Does arterial spin-labeling MR imaging-measured tumor perfusion correlate with renal cell cancer response to antiangiogenic therapy in a mouse model?

Abstract

Purpose: To determine whether arterial spin-labeling (ASL) magnetic resonance (MR) imaging findings at baseline and early during antiangiogenic therapy can predict later resistance to therapy.

Materials and methods: Protocol was approved by an institutional animal care and use committee. Caki-1, A498, and 786-0 human renal cell carcinoma (RCC) xenografts were implanted in 39 nude mice. Animals received 80 mg sorafenib per kilogram of body weight once daily once tumors measured 12 mm. ASL imaging was performed at baseline and day 14, with additional imaging performed for 786-0 and A498 (3 days to 12 weeks). Mean blood flow values and qualitative differences in spatial distribution of blood flow were analyzed and compared with histopathologic findings for viability and microvascular density. t Tests were used to compare differences in mean tumor blood flow. Bonferroni-adjusted P values less than .05 denoted significant differences.

Results: Baseline blood flow was 80.1 mL/100 g/min +/- 23.3 (standard deviation) for A498, 75.1 mL/100 g/min +/- 28.6 for 786-0, and 10.2 mL/100 g/min +/- 9.0 for Caki-1. Treated Caki-1 showed no significant change (14.9 mL/100 g/min +/- 7.6) in flow, whereas flow decreased in all treated A498 on day 14 (47.9 mL/100 g/min +/- 21.1) and in 786-0 on day 3 (20.3 mL/100 g/min +/- 8.7) (P = .003 and .03, respectively). For A498, lowest values were measured at 28-42 days of receiving sorafenib. Regions of increased flow occurred on days 35-49, 17-32 days before documented tumor growth and before significant increases in mean flow (day 77). Although 786-0 showed new, progressive regions with signal intensity detected as early as day 5 that correlated to viable tumor at histopathologic examination, no significant changes in mean flow were noted when day 3 was compared with all subsequent days (P > .99).

Conclusion: ASL imaging provides clinically relevant information regarding tumor viability in RCC lines that respond to sorafenib.

Figure 1:

Experimental groups and design. Rad-path denotes that both ASL MR imaging and histopathologic examination were performed, as described in the text. For the A498 follow-up (f/u) group, all five animals were imaged weekly for various periods until growth of 3 mm was noted.

Figure 2:

Average growth curve of 786-0 cell line. Size was measured daily with calipers; mean blood flow values (in mL/100 g/min) in shaded boxes were calculated from ASL MR images. Unlike changes in spatial blood flow distribution (see Fig 3), no significant change was seen in mean blood flow from day 3 to 22, despite measured tumor growth.

Figure 3:

Histopathologic correlation of sorafenib-treated 786-0 tumors with ASL perfusion maps. Top row: ASL perfusion maps (with yellow region of interest outlining the tumor). Middle row: Hematoxylin-eosin–stained specimens (entire specimen is tumor) (original magnification, ×2). Bottom row: Immunohistochemistry images. (CD34 stain; original magnification, ×20.) ASL map shows decreased signal intensity on day 3 with return of peripheral signal intensity (arrow in top row) from day 9 to day 22. Viability (dark pink on day 9 that surrounds lighter necrosis, arrow in middle row) and vessels (brown-stained structures on immunohistochemistry images, arrows in bottom row) correlated with new ASL signal intensity on days 9 and 22.

Figure 4:

Average growth curve of A498 cell line. Data to day 60 show decreased blood flow and stable or decreased tumor size. Beyond 60 days, growth measurements of a single representative tumor are presented (denoted by lighter gray diamonds), with average blood flow values in animals regrowing to 15 mm. Size was measured daily with calipers; mean blood flow values (in mL/100 g/min) presented in the shaded boxes were calculated from ASL MR images.

Figure 5:

ASL perfusion maps in sorafenib-treated A498 tumors. Yellow region of interest outlines the tumor in each image. Mean blood flow (in mL/100 g/min) is noted below the image. Color table represents flow values. ASL signal intensity is high at baseline, with decrease of both signal intensity and tumor size following therapy. Minimum values are seen at 6 weeks. Yet at 7 weeks, blood flow was up to 19.362 mL/100 g/min, with stable tumor size. Definitive measurable tumor growth was seen at 11 weeks, which was 4 weeks after the reversal of blood flow. gm = Gram.

Figure 6:

Histopathologic correlation of sorafenib-treated A498 tumors. Left: ASL perfusion maps (with yellow region of interest outlining the tumor). Middle: Hematoxylin-eosin–stained specimens. (Original magnification, ×2.) Right: Immunohistochemistry images. (CD34 stain; original magnification, ×20.) Color table represents flow values. gm = Gram.

Figure 7:

Average growth curve of Caki-1 cell line. Size was measured daily with calipers; mean blood flow values (in mL/100 g/min) presented in the shaded boxes were calculated from ASL MR images. No effect of sorafenib was seen for this cell line.

Figure 8:

Histopathologic correlation of sorafenib-treated Caki-1 tumors. Left: ASL perfusion maps (with yellow region of interest outlining the tumor). Middle: Hematoxylin-eosin–stained specimen (entire specimen is tumor). (Original magnification, ×2.) Right: Immunohistochemistry images. (CD34 stain; original magnification, ×20.) Color table represents flow values. gm = Gram.