogeneous contrast. Various versions of ASL have been validated in animals using microspheres, and in humans using 15O-PET in the brain. ASL also has a number of advantages compared to dynamic contrast enhanced (DCE) and dynamic susceptibility contrast (DSC) based MR perfusion measurements. Specifically, ASL does not require exogenous agent alleviating the concerns of gadolinium accumulation or nephrogenic systemic fibrosis (NSF) in patients with impaired renal function and, unlike DCE/DSC, the contribution of vascular permeability to ASL measured perfusion is negligible enabling absolute perfusion quantification in physiological units.