Orthotopic Versus Allotopic Implantation: Comparison of Radiological and Pathological Characteristics

Abstract

Background: In experimental animal models, implantation location might influence the heterogeneity and overall development of the tumor, leading to an interpretation bias.

Purpose: To investigate the effects of implantation location in experimental tumor model using magnetic resonance imaging (MRI) and pathological findings.

Study type: Prospective.

Subjects: Forty-five breast cancer-bearing mice underwent orthotopic (N = 15) and heterotopic (intrahepatic [N = 15] and subcutaneous [N = 15]) implantation.

Field strength/sequence: Sequences including: T1-weighted turbo spin echo sequence, T2-weighted blade sequence, diffusion-weighted imaging, pre- and post-contrast T1 mapping, multi-echo T2 mapping at 3.0 T.

Assessment: MRI was performed at 7, 14, and 21 days after implantation. Native T1, post-contrast T1, T2, and apparent diffusion coefficient (ADC) of tumors, the tumor volume and necrosis volume within tumor were obtained. Lymphocyte cells from H&E staining, Ki67-positive, and CD31-positive cells from immunohistochemistry were determined.

Statistical tests: One-way analysis of variance and Spearman's rank correlation were performed. P value <0.05 was considered statistically significant.

Results: The tumor volume (intrahepatic vs. orthotopic vs. subcutaneous: 587.50 ± 77.62 mm³ vs. 814.00 ± 43.85 mm³ vs. 956.13 ± 119.22 mm³ ), necrosis volume within tumor (89.10 ± 26.60 mm³ vs. 292.41 ± 57.92 mm³ vs. 179.91 ± 31.73 mm³ , respectively), ADC at day 21 (543.41 ± 42.28 vs. 542.92 ± 99.67 vs. 369.83 ± 42.90, respectively), and post-contrast T1 at all timepoints (day 7: 442.00 ± 11.52 vs. 435.00 ± 22.90 vs. 394.33 ± 29.95; day 14: 459.00 ± 26.11 vs. 436.83 ± 26.01 vs. 377.00 ± 27.83; day 21: 463.50 ± 23.49 vs. 458.00 ± 34.28 vs. 375.00 ± 30.55) were significantly different between three groups. Necrosis volumes of subcutaneous and intrahepatic tumors were significantly lower than those of orthotopic tumors. The CD31-positive rate in the intrahepatic implantation was significantly higher than in orthotopic and subcutaneous groups. Necrosis volume (r = -0.71), ADC (r = -0.85), and post-contrast T1 (r = -0.75) were strongly correlated with vascular invasion index.

Data conclusion: Orthotopic and heterotopic tumors have their unique growth kinetics, necrosis volume, and vascular invasion. Non-invasive MR quantitative parameters, including ADC and post-contrast T1, may reflect vascular invasion in mice.

Level of evidence: 1 TECHNICAL EFFICACY: Stage 3.

Keywords: animal model; heterotopic; orthotopic; tumor microenvironment; vasculature.