Feasibility study of blood oxygen level-dependent magnetic resonance imaging in evaluating the response of metastatic lymph nodes of rabbit VX2 tumor to radiotherapy

Abstract

Objective: To investigate the feasibility of blood oxygen level-dependent magnetic resonance imaging (MRI) in evaluating the response of metastatic lymph nodes of rabbit VX2 tumor to radiotherapy. Methods: Twenty-eight healthy New Zealand white rabbits which were provided by the Laboratory Animal Center of Soochow University, male or female, 2 to 3 months, weighing 2 to 3 kg, were used to establish the animal model of VX2 tumor popliteal fossa metastatic lymph node, and then were divided into either the radiotherapy group (n=16) or the control group (n=12). The radiotherapy group received a 20 Gy radiotherapy per rabbit, the control group received sham radiotherapy. All rabbits underwent MRI scan on four time points, including before (0 day), 3rd, 7th and 14th days after radiotherapy. The two parameters of size and R(2*) value (s(-1)) of lymph node were measured. At each time point,two rabbits in each group were sacrificed randomly to resect lymph nodes for pathological examination, and two parameters of microvessel density (MVD, strip/HP) and apoptosis index (AI, %) were analyzed. The parameters among the four time points in each group or between the two groups were compared. The correlation of lymph node size and R(2*) value with MVD or AI was analyzed, respectively. Results: A significant size difference was neither between the two groups or among the each time points in each group (P>0.05). The R(2*) of lymph node in the radiotherapy group was (29.6±1.7),(36.8±2.6),(44.8±5.8) and (57.7±6.2) s(-1) at the time points of 0, 3, 7 and 14 days, respectively, showing a gradual increase trend; MVD was (52.3±2.5),(41.0±3.6),(34.0±3.6) and (22.7±2.5) strip/HP respectively, showing a decreasing trend; AI was 12.8%±0.5%,14.9%±0.6%,20.6%±0.5% and 27.5%±0.7% respectively, showing a gradual increase trend (all P<0.05). In the control group, both R(2*) value and AI among the four time points did not change statistically (all P>0.05), but MVD showed a gradual increase trend,(50.0±3.0),(53.0±1.7),(60.3±2.5) and (70.0±2.0) strip/HP, respectively, P<0.05. There were significant differences in R(2*) and MVD at 3, 7 and 14 days, in AI at 7 and 14 days between the two groups (all P<0.05). There was a linear correlation of R(2*) value, but not of size, with MVD and AI (r=-0.87 and 0.94, respectively). Conclusion: Blood oxygen level-dependent MRI can indirectly reflect the hypoxic status of metastatic lymph nodes after radiotherapy, and has potential value in evaluating the response of metastatic lymph nodes to radiotherapy.