Intraosseous extent in rabbit VX2 malignant bone neoplasm: comparison of MRI and pathology

Objective To investigate the value of MRI in displaying intraosseous extent and positive rate of rabbit VX2 malignant bone tumor. Methods Twenty-six male rabbits with the same age (eight weeks) and weight (2.0-3.0 kg) were selected to erect experimental phantom from 2010 to 2012, and were retrospectively reviewed. The neoplastic tissue of VX2 sarcoma was inoculated into the rabbit subtibial plateau marrow cavity to prepare the phantom. The rabbits were taken MR scan with general anesthesia state. The sweep sequences included spin echo (SE) T1WI, fat saturation fast spin echo (Fat-Sat FSE) T2WI, short time inversion recovery (STIR) and Fat-Sat SE T1WI with contrast. The intraosseous extent of tumor was observed and measured from the most distal site to the tibial plateau in the maximum longitudinal diameter of tumor. The model rabbits were executed after MR scan. The right lower extremity was incised continuously with Heavy-duty sliding microtome from MR scan begging with interval of 4 mm. The section specimen was corresponded to MR image with the maximum longitudinal diameter of tumor. Then, measurement and pathology observation were continued. One-way analysis of variance and paired sample t test were used to compare the difference between MRI and section specimen in displaying tumor intraosseous extent. The positive rate was analyzed among each MR sweep sequence with Chi square test, respectively. Results No statistical significance of the value of tumor intraosseous extent was found among the sweep sequence of MR(F=0.802, P>0.05). The average value of group SE T1WI, Fat-Sat FSE T2WI, STIR and Fat-Sat SE T1WI with contrast were (44.5±10.8) mm, (41.0±9.7) mm, (40.7±9.4)mm and (40.3±9.5) mm, respectively. The value of measuring tumor intraosseous extent in group SE T1WI, Fat-Sat FSE T2WI, STIR and group Fat-Sat SE T1WI with contrast were all bigger than the specimen measurement, which was (39.3±9.3)mm. And there was statistical significance between the four MR sequences and section specimen (t=7.053, 6.334, 6.455, 8.150, all P values<0.05). Statistical significance was found between the four MRI groups in displaying the positive rate of tumor intraosseous extent (χ2=16.857, P<0.05). The positive rate ranked from great to small as follows: group Fat-Sat SE T1WI with contrast (76.54%, 62/81), STIR (57.69%, 45/78), Fat-Sat FSE T2WI (52.05%, 38/73), SE T1WI (45.71%, 32/70). The positive rate of group Fat-Sat SE T1WI with contrast in showing tumor intraosseous extent was apparently higher than the other three groups. Statistical significance were found between group Fat-Sat SE T1WI with contrast and groups SE, T1WI, Fat-Sat FSE T2WI, respectively(all P values<0.05). Conclusions Of all the MR sequence above, Fat-Sat SE T1WI with contrast is the most similar to pathology in measuring the tumor intraosseous extent. The positive rate of Fat-Sat SE T1WI with contrast is the highest among all the four sequences of MR. Fat-Sat SE T1WI with contrast is the optical MR sweep sequence in displaying intraosseous extent and the positive rate of malignant bone tumor. Key words: Bone tumor; Intraosseous extent; Rabbit; Magnetic resonance imaging; Pathology